Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Tracing Black Hole and Galaxy Growth Across Environments Since Cosmic Noon
Authors: Emma Jane Weller et al.
First Author’s Institution: Yale University
Status: Published in ApJ

They say you can’t choose your hometown, but it still ends up shaping who you become. Turns out the same is true for black holes. The galaxy a black hole grows up in will have a major impact on how it will live out the rest of its life. Similarly, the black hole can leave a lasting impression on its galaxy!

Astronomers have long known that a galaxy’s stellar mass (the combined mass of all its stars) tracks pretty tightly with the mass of the black hole sitting at its centre: bigger galaxy, bigger black hole. This is one pillar of the idea that galaxies and their central black holes coevolve; that is, their life stories are tangled up with one another from early on. But ever since JWST started spotting suspiciously large black holes in the early universe, astronomers have been debating whether this relationship holds steady across cosmic time, or whether the rules were different when the universe and everything in it was younger.

Not Everyone Stays in the Town Centre

Most previous studies of the relationship between black holes and their galaxies have focused only on the central black hole: the biggest, most famous resident of a galaxy, sitting right downtown at its centre. That’s a reasonable place to start, since the central black hole is usually the most massive one around and tends to dominate the story. But this article’s authors wanted to know what happens to everyone else in town, too!

That includes satellite black holes, which are the central black holes of smaller “satellite” galaxies that orbit a bigger “primary” galaxy, a bit like a small town around a big city. It also includes a stranger, more recently discovered population: wandering black holes. These are the black hole equivalent of someone whose hometown got swallowed up by an expanding metropolis, and who never quite settled into their new surroundings.

Here’s how it happens: when a small satellite galaxy merges with a larger one, the satellite gets torn apart by tidal forces and its gas and stars are stripped away. However, its central black hole is often left behind, drifting through the new, much bigger galaxy on a wide orbit. The process that would normally drag a black hole down to the centre of a galaxy, called dynamical friction, is painfully slow, so wanderers can spend billions of years roaming the outskirts.

Two Cities, Two Sets of Rules

To study how these different black hole “life paths” shape, and are shaped by, their host galaxies, the authors turned to two cosmological simulations, ASTRID and TNG300, which handle black hole physics quite differently.

In TNG300, black holes are assumed to start fairly massive and are snapped straight to the centre of their host galaxy at every timestep. This makes mergers happen quickly and often, but it also means there’s no such thing as a wandering black hole in TNG300.

ASTRID, on the other hand, seeds smaller black holes and lets them move more realistically through their galaxy using a subgrid dynamical friction model. This is what allows wandering black holes, with their slow, aimless orbits, to exist and be studied in the first place.

Figure 1 reveals a lot at once. First, ASTRID (the simulation that allows wandering black holes and tidally stripped satellites) has whole new neighbourhoods on the map that simply don’t exist in TNG300. Those populations clearly aren’t following the same tidy, linear coevolution that central black holes enjoy with their galaxies. Second, the most massive black holes tend to be the most yellow, meaning mergers did the heavy lifting for their growth. This is especially pronounced in TNG300, where roughly 80–90% of the growth of the most massive black holes comes from mergers. Meanwhile, the wanderers and stripped satellites sit at the deep-purple end: these black holes have barely merged with anyone at all.

Four Kinds of Black Hole Life Stories

Pulling all of this together, the authors sketch out a kind of “neighborhood map” of black hole fates, sorted by where a black hole ends up on the mass-relation plot and how it got there (see Figure 2).

• Normal centrals are the homebodies. They sit at the centre of primary or satellite galaxies and grow up steadily alongside their hometown, climbing the mass relation together through a healthy mix of accretion and the occasional merger. Their galaxies keep forming stars the whole time in a functional, unremarkable, thoroughly nice place to live.
• High-mass centrals are the ones who “made it.” These are central black holes in big primary galaxies, and their growth comes overwhelmingly from mergers with other black holes. Once they get big enough, they start throwing their weight around: their jets blast gas out of the galaxy, shutting down both star formation and their own further growth.
• Stripped satellites are the annexed suburbs. They start out on the normal mass relation, but a merger with a bigger galaxy tears away their gas and stars, shrinking their stellar mass while their black hole is left comparatively overweight for its new, smaller home. With their gas reserves gone, star formation grinds to a halt, and their black holes, starved of fuel, barely grow either.
• Wanderers are the ones who moved to the big city and never quite settled in. They start out on the normal relation inside a satellite galaxy, but when that galaxy merges into something much bigger, they’re left drifting through unfamiliar, low-density territory. With little gas around to feed on and almost no chance of merging with anyone else, their growth basically stalls.

This four-way map is a genuinely useful new way to read the black hole–galaxy relation: a black hole’s exact position on the plot isn’t just a number, it’s a fossil record of everywhere it’s ever “lived.”

There’s plenty more in the article we haven’t covered here, including how quenching plays out over time, how these results stack up against the local universe, and a closer look at how star formation rates and black hole accretion rates track each other across all four populations. If any of this piqued your interest, the full article is well worth a read.

Original astrobite edited by Ryan White and Chloe Klare.

About the author, Jayde Willingham:

I am a first-year PhD student at Swinburne University of Technology studying the early universe and how the first galaxies came to be. My work combines observations and theoretical modelling to explore how galaxies form, evolve, and shape the universe we see today. When I’m not at my desk or doing SciComm, I’m probably reading a good book with a hot beverage under a cozy blanket.

The post Not All Black Holes Who Wander Are Lost appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Phase-Dependent Magnetic Coherence in the Turbulent Interstellar Medium
Authors: Iryna S. Butsky et al.
First Author’s Institution: Stanford University
Status: Accepted to ApJ

The space between stars isn’t empty, because it’s threaded with magnetic fields that shape how gas moves and steer cosmic rays as they race across the galaxy. As a result, understanding how the magnetic field is structured within different components of the interstellar medium, specifically clouds of different temperatures, is a key part of understanding how the interstellar medium we see today takes shape.

Because magnetic fields are invisible, astronomers need tracers to reveal their structure. One such tracer is the emission from dust grains. Dust grains are not spherical, and a dust grain’s long axis tends to align at a right angle to the magnetic field, causing dust grains to emit light that vibrates in a particular direction. This is called polarized emission. If the magnetic field in a region is very ordered, the grains all align the same way, and a large fraction of their emitted light vibrates in the same direction, giving a high polarization fraction. This fraction is therefore used as a proxy for whether the field can maintain a consistent direction on large scales (an ordered/regular magnetic field).

Comparing the polarization fraction with other tracers of gas lets astronomers infer the field’s structure in the interstellar medium. Previous observations found that the polarization fraction correlates with the fraction of cold gas, but not with the total amount of gas along the line of sight. This suggests that the field is preferentially ordered within cold clouds rather than warm clouds. Curiously, this seemed to contradict other studies that found the field to be more ordered in the warm clouds. Such disagreements are where simulations become useful: because the true magnetic field is known in a simulation, astronomers can test which interpretation the observations actually support.

In this article, the authors use high-resolution numerical simulations to reproduce the relationship between polarization fraction and cold cloud fraction. They find that as long as the line of sight crosses fewer than about 20 clouds, the polarization fraction correlates with the cold cloud fraction, but not with the total neutral gas (Figure 1). This matches earlier observations, where the correlation shows up at high galactic latitudes, which point out of the galactic plane and therefore cross only a few cold structures. When too many clouds lie along the line of sight, each cloud carries an ordered field pointing in a different direction. Averaged together, these vibration directions cancel out, lowering the polarization fraction and washing the correlation away.

Since the simulation reproduces the observational result, does it therefore agree with the conclusion that magnetic fields are genuinely more ordered in the cold neutral medium? The authors argue that the answer depends on how you measure “order.” Comparing the magnetic field direction in individual dense clouds, they find that cold clouds have more ordered fields when compared against warm clouds of similar mass, but the conclusion flips when comparing clouds of similar length (Figure 2).

This apparent paradox comes from the different nature of cold and warm clouds. Cold clouds are denser and more compact, occupying much smaller physical scales than warm clouds. So to match the length of a warm cloud, you have to string together several cold structures, sampling more of the field’s variation and making the cold gas look disordered. Measured per unit length, then, warm clouds appear more ordered. Measured per unit mass, cold clouds appear more ordered. Crucially, when we observe dust emission, we are naturally weighting by mass, so real observations effectively trace the polarization fraction per unit mass, the regime in which the cold clouds are the more coherent phase.

In the end, this work shows that the ordering of interstellar magnetic fields depends on both the gas phase and how we choose to measure it. Cold clouds carry more coherent fields per unit mass, which is exactly the quantity dust observations trace, resolving an apparent contradiction between earlier studies. It is a reminder that even our best observations come with hidden assumptions, and that simulations, where the true field is known, are essential for interpreting what the sky is really telling us.

Original astrobite edited by Viviana Cáceres.

About the author, Sandy Chiu:

I’m a PhD candidate at the University of Michigan, Ann Arbor. I’m interested in numerical simulations of cosmic rays feedback in galaxies and their comparison with observation.

The post The Invisible Architecture Between the Stars appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The SPHEREx Satellite Mission
Authors: James J. Bock et al.
First Author’s Institution: California Institute of Technology; Jet Propulsion Laboratory
Status: Published in ApJ

You know JWST. You’ve likely heard about the recently launched Nancy Grace Roman Space Telescope. But did you know that NASA launched a third space telescope focused on infrared astronomy just last year? That telescope, called SPHEREx (short for Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer, but we’ll forgive the tortured acronym), is currently carrying out its ambitious mission to record an infrared spectrum for every point in the sky. Let’s break down SPHEREx’s three main science goals and then discuss how SPHEREx will accomplish its mission.

Goal #1: Stress-Testing Cosmic Inflation

According to our best current models of cosmology, the very early universe experienced a rapid burst of accelerating expansion called cosmic inflation. During inflation, small regions of the universe that by chance had slightly higher energy density than their neighbors (due to quantum fluctuations) expanded to huge scales. Because these regions were very gravitationally attractive, once inflation ended, they became the seeds where matter accumulated and eventually formed galaxies.

This means that the distribution of galaxies that we see today is a reflection of the properties of inflation (i.e., what the quantum fluctuations were like and how inflation supersized them). SPHEREx will assemble the largest atlas of galaxies ever, recording the three-dimensional positions of hundreds of millions of galaxies across the entire sky. To do this, SPHEREx must figure out how far away these galaxies are from Earth, using their redshifts. SPHEREx’s low-resolution infrared spectroscopy allows for moderately precise measurements of these redshifts. By performing statistical analyses of how galaxies are separated from each other, SPHEREx will test different theories of inflation, like whether inflation was driven by multiple fields.

Goal #2: Frozen Assets

Astronomers are racing to find evidence for life (or at least habitability) in the atmospheres of exoplanets through transmission spectroscopy. However, we don’t know for sure how molecules that could be used in biological processes (called biogenic molecules, these include water, carbon dioxide, carbon monoxide, and methanol) even end up on planets.

We think that a large fraction of these molecules are frozen onto dust grains throughout the interstellar medium. When a new star forms in the interstellar medium, this ice and dust would become part of the star’s protoplanetary disk. The ice species can help create planetesimals (which may grow into planets) or can later deliver biogenic molecules to planetary surfaces. Earth’s oceans likely came (at least in part) from collisions with icy asteroids. As Figure 1 shows, SPHEREx will measure the abundance of ice species in different environments that track the history of solar system formation, from dense clouds in the interstellar medium that may soon form stars to regions around newborn stars and evolving protoplanetary disks. In this way, SPHEREx will track how biogenic molecules from the interstellar medium can end up on planets.

Goal #3: Your Noise Is SPHEREx’s Signal

SPHEREx will measure an infrared spectrum for every point in the sky — even those that don’t contain galaxies or stars that we can resolve. These points are instead dominated by diffuse extragalactic background light. This light comes from many different sources, including stars, dust, and quasars, from the early universe to the present day, all added together incoherently. For most telescopes, the extragalactic background light is just a frustrating source of noise. For SPHEREx, it’s a treasure trove. We can compare the properties of the extragalactic background light that SPHEREx measures (its spectrum and how it fluctuates across the sky) to theoretical predictions and see if we seem to be missing anything. SPHEREx’s very broad infrared spectra — capturing redshifted light from the very early universe — and total sky coverage are crucial for this purpose. For instance, the SPHEREx team will try to isolate emission from the first stars and galaxies, which ionized most of the gas in the universe in the epoch of reionization. By understanding this and other contributions to the extragalactic background light, we can test hypotheses about how galaxies form and evolve.

Making It Happen

Beyond inflation, ices, and extragalactic background light, SPHEREx’s all-sky infrared spectra will be invaluable for countless other science cases, like studying asteroids, characterizing the atmospheres of stars that host exoplanets, and even understanding the upper atmosphere of Earth, through which SPHEREx orbits.

But how can SPHEREx even make a spectrum for every point in the sky? An image of some part of the sky is fundamentally two-dimensional, and measuring how the brightness of every point changes with wavelength adds a third dimension. Astronomers have come up with many ways to solve this problem. Integral field spectroscopy approaches typically involve splitting an image into small regions or strips, then using a dispersive element like a prism or grating to make a spectrum of each region, and finally putting these spectra side by side on a camera. Figure 2 shows an example of this strategy, as well as what the final three-dimensional “datacube” looks like after processing.

This approach is fairly complicated, relying on fancy optics, a prism or grating, and oftentimes optical fibers. From Figure 2, note that you could also make the final “datacube” product if you simply took many images with narrow filters that let through different wavelengths of light. This takes to an extreme the approach astronomers use for spectral energy distribution fitting. Of course, this would require an exorbitant number of filters. Here, the SPHEREx team had a stroke of genius: instead of having many different filters, what if you had a filter where the transmitted wavelength changes linearly across the filter? Each row would let through a different wavelength. To get a full spectrum for all points in an image, you could take many exposures, moving the telescope slightly between each exposure so points in the image fall on different rows each time.

This is the SPHEREx survey strategy, with a few complications. First, before any filter, SPHEREx splits incoming light into two beams, with short-wave infrared light (SWIR, with 0.75 μm < λ < 2.42 μm) going in one direction and mid-wave infrared light (MWIR, 2.42 μm < λ < 5.00 μm) going in another. SPHEREx makes images with these two beams on different sets of detectors, so it can measure the brightness of an object at one SWIR wavelength and one MWIR wavelength simultaneously. The SPHEREx field of view covers 10.5° by 3.5°, with the SWIR and MWIR images each spread across three detectors. Every detector has a different linearly variable filter. To get a complete infrared spectrum of a given point in the sky, SPHEREx moves that point to 17 different positions on each detector. This requires 51 different telescope (17 × 3 detectors) pointings in all and yields 102 spectral values (51 × 2 for the SWIR and MWIR beams). Figure 3 shows the 17 bands on each of the six SPHEREx detectors, each corresponding to a different wavelength value.

While it would take decades to collect spectra for every point on the sky with conventional methods for integral field spectroscopy, the SPHEREx team devised a nifty solution that will collect this dataset in a few years. Soon, SPHEREx will start answering questions about cosmic inflation, interstellar ices, and extragalactic background light, while its rich dataset will be relevant for decades to come. So tell all your friends about SPHEREx (even if you can’t remember what the acronym stands for)!

Original astrobite edited by Veronika Dornan.

About the author, Chris Layden:

Chris is a PhD candidate in astrophysics at the Massachusetts Institute of Technology. His primary research interest is developing instrumentation that enables high-speed astronomical observations. With such instruments, he studies the most rapidly varying phenomena in the sky, including millisecond pulsars, accreting black holes, and solar system objects whizzing past background stars. Outside of astronomy, Chris enjoys reading classic novels, playing ultimate frisbee, and hanging out with his wife and dog.

The post SPHEREx: The Niftiest Space Telescope You May Not Have Heard Of appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Discovery of the Distant, Ultra-Faint Milky Way Satellite Aquarius IV with the Vera C. Rubin Observatory Early Data Preview 2
Authors: William Cerny et al.
First Author’s Institution: Yale University
Status: Published in RNAAS

Warming Up the World’s Largest Camera

Today’s bite covers one of the NSF–DOE Vera C. Rubin Observatory‘s first major discoveries, which was found in testing data before the observatory began its decade of observations. Perched atop a mountain in Chile, the brand-new Rubin Observatory is just beginning a 10-year survey called the Legacy Survey of Space and Time (LSST). LSST will be the deepest and widest sky survey ever conducted, taking images of the entire southern night sky every three nights. This ultra-wide, ultra-high-definition time-lapse of the universe will help answer fundamental questions about dark matter and dark energy, study objects in our solar system, find distant supernova explosions, and more.

LSST is a photometric survey, meaning its main data product is images in several filters. These images are taken with the largest digital camera ever constructed, about the size of a Mini Cooper. The LSST camera has a resolution of 3.2 gigapixels. To put that number into perspective, you would need about 13 copies of the Las Vegas Sphere to display just a single LSST image… and hundreds of these images will be taken every night!

The authors of today’s article use the first dataset from the LSST camera, called Early Data Preview 2 (EDP2). EDP2 was taken from April 2025 to January 2026, covered about 3,000 square degrees (about 7% of the entire night sky), and primarily served as a test before beginning the 10-year LSST (which began in June 2026).

Small Galaxies, Big Questions

Since LSST will survey a large area at unprecedented depths, one field of interest is to study extremely dim and small galaxies called ultra-faint dwarf galaxies (UFDs). These galaxies are so faint that we can only find them close to home, orbiting the Milky Way and other nearby galaxies as satellites. UFDs don’t have much luminous matter, meaning they are likely dominated by their dark matter halos. This makes them useful test beds for our theories of dark matter and galaxy formation, if we can spot them.

Many UFDs don’t look like normal galaxies that resemble blobs of diffuse light with stars; instead, they’re more like a handful of individual stars in an image that also contains foreground stars and background galaxies. So how do you identify them? The trick is that stars born together at the same time from the same gas follow a predictable track in color and brightness called an isochrone. The authors slide a model isochrone for an ancient, metal-poor population through the data at a range of assumed distances and ask: at any spot in EDP2, are there more stars sitting on that track than random chance would predict? Cerny and coauthors found a new spot, which they call Aquarius IV, a new UFD candidate (Figure 1).

Aquarius IV had not been identified previously by any other observatory. However, the authors combed through data from LSST’s predecessor (the Dark Energy Survey, which concluded taking data in 2019) and found previously missed evidence that supports their discovery, namely an excess of faint, blue, marginally resolved stars. As a sanity check, they confirm that all previously known UFDs within the EDP2 footprint (Sagittarius II, Aquarius II, Aquarius III, and Virgo III) are also detected using their methods.

The authors also infer several properties of Aquarius IV, including its centroid coordinates, half-light radius, ellipticity, distance, and absolute magnitude. They find that its radius is larger than almost all Milky Way globular clusters (tight gravitationally bound systems of old stars that don’t reside in their own dark matter halos), suggesting that Aquarius IV is a true dwarf galaxy.

Currently, there are about 40 known UFDs around the Milky Way, and LSST is expected to roughly double this number (see this Astrobite), allowing us to study these tiny galaxies at a population level. This article proves that LSST has the potential to revolutionize many areas of astrophysics! If you are a researcher in the US or Chile and want to play with the EDP2 data yourself, you can access it via the Rubin Science Platform. For researchers from other countries, see more info here.

Original astrobite edited by Katya Gozman.

About the author, Ben Sherwin:

I am a physics PhD student and NSF Graduate Research Fellow at Stanford University. I am interested in theoretical and observational cosmology, specifically in cross-correlations between the cosmic microwave background and tracers of large-scale structure. Outside of work, I enjoy seeing the latest movies in theaters and exploring the San Francisco Bay Area.

The post An Early Discovery by Rubin Observatory appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Probing the IMF in the Early Universe — Direct Measurements in the Boötes I UFD with JWST/NIRCam
Authors: Keyi Ding et al.
First Author’s Institution: University of Maryland
Status: Published in ApJ

The Stellar Initial Mass Function

A fundamental concept in astronomy is the stellar initial mass function (IMF). The IMF describes the number of stars of each mass that form from a single birth cloud. If the IMF has a negative slope, it means most stars are low in mass; if it has a positive slope, it means most stars are high in mass. Figure 1 shows some examples of commonly used IMFs, all of which have a negative slope, since we see far more low-mass stars than high-mass stars in the Milky Way. You may notice that several models deviate from a straight line at about 0.5 solar mass. This point, known as the turnover or critical mass, is crucial in determining the exact shape of the IMF.

For a relatively simple concept, the IMF is shaped by incredibly complicated underlying physical mechanisms. Things like turbulence, magnetic fields, and chemical enrichment all play a role in shaping the observed IMF. Additionally, the IMF is a pretty fundamental quantity. An enormous amount of astronomy research relies on assumptions made about the IMF. For example, since most stars are low in mass, and low-mass stars are dimmer, astronomers use the IMF to convert the amount of light in a galaxy to the number of stars; if there are more or fewer low-mass stars than we expect, our measurements will be wrong.

A big question surrounding the IMF is whether or not it’s universal. In the Milky Way, astronomers have been able to measure the IMF accurately, and they have found that it seems to be the same regardless of which bunch of stars we use to measure it.¹ However, we know that galaxies in the early universe were very different compared to today. Things get tricky when you acknowledge that most measurements made of early-universe galaxies rely on modeling tools that are entirely reliant on assuming an IMF. This gnarly little detail makes measuring the IMF in the early universe especially valuable to astronomers.

What Makes Ultra-Faint Dwarfs So Special?

Today’s authors attempt to measure the early-universe IMF using a local relic, an ultra-faint dwarf galaxy (UFD). You might describe UFDs as “incredibly funky little galaxies.” They’re much less massive than the Milky Way, with about 10,000 times less stellar mass. What stars they do have tend to be very old and metal poor. The nature of UFDs has led many astronomers to think of them as fossils: relatively untouched galaxies formed in the early universe. Since we think UFDs are fossils of earlier galaxies, measuring the IMF in a UFD tells us whether the IMF was the same in the early universe as it is today. The authors focus on Boötes I, a relatively luminous UFD orbiting the Milky Way. Figure 2 shows Boötes I as seen by the Sloan Digital Sky Survey.

The IMF in Boötes I

Measuring the IMF can get tricky — it’s typically pretty difficult to measure the mass of each individual star in a galaxy. Thankfully, Boötes I is close enough that we can do exactly that! Using JWST’s NIRCam instrument, today’s authors obtain imaging of Boötes I that is sensitive enough to extract roughly 10,000 stars belonging to the galaxy.

To measure the IMF from the observed population of stars, the authors use a modified version of Starwave, a Bayesian inference tool. In short, the tool takes in some assumptions about the population of stars in the galaxy, then generates many potential color–magnitude diagrams for various parameter selections. You can then assess how well each simulated color–magnitude diagram fits the observed data, thus determining likely parameters for the stellar population. The authors apply their tool for three different IMF models, testing how close the IMF in Boötes I is to that of the Milky Way. This allows them to determine how well the IMF compares to that of the Milky Way, which is typically thought of as a broken power law or log-normal distribution. If the IMF in Boötes I aligns with the Milky Way’s IMF, we’ll have a solid piece of evidence for a truly invariant IMF across cosmic time, allowing astronomers to rest easy knowing our modeling efforts haven’t been bunk this whole time.

So… What Did We Learn?

The authors find that a single power law can be ruled out to a good degree of confidence. This is good, as Milky Way studies show undeniable evidence of a turnover in the distribution. The broken power law and log-normal models both fit the observed data relatively well, aligning well with Milky Way–derived IMFs. All in all, they find solid evidence for an invariant IMF in the early universe.

However, they are unable to say with absolute certainty that the IMF is invariant. Given that their data are nearly perfect (that is, it’s pretty much impossible to get better data for Boötes I), the authors emphasize that a larger sample of UFD IMFs is needed to truly rule out an invariant IMF, but we’re certainly taking steps in the right direction! As the age-old saying goes, “more data are needed!”

Original astrobite edited by Maggie Verrico.

1. Editor’s Note: The Milky Way IMF may vary from star cluster to star cluster; as described in this AAS Nova highlight from 8 July 2026, recent research using data from the Gaia spacecraft has found evidence for IMF variation in our galaxy. ︎

About the author, Drew Lapeer:

Drew is a first-year PhD student at the University of Massachusetts Amherst. They are broadly interested in the evolution of galaxies, with a focus on the impact of cosmic feedback on the galactic ecosystem. In their free time, they enjoy reading, rock climbing, hiking, and baking!

The post A Sneak Peek into Early-Universe Star Formation with Boötes I appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Exozodi Spectral Effect: Residual Habitable Zone Dust May Bias exoEarth Characterization
Authors: Miles H. Currie et al.
First Author’s Institution: NASA Goddard Space Flight Center
Status: Published in AJ

We have dust in our solar system — a huge cloud of it that permeates every corner. The dust itself is thought to be the product of many different processes including comet tails, collisions between asteroids, leftover debris from planetary formation, and gravitational collisions induced by planets. It’s small (on the order of 10–100 microns), made up of different silicates, and creates a noticeable triangle-shaped light that can be seen on the darkest nights (see Fig.1). This zodiacal dust is aptly named since the scattered light from the dust grains follows the line of the zodiac, or the ecliptic plane, across the sky. Its structure is quite complicated, with dust rings and trails and clumps near larger planetary bodies. For more information on the solar system’s zodiacal dust, you can read these bites here and here!

This zodiacal dust is not exclusive to the solar system. Other star systems, such as Tau Ceti and Vega, have been found to host similar clouds of interplanetary dust. This means planets imaged in these systems will also have emission from this dust mixed into the light that we observe from the planets. The Habitable Worlds Observatory (HWO) is a future space telescope mission under development by NASA. One of its primary science goals is to directly image 25 Earth-like planets in the habitable zones of nearby stars and look for signs of life (or biosignatures) in these planets’ atmospheres. Exozodiacal dust (aka exozodi) represents the biggest source of astrophysical noise that will need to be subtracted out from HWO observations of exoEarths. The authors of today’s article look at the potential impact of exozodi emission on the spectra of exoEarth atmospheres.

Show Me the Zodi

The authors first assume four different models of exozodi dust and how the dust’s emission changes with wavelength. The first model assumes a dust cloud that acts as a gray scatterer, meaning the particles scatter all wavelengths of light with no preference, and the second model’s dust acts just like the solar system’s does. However, it’s unknown whether the composition of exozodi will be the same as the solar system’s. Therefore, they adopt two more models of exozodi that act just like debris disks that have different compositions. One model is bluer in color because it is made up of smaller particles that preferentially scatter blue light, and the other model is redder in color because it is made up of bigger dust particles or different chemical compositions. Each of these models over HWO’s wavelength range can be seen in Figure 2. The authors then simulate a modern Earth atmosphere (both clear and 50% cloudy) as well as an Archean Earth atmosphere to understand how different exozodi models will affect observations of different exoplanet atmosphere conditions and compositions.

There’s Dust in My Spectra

In general, the effect that zodiacal dust has on the spectra of Earth-like planets can be broken down into two categories: its effect on the continuum and on the spectral features. The continuum is the object’s light emitted over all wavelengths and uninterrupted by spectral features. Spectral features are absorption and emission lines. These create either a dip or peak in the spectrum where atoms have either absorbed or emitted light at that particular wavelength. Lines at specific wavelengths tell us that a certain material is there, providing a list of possible chemicals that could be present in the planet’s atmosphere. For a brief summary of spectroscopy more generally, see here.

In the case of the continuum, exozodiacal emission raises the continuum, meaning the base flux of the planet is raised and absorption lines become more shallow. These effects can be seen in Figure 3, where the authors’ simulated spectra are plotted with and without zodiacal dust included. Not only is the continuum elevated for all models including zodiacal dust, notice how the dips become shallower at longer wavelengths. Because dust primarily radiates thermally, the red debris disk model nearly smooths out all dips beyond 1.25 microns! Molecules that absorb at these longer wavelengths, such as water or methane, may not be correctly identified with this type of exozodiacal emission.

Additionally, the slope of the spectra between 0.25 and 0.75 microns (called the Rayleigh scattering slope) becomes shallower with zodiacal dust emission, which may bias our models toward lower-molecular-weight atmospheres. The slope at these wavelengths is dependent on the size distribution of molecules in the atmosphere, meaning it can tell us how much the average molecule in an atmosphere weighs.

From Figure 3, the effects of zodiacal dust are minimized with a bluer zodiacal dust model. This means systems with “bluer” zodiacal dust may be more optimal for HWO observations, as bluer dust does not scatter as much starlight as gray, solar system–like, or red dust. From their results, the authors recommend that the residual exozodi-to-planet flux ratio (in the V-band wavelengths) be kept to less than 10^-2 in order to correctly understand the line depths (identify if a molecule is present or not) and 10^-3 in order to accurately retrieve molecular abundances (understand how much of a molecule is present in the atmosphere). Dust is pesky enough here on Earth, but to find life on another planet like ours, we’ll dust have to deal with it.

Original astrobite edited by Anavi Uppal.

About the author, Kaz Gary:

I am a fourth-year PhD candidate at The Ohio State University with a passion for planets. My current work focuses on modeling exoplanet observations for the Habitable Worlds Observatory and understanding planetary atmospheres. Outside of research, I help develop planetarium shows and love all forms of science communication. In my free time, I enjoy playing tabletop RPGs, painting, watching terrible reality TV, and hanging out with my pet hedgehog.

The post Dust Deal with It appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Thermal Electrons in the Radio Afterglow of Relativistic Tidal Disruption Event ZTF22aaajecp/AT 2022cmc
Authors: Lauren Rhodes et al.
First Author’s Institution: McGill University
Status: Published in ApJ

Tidal disruption events (TDEs, see Figure 1), which occur when a star gets a little too close to a supermassive black hole (SMBH) and gets ripped apart, are a hot topic across the electromagnetic spectrum. During a TDE, the star can no longer hold itself together under the gravitational pull of the SMBH. Some bits of the star end up accreting onto the black hole, which can launch a temporary outflow, or, in especially dramatic cases, a relativistic jet.

Aside from being cool transients (as far as spectacular stellar deaths go, they are rivaled only by the supernova), TDEs are valuable to study for a variety of reasons. They likely play a role in galaxy evolution, SMBH growth, and even the life cycles of active galactic nuclei (AGN, in which material is constantly swirling onto the SMBH). Not to mention, they are a great tool for eyeballing the mass of the SMBH responsible for the TDE; if the black hole is any bigger than about 1 billion times the mass of the Sun, its event horizon — the point at which not even light can escape its gravity — extends too far, and a star will disappear across the event horizon before it ever gets ripped apart.

How to Spot a TDE

Usually, TDEs are discovered in the X-ray (after all, not many transients are powerful enough to generate an X-ray flare) or in the optical (thanks to high-cadence, wide-field surveys like the Zwicky Transient Facility and the Legacy Survey of Space and Time). However, radio is a particularly useful regime for TDE searches, especially in dusty host galaxies where we cannot see the optical TDE signatures at all; while dust obscures optical light, the radio is unaffected! There is just one catch: how exactly can we identify a TDE based on only its radio emission?

The radio light in a TDE is generated via synchrotron radiation — which is caused by fast moving electrons in strong magnetic fields — and originates in jets or shocks. Unfortunately, this is also the case in AGN, the primary extragalactic source of variable radio emission, and it can be difficult to distinguish between the two. Additionally, the amount of light we see from synchrotron radiation at a given frequency depends on how energy is distributed among the electrons in the source, which is unknown. Perhaps the simplest case — or at least the one you’ll learn about first in your undergraduate statistical mechanics course — is that the energy is distributed among the electrons thermally (left panel in Figure 2). However, most radio transients associated with black holes are assumed to have non-thermal electron energy distributions, usually where there are exponentially many more low-energy electrons than high-energy electrons (right panel in Figure 2).

As a further challenge, very few TDEs have had extensive follow-up observations in the radio, so we don’t have strong constraints on how most TDEs should behave, especially in the long term. Luckily, some astronomers are working to change this.

This includes today’s authors, who have focused on one particular TDE, ZTF22aaajecp/AT2022cmc (I’m sure everyone mispronounces her name), or 2022cmc for short. 2022cmc is a special TDE: it is one of only five TDEs suspected to have launched a relativistic jet, and it has the highest redshift (z ~ 1.2) of the five. It was observed first in the optical and then in the X-ray and radio. The X-ray emission was highly variable for 400 days post-TDE, and then disappeared, and the first 100 days of radio emission indicated there was a relativistic outflow. At the time, it was concluded that the jet shut off and the X-ray emission ceased. However, the story doesn’t end there; the authors of today’s article continued to observe 2022cmc in the radio for an additional 900 days at frequencies spanning about 1–100 GHz.

Our authors had two key observing strategies: wide frequency coverage and long duration. Measuring the change in brightness with observing frequency, or the spectral energy distribution (SED), is important for determining which processes are responsible for the radio emission, since thermal electrons and non-thermal electrons will produce different SEDs. On the other hand, measuring the change in brightness over time, or the light curve, is important for determining what is happening at the source, such as whether there is a jet and, if so, how long it lasts. Here, our authors combine light curves at six different frequencies (Figure 3) and SEDs at nine different epochs (Figure 4) to investigate both.

The Findings

Our authors find a couple of interesting results. First, in the time domain, they find the highest-frequency light curves fade with time, while the lower-frequency light curves increase initially, hit peaks at different times (after 300 days at 15.5 GHz, and after 800 days at 1.3 GHz), and then begin to decay. The spectral evolution is even more intriguing; the SED is always peaked, and the peak shifts to lower frequencies over time, which is typical for an expanding emission region. At frequencies below the peak, the SED doesn’t change much. However, the high-frequency side of the SED shows confusing behavior: early on, it has a very steep slope (the amount of light drops off sharply at higher frequencies), but it eventually flattens.

Investigations Begin

Next, our authors consider three possibilities to explain these observations. They first consider synchrotron radiation from only non-thermal electrons in a relativistic jet (Figure 5), which is the canonical model for a gamma-ray burst radio afterglow. Initially, the jet is so fast that all its radio light is relativistically beamed in one direction (like a flashlight), and we can only see the jet if it’s pointed directly at us. Eventually, the jet slows down enough that it is no longer beamed, and light is emitted in all directions (like a light bulb). At this point, we see the light fade over time, which is exactly what happened in the high-frequency light curves. However, there is one glaring problem: this model cannot possibly explain the observed long-lasting steep SED at high frequencies. So, our authors discard this theory.

Instead, our authors consider a jetted outflow that has non-thermal and thermal electrons both independently generating synchrotron emission¹, giving us the superposition of two radio spectra. This model also doesn’t fit the data well. The peak of the SED moves too slowly, a very dense local environment is required, and, once again, the authors cannot reconcile that steep high-frequency SED.

Lastly, our authors keep the thermal/non-thermal electrons combo and ditch the jet entirely, modeling a spherical outflow. Excitingly, this model best fits the observations, and, most importantly, can explain the mysterious high-frequency behavior. Initially, most of the radio emission originates in the thermal electron population, which produces more light at higher frequencies. As the outflow expands, the non-thermal electrons begin to dominate, and the SED transforms into the classic non-thermal synchrotron shape.

Next Steps

Ideally, our authors want to compare 2022cmc to other TDEs, but few TDEs have such thorough follow-up observations. They speculate that thermal electrons were important in the radio evolution of one of the few such TDEs, Swift J1644, and recommend we revisit its emission models. More broadly, they recommend two future priorities: first, we need extensive radio observations for larger samples of TDEs, especially around 100 GHz, where the thermal electrons contribute most to the SED; secondly, we should stop neglecting thermal electrons not just in TDE models, but in models of any black hole jets, including gamma-ray bursts and X-ray binaries. While this work addresses only one event, one thing is clear. For theorists and observers alike, there is a myriad of exciting work to be done, so it’s no wonder TDEs have garnered so much attention lately.

1. A convenient property of extragalactic synchrotron sources is that all their electrons radiate independently of one another, even if they are part of the same electron population. This is called incoherent emission. (Galactic sources are another story, but I’ll save that for another time.) ︎

Original astrobite edited by Flavia Pascal.

About the author, Chloe Klare:

I’m a PhD student in astronomy and astrophysics at Penn State (with a physics minor, so I get to use my semester spent in QFT for something!). I study active galactic nuclei (in the radio!), and I’m currently looking for baby synchrotron jets in active galactic nuclei.

The post Thermal Electrons Can Explain the Mysterious Radio Behavior of a Tidal Disruption Event appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: JWST Discovery of Warm Dust in the Circumgalactic Medium of the Makani Galaxy
Authors: Sylvain Veilleux et al.
First Author’s Institution: University of Maryland
Status: Published in ApJ

The Windy Galaxy

Billions of light-years away lies Makani, a record-breaking galaxy discovered in 2019 by researchers at the Keck Observatory atop Maunakea, Hawaiʻi. The galaxy shows evidence of a large merger event, where two galaxies collided to form a larger galaxy. This event triggered starburst activity, or several waves of intense star formation, that changed the makeup of not just Makani, but the region around it.

There are several processes that can create galactic winds. An active supermassive black hole at the center of the galaxy can slingshot particles out of the galaxy at close to the speed of light, or supernovae leftover from previous starburst activity can help to expel material from the galaxy. Makani, whose name in Hawaiian just so happens to mean “wind,” has both factors contributing to its galactic winds: massive amounts of star formation and a very active black hole in its center.

This contributes to some of the strongest galactic winds discovered to date, stretching approximately 100 kiloparsecs (330,000 light-years) beyond the galaxy, roughly ten times the extent of a typical galactic wind. These galactic winds remove gas from the galaxy and help to enrich the circumgalactic medium around the galaxy. There is still much to discover about how galactic winds change the makeup of such galaxies, and what exactly they take with them on their way out.

Earth’s Favorite Carcinogen: Polycyclic Aromatic Hydrocarbons

Polycyclic aromatic hydrocarbons (PAHs, pronounced P-A-Hs or “paws”) are small flat dust grains scattered into every corner of the universe, from the atmosphere of Earth in the form of exhaust and wildfire smoke to the circumgalactic medium (the mostly empty space between galaxies). These PAHs contribute to regulating the temperature of the interstellar medium, but astronomers have long wondered how they migrate across galaxies.

Even though they are smaller than a human hair, PAHs are still easily observable even in very distant galaxies.  These dust grains emit infrared light, and if there are enough PAHs in a region, they are bright enough to be observed with modern telescopes, such as JWST. By combining multiple observations across multiple filters, astronomers can calculate ratios that reveal information about the PAHs’ size, temperature, and charge, allowing reserachers to further discern properties about the regions in which these PAHs reside.

With redshifted galaxies like Makani, the light from PAHs is stretched beyond its usual wavelength, but fortunately for today’s authors, the light was stretched just enough to land known PAH features well within JWST filters, as seen in Figure 1. The authors were then able to use this information to track the distribution of these PAHs across Makani.

PAHs Gone Extragalactic

Using JWST, the authors scrutinized Makani for any signs of PAHs. They found that not only did PAHs exist within the galaxy, these molecules were found up to 30 kpc (~100,000 light-years) outside the galaxy. Makani’s strong galactic winds were ejecting the dust grains from the galaxy over an estimated timescale of a billion years.

Previously, astronomers had not expected to find high quantities of dust grains outside the protection of a galaxy. The journey across a galaxy is long and hot, with temperatures in the millions of degrees. A dust grain would likely not survive the trip, as it would be destroyed by high-energy photons. This suggests that the PAHs were somehow shielded from the hotter parts of the interstellar medium and circumgalactic medium in large enough volumes to make it beyond the edge of Makani.

This answered questions about how galaxies enrich the circumgalactic medium, but it also added several more. Since today’s authors were limited by the sensitivity and range of their instruments, they were unable to confirm if there were PAHs along Makani’s entire wind structure, or just the first 35 kpc (114,000 light-years) of it. They also would like to turn their sights towards the intergalactic medium to see if PAHs could make it even farther beyond the reach of their host galaxy.

Original astrobite edited by Chloe Klare.

About the author, Natalie Price:

As a first-year master’s student at Wesleyan University, I study how stellar winds interact with the local interstellar medium. Outside of the observatory, you can find me dancing, with my nose in a book, or running at non-relativistic speeds.

The post Hitchhiking PAHs Caught Catching a Ride on Galactic Winds appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Unbreaking the Universe: MINERVA Measurements of Color Gradients in Massive Quiescent Galaxies Can Help Ease Too-Early Star Formation Tensions
Authors: Sam E. Cutler et al.
First Author’s Institution: Tufts University
Status: Published in ApJL

Universe Breakers

JWST launched in 2021, and one of its key goals was to observe the first galaxies as they formed. Early results from JWST found something surprising: extremely massive galaxies in the early universe that had “quenched,” or stopped making new stars, much faster than was expected. These early results led some to believe that our understanding of how quickly massive galaxies can form and quench was fundamentally wrong.

Before concluding that our understanding of the universe is broken, it’s important to find other reasons why we might see very massive galaxies in the early universe. One theory is that these galaxies are actually less massive than we think. To “weigh” a galaxy, astronomers first measure the amount of light being emitted by that galaxy. If all the light in a galaxy comes from stars (which should be true in the early universe, though active black holes can contribute a lot of light in some galaxies), you can infer the total number of stars by accounting for the average amount of light emitted per star of a certain mass; this is called the “mass-to-light ratio.” Since massive stars emit a lot more light than low-mass stars, and high-mass stars die out faster than low-mass stars, the mass-to-light ratio is higher for older stellar populations. To accurately “weigh” a galaxy, scientists therefore need both a good measurement of the total amount of light in the galaxy and how old its stellar population is. The authors of today’s article suggest that the uncertainty in these measurements might be leading to overestimates of galaxy masses and underestimates of galaxy star formation rates, possibly explaining the existence of the earliest massive quiescent (non-star-forming) galaxies.

The Impact of Color Gradients

Previous works about the massive quiescent galaxies in this study used spectroscopy to determine the galaxies’ masses and star formation rates. These spectra were taken with a “slit,” which introduces a source of error: if the slit is smaller than the image of the galaxy, some of the galaxy’s light will be missed in the spectrum. Astronomers usually correct for this by checking how much of the total light of a galaxy image is covered by the slit and scaling up the spectrum to account for the missing light, but this correction assumes that the spectrum is basically the same across the entire galaxy.

However, this assumption is rarely correct. In the local universe, star-forming spiral galaxies like the Milky Way tend to be made of a central “bulge” surrounded by a “disk” (see Figure 1). The bulge is no longer actively forming new stars, leading to a red-to-blue color gradient as you move out from the center of the galaxy. In astronomy, this is considered a “negative” color gradient. A slit that only covers the galaxy center would lead to an overall overestimate of the stellar mass and underestimate of the star formation rate. If the galaxies instead had a positive color gradient, with more star formation in the center of the galaxy, the mass would be underestimated and the star formation rate would be overestimated. Galaxies with both positive and negative color gradients have been found in the distant universe. By measuring the color gradient for each individual galaxy in their sample, the authors can correct the mass and star formation rate calculations from spectra taken with a slit spectrograph.

Results

Instead of using spectra, the authors of today’s article use imaging in different filters to measure color gradients for a sample of four galaxies in the early universe that had previously been found to be extremely massive and no longer forming stars. They use medium-band filters, or filters that let in only a small part of a galaxy’s spectrum, to produce a spectral energy distribution at each radius. This spectral energy distribution is less informative than a spectrum (think a handprint versus a fingerprint), but it’s detailed enough to model the likely stellar population at each radius to determine whether the galaxies have positive, negative, or no color gradients, allowing for better corrections to the properties measured from the galaxies’ centers. They also model the stellar population from the central part of the galaxy that would normally be covered by a slit, as well as from the actual spectrum, to test whether any differences in their results come from their choice of modeling techniques.

The authors find that three of the four galaxies have a negative color gradient in at least some areas. The fourth galaxy has a relatively flat color gradient, though this measurement is less certain due to the object’s redshift. When the authors plot the galaxies’ colors on a diagnostic diagram that separates red and old galaxies from young and blue galaxies, they find that three of the four galaxies have centers that appear red and old but outskirts that appear young and blue (Figure 2). This means previous measurements of the mass may have been overestimated, and previous measurements of the star formation rate may have been underestimated.

Next, the authors model the stellar ages across the galaxies using spectral energy distribution modeling. This technique compares the measured spectral energy distribution to libraries of stellar spectra, rules about how different dust or chemical composition impact galaxy appearance, and observational effects to predict the actual stellar makeup of a galaxy. In this case, the authors can’t know for sure whether the redder colors at galaxy centers come from an older stellar population, more dust, or changes in the chemical composition (the so-called “dust–age–metallicity degeneracy”). To account for this, the authors try modeling the stellar population two ways: once by assuming all change in color comes from a change in stellar age, and again by allowing the dust and chemical composition of the galaxy to change with radius. They find that if only the stellar age varies across the galaxy, age gradients in the galaxy stellar population can bring the estimated galaxy masses and star formation rates back toward agreement with models of galaxy evolution in the early universe, though not all the way. However, allowing dust and chemical composition to change across the face of the galaxy lead to higher stellar masses and less star formation, meaning the mystery may not yet be solved.

Is the Universe Broken?

To determine whether these galaxies still count as “universe breakers,” the authors use a cosmological model to predict the largest expected galaxy mass in the observed area at different periods in cosmic time. The previous results had less than a 0.3% chance of occurring under current cosmological models; with their updated numbers and stellar population models, the authors find that the observed galaxies sometimes have more than a 5% chance of occurring, though there is still tension. They point out that their analysis ignores processes that could help galaxies grow and quench in the early universe, like galaxy mergers; still, the existence of these massive galaxies with such low star formation rates remains a puzzle.

Original astrobite edited by Ansh Gupta.

About the author, Margaret Verrico:

I am a fourth-year graduate student at the University of Illinois Urbana-Champaign. I study the connection between supermassive black hole transients and their host galaxies. I am also an avid knitter and reader, and I am passionate about opening up STEM opportunities for people of all backgrounds.

The post Galaxies of Many Colors appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Geometry, Not Calorimetry, Drives the Radio–Infrared–γ Ray Correlation
Authors: T. A. Porter, I. V. Moskalenko, and G. Jóhannesson
First Author’s Institution: Stanford University
Status: Published in ApJ

Original astrobite edited by Kelsie Taylor.

About the author, Sandy Chiu:

I’m a PhD candidate at the University of Michigan, Ann Arbor. I’m interested in numerical simulations of cosmic rays feedback in galaxies and their comparison with observation.

The post Do Galaxies Really Glow in Sync — or Are We Just Tilting Our Heads? appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The JWST EXCELS Survey: Insights into the Nature of Quenching at Cosmic Noon
Authors: Maya Skarbinski et al.
First Author’s Institution: Johns Hopkins University
Status: Published in ApJ

Original astrobite edited by Anavi Uppal.

About the author, Margaret Verrico:

I am a fourth-year graduate student at the University of Illinois Urbana-Champaign. I study the connection between supermassive black hole transients and their host galaxies. I am also an avid knitter and reader, and I am passionate about opening up STEM opportunities for people of all backgrounds.

The post EXCELlent Work, Detectives! Solving the Murder of Star Formation in Galaxies with JWST appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Rhythm of the ISM: Tracing the Timescales of Gas Evolution and Star Formation Across Galactic Environments
Authors: Zuzanna Kocjan and Vadim A. Semenov
First Author’s Institution: University of Maryland
Status: Published in ApJ

The Connection Between Gas and Star Formation in Galaxies

Stars are born in the interstellar medium (ISM) when cold, dense clouds of gas become unstable and collapse under gravity. Despite their plentiful reservoirs of gas, galaxies convert only a small fraction of this material into stars, making star formation a surprisingly inefficient process. A key factor behind this inefficiency is stellar feedback: radiation, stellar winds, and supernova explosions from young stars can inject energy and momentum into the surrounding gas, heating, stirring, and dispersing it. In this way, stellar feedback shapes future star formation across scales ranging from individual regions in the ISM to whole galaxies.

An important tool astronomers use to study star formation is the Kennicutt–Schmidt relation, which links the amount of gas in a galaxy to the rate at which stars form. More specifically, it relates the gas surface density, Σ_gas, to the star formation rate surface density, Σ_SFR. (For more on this relation, see this Astrobites article on a classic research article.) However, this law is underpinned by a crucial ingredient: timescales. In particular, the pace of star formation depends both on how quickly galactic gas is cycled into star-forming material and on how rapidly those regions convert gas into stars once they form. Today’s article investigates the physical origin of star-formation scaling relations behind the Kennicutt–Schmidt law by building on a simple theoretical framework for the ISM.

Consider a Kettle of Boiling Water…

In a given region of a galaxy, the ISM consists of gas in either an actively star-forming state or an inert, non-star-forming state, depending on whether it is dense and unstable enough to collapse (as illustrated in Figure 1). The transition of non-star-forming gas into the star-forming state occurs on the supply timescale, τ₊. Conversely, the dispersal of star-forming gas back into a non-star-forming state is characterized by the removal timescale, τ_–. As a helpful analogy, the authors compare this process to water boiling in a kettle: the ISM is continuously “boiling,” with gas moving between active and inactive states. Meanwhile, the total gas reservoir gradually decreases, analogous to water slowly evaporating as the kettle boils. The gas depletion time, τ_*, therefore represents the timescale over which the available gas would be exhausted by star formation.

Original astrobite edited by Jayde Willingham.

About the author, Shalini Kurinchi-Vendhan:

After studying astrophysics and literature at Caltech, I moved onto a Fulbright Fellowship in Heidelberg, Germany. I’m passionate about using computer simulations to explore supermassive black holes and galaxy evolution — but I also love poetry and traveling.

The post The “Rhythm” of the Interstellar Medium appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Mars as an Exoplanet: Lessons from a Planet at the Edge of Habitability
Authors: Stephen R. Kane et al.
First Author’s Institution: University of California, Riverside
Status: Published in PSJ

Mars in the Upside Down

If you’re a fan of Stranger Things like me, you’ll know of the Upside Down: a mirror world similar to our own, but with very different rules. Imagine Mars in the Upside Down, where it is no longer our next-door neighbor but a planet hundreds of light-years away. An astronomer on this Upside Down Earth would be looking at a distant speck with a transit signal barely distinguishable from noise. In this world, Mars would no longer be familiar but completely foreign, with unknown properties. Kane et al. suggest that treating Mars as if it were a stranger is a useful way to think about exoplanet science today.

JWST is hunting for atmospheres on small rocky planets around other stars. However, only a select few Mars-like exoplanets have been discovered (Figure 1). This is because sub-Earth planets are small and hard to detect, testing the bounds of current technology. The ones that have been discovered, such as TRAPPIST-1h and Kepler-138b, were detected due to favorable geometry. They happen to orbit very close to a low-mass star or sit in resonant multi-planet systems, which amplify the signal we are looking for. True Mars analogs with low flux and moderate orbital periods remain out of reach. This is precisely what makes Mars itself so important scientifically — it is the only planet of this type we can study up close.

Getting to Know Mars, Getting to Know All About Mars

We know a lot about Mars in comparison to other planets thanks to rovers (such as Curiosity and Perseverance), atmospheric orbiters (such as Mars Atmosphere and Volatile EvolutioN and the Mars Orbiter Mission), and other science missions (Figure 2). We know it once had water features with neutral pH and favorable chemistry for life from sedimentary evidence at the Gale and Jezero craters. We know its atmosphere is 95% CO₂ with a surface pressure less than 1% of Earth’s. We know it once had an active magnetic dynamo, but the dynamo died out about 4 billion years ago, leading to solar wind steadily stripping away Mars’s atmosphere over time.

But in the Upside Down, we would know none of this since Mars would be so distant. With current technology, this Upside Down Mars would be essentially undetectable using transit signals, radial velocity measurements, and spectrographs.

Beyond Our Backyard

JWST is currently studying the small rocky TRAPPIST-1 planets and is finding little evidence of atmospheres. The authors of today’s article explore the idea that these exoplanets are similar to Mars and perhaps are undergoing processes that strip away their atmospheres and create optimal conditions for life only for short durations. Using Mars as a blueprint, the authors show how we can better understand exoplanet processes, including volatile delivery and loss, photochemistry, climate evolution, obliquity forcing, planetary architecture, and intrinsic magnetism. For example, measurements of noble gases directly fingerprint Mars’s atmospheric escape process, and the authors argue these give us a calibration framework for interpreting future exoplanet spectra.

The upcoming Nancy Grace Roman Space Telescope’s microlensing survey will start revealing how common true Mars analogs actually are throughout the galaxy, while the future Habitable Worlds Observatory will attempt direct imaging of nearby candidates with unprecedented precision. Together, these missions will tell us whether Mars-like outcomes (brief habitability, catastrophic atmospheric loss, barren surfaces, etc.) are the default fate for small rocky planets. Interpreting that data will require the exact framework this article builds: one grounded in the only Mars analog we can study in detail.

Original astrobite edited by Sandy Chiu.

About the author, Ben Sherwin:

I am a physics PhD student and a National Science Foundation Graduate Research Fellow at Stanford University, working with Josh Frieman. I am interested in theoretical and observational cosmology, specifically in cross-correlations between the cosmic microwave background and tracers of large-scale structure.

The post What if Mars Were a Stranger (Thing)? appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: Carbon Cycle Imbalances on Arid Terrestrial Planets with Implications for Venus
Authors: Haskelle T. White-Gianella and Joshua Krissansen-Totton
Authors’ Institution: University of Washington
Status: Published in PSJ

The Carbon-Cycle Thermostat

When discussing the habitability of planets, we usually focus on the habitable zone, the region around a star where a planet can host a temperature suitable for supporting liquid water on its surface. Yet, being in the right place doesn’t necessarily guarantee that a planet will be habitable.

A planet also needs the right atmospheric composition to sustain the necessary temperatures for hosting liquid water. A key mechanism found on Earth for maintaining this composition is the geologic carbon cycle, which acts as a built-in climate control system. This cycle begins with volcanic eruptions releasing carbon dioxide (CO₂₎, which then dissolves in rainwater and forms a weak acid that weathers rocks on continents. The weathering products wash into oceans, where they form carbonate rocks, locking carbon away. Eventually, plate tectonics recycles some of that carbon back to volcanoes. The weathering process works faster when the planet is warmer, thus regulating the amount of CO₂ and stabilizing the climate over long periods of time (but it should be noted that it can take up to a few hundred thousand years to rebalance this slow carbon cycle through the weathering process).

An important caveat is that the weathering process requires sufficient liquid water on the planet’s surface, so what happens in the case for planets with shallower oceans? Low-mass M dwarfs, the most common type of star found in the galaxy, are expected to host less-massive disks and therefore lower water inventories for forming planets. If more “dry” planets are indeed a more likely outcome of planet formation, we’d want to know how their arid conditions influence their long-term evolution and habitability.

Creating Carbon-Tracking Models

The authors built a model that tracks how water and CO₂ move between a planet’s interior, oceans, and atmosphere over 4.5 billion years. The model notably includes the following:

• A sophisticated weathering model that accounts for how runoff can limit rock weathering, improving upon previous models that only included a temperature dependence on weathering
• Wind-driven evaporation limits that influence the amount of water evaporation in addition to sunlight-driven evaporation
• Multiple deep water cycle parameterizations that explore how water moves between the interior and the surface

The model tests for four initial surface water inventories (0.1%, 1%, 10%, and 100% of Earth’s oceans) and then simulates how the surface water, atmosphere, and climate evolve over time.

A Critical Water Threshold

The models reveal that planets require at least 20–50% of Earth’s ocean mass to maintain a balanced carbon cycle. Below this threshold, the carbon cycle becomes unbalanced, leading to devastating consequences for habitability (Figure 1).

For the more arid planets, models reveal a concerning runaway process. Limited surface water reduces precipitation and slows rock weathering. Weathering cannot keep up with volcanic CO₂ release, causing CO₂ buildup and warming of the atmosphere. The cycle repeats, driving runaway warming until all water evaporates.

Important Implications (Within and Outside Our Solar System)

Looking within the solar system, the results offer an explanation for how a potentially habitable Venus could have transitioned into its current inhospitable state. If Venus had formed with an initial water inventory below this critical threshold of 20–50% of Earth’s ocean mass, then its carbon cycle would have become unbalanced, creating the CO₂ inferno seen on Venus today.

In future exoplanet studies, telescopes like the Habitable Worlds Observatory might detect ocean glint (specular reflection from liquid water) or measure land fraction from light curves. Finding a planet in the habitable zone but with limited ocean coverage would likely be bad news, as the planet could be quietly losing its grip on habitability. Astronomers may want to be more careful when looking at seemingly promising planets within their habitable zones.

Original astrobite edited by Natalie Price.

About the author, Jared Bull:

I am a 2nd-year PhD student at Johns Hopkins University. I study brown-dwarf variability and am interested in using time-series observations to uncover dynamic processes within their atmospheres. In my free time I like to read, cook, and do astrophotography.

The post Can Arid Planets Keep Their Cool? appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: MAGAZ3NE: Confirming Dust Deficiency and Quiescent Nature of Ultramassive Galaxies at 3 < z < 4 with ALMA Observations
Authors: Wenjun Chang et al.
First Author’s Institution: University of California, Riverside
Status: Published in ApJ

Distant galaxies offer a unique window into how stars, gas, and dust evolve over cosmic time. Tracing this evolution requires understanding not only how galaxies form, but also how and when they stop forming stars, a process known as quenching. Understanding how, when, and why galaxies quench is a fundamental question in astrophysics, and one that requires observations of star-forming, quenching, and fully quenched galaxies alike. Unfortunately, one complication is that identifying truly quenched galaxies is challenging: galaxies that appear “dead” may instead be actively forming stars, hidden behind a thick veil of dust.

In today’s article, the authors use observations from the Atacama Large Millimetre/submillimetre Array (ALMA) to investigate five ultramassive galaxies at redshifts, z, between 3 and 4, and ask a deceptively simple question: are these massive red galaxies genuinely quenched, or are they secretly forming stars behind the scenes?

Meet the Suspects: Ultramassive Galaxies at the Edge of Cosmic Noon

The five galaxies in this study are drawn from the Massive Ancient Galaxies at z > 3 NEar-infrared (MAGAZ3NE) survey, which targets some of the most massive galaxies known at early cosmic times. All five have stellar masses exceeding 100 billion solar masses and have been confirmed at z > 3. At these redshifts, we are observing the galaxies as they were when the universe was less than 2 billion years old. This is just before an epoch known as cosmic noon, when star formation across the universe reached its peak.

These galaxies also benefit from extensive multiwavelength observations from a range of observatories, including the ground-based Visible and Infrared Survey Telescope for Astronomy and the Spitzer Space Telescope. By combining imaging across wavelengths from the ultraviolet to the near-infrared, astronomers can measure the “colours” of galaxies and use these colours to infer the galaxies’ star-forming activity.

Galaxies that have quenched their star formation are dominated by older stellar populations, which makes them appear red (hence why they are often referred to as “red and dead” galaxies). However, dust can redden galaxies in a similar way by absorbing blue light and re-emitting it at longer wavelengths — meaning that a dusty, star-forming galaxy can easily masquerade as a quenched one. To uncover any hidden star formation, the authors turn to ALMA to search for far-infrared dust emission. The sample of galaxies investigated with ALMA is shown in Figure 1.

ALMA on the Scene

ALMA observes light at sub-millimetre wavelengths, which at these redshifts traces emission from star-forming regions that are obscured by dust. Of the five ultramassive galaxies in this sample, only one is detected with ALMA. Even when the remaining four galaxies are stacked together, no dust emission is recovered, indicating that if any dust is present, it must be extremely faint.

To better quantify what these ALMA non-detections imply, the authors perform spectral energy distribution fitting (see a recent overview bite on spectral energy distribution fitting) using the Code Investigating GALaxy Emission, or CIGALE, a code that enforces energy balance between absorbed starlight and dust emission to estimate physical properties such as a galaxy’s star formation rate and dust content. With the ALMA constraints included, all five galaxies are found to lie more than 10 times below the star-forming main sequence. Even the single ALMA-detected galaxy remains formally quenched, showing only weak residual star formation.

In other words, these galaxies really are dead (or at least extremely dormant).

Extremely Dust-Poor Galaxies

The spectral energy distribution fitting also allows the authors to measure how much dust these galaxies contain relative to their stellar mass. This ratio provides a simple but powerful way to assess how much interstellar material remains in a galaxy — and therefore how much fuel is left for future star formation.

Three of the five ultramassive galaxies have ratios of M_dust/M_star < 10^-4 (Figure 2), placing them among the most dust-poor quenched galaxies confirmed at z > 3. The lone galaxy detected by ALMA contains slightly more dust, with M_dust/M_star = 10^-3, but even this is far below what would be expected for an actively star-forming galaxy. For comparison, typical star-forming galaxies at similar stellar masses host more than 100 times more dust.

These results raise important questions about how galaxies can become ultramassive, quenched, and nearly dust-free within the first two billion years of cosmic history.

For now, ALMA has delivered a clear answer to the question of “dead or in disguise?”: at least some candidate ultramassive galaxies in the early universe really are quenched, and they are strikingly dust poor. By ruling out the dusty impostor scenario, this study shows that deep ALMA observations can cleanly distinguish genuinely quenched galaxies from dusty star-forming ones, even at the highest stellar masses and earliest cosmic times. How these galaxies lost their dust remains an open question, but one thing is clear: by the time cosmic noon arrived, some galaxies had already finished forming stars and quietly faded into dormancy.

Original astrobite edited by Viviana Cáceres.

About the author, Lucie Rowland:

I’m a fourth (and final!) year PhD student at Leiden Observatory in the Netherlands, studying massive, star forming galaxies in the early universe with ALMA and JWST. It’s a really exciting time to be interested in astronomy, so I hope to make groundbreaking new research more accessible!

The post Distant Galaxies: Dead or in Disguise? appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: The Wandering Supermassive Black Hole Powering the Off-Nuclear Tidal Disruption Event AT2024tvd
Authors: M. Guolo et al.
First Author’s Institution: Johns Hopkins University
Status: Published in ApJL

A Star Gets Eaten in the Wrong Neighborhood

We find supermassive black holes at the centers of most large galaxies. They end up there because of how galaxies form: as matter collapses and merges over time, material sinks to the gravitational center, and the black hole settles in.

So, what happens when we catch a star being ripped apart by a supermassive black hole that is not at the center of its galaxy?

That is exactly the puzzle posed by AT2024tvd, a tidal disruption event (TDE) spotted roughly 2,600 light-years from the center of a massive galaxy located 600 million light-years away from us (see Figure 1).

We see TDEs when a black hole tears apart a star that gets too close to it. It happens because the black hole’s gravity pulls harder on the near side of the star than the far side, stretching the star until it comes apart. The stellar debris then forms a hot accretion disk, a ring of material swirling around the black hole that spirals inward and releases a burst of electromagnetic energy across many wavelengths — from visible light to X-rays. These events are valuable to astronomers because they briefly light up black holes that would otherwise be invisible, giving us a rare window to measure their properties. Since supermassive black holes live at galactic centers, that is also where we expect TDEs to happen, which makes a TDE found away from a galactic center rare and puzzling. The galactic center is often called the nucleus of a galaxy, so astronomers refer to these displaced events as “off-nuclear” TDEs. AT2024tvd is only the third known off-nuclear TDE, and this research article makes a strong case that it is the most remarkable one yet.

Measuring Mass Using Light

To figure out the mass of the black hole responsible for AT2024tvd, the authors needed to get creative. You cannot weigh a black hole directly, so astronomers have to work backward from what they can see. The key idea is that a black hole’s mass controls how its accretion disk behaves. A more massive black hole produces a larger, cooler disk, while a less massive one produces a smaller, hotter disk. By measuring how bright the disk is at different wavelengths and how hot it gets, you can figure out how massive the black hole must be.

The authors did this by modeling the light the event produced across many wavelengths. They combined data from several telescopes: the Zwicky Transient Facility, the Neil Gehrels Swift Observatory, Pan-STARRS, and two rounds of high-quality X-ray data from XMM-Newton. TDE light curves go through different phases. The early flare in visible and ultraviolet light is bright but complicated, and the physical processes behind it are not fully understood. But after a few hundred days, TDEs settle into a quieter “plateau phase,” where the ultraviolet and visible light come directly from the accretion disk. At this stage, the emission follows well-understood physics, and astronomers can model it reliably.

The authors used a model called kerrSED, which describes the spectral energy distribution (SED) of a spinning black hole’s accretion disk (kerr). It accounts for the disk’s temperature, its physical size, the black hole’s spin (how fast it rotates), and the angle at which we are viewing the system. It also accounts for a process called Comptonization, where hot electrons near the black hole boost lower-energy photons (particles of light) up to X-ray energies. By fitting this model to the observed light at multiple wavelengths simultaneously, the authors could pin down the disk properties and extract the black hole mass. The result was a clean fit: the disk model alone could explain all the observed light during the plateau phase, with nothing significant left over (see Figure 2).

Not an Intermediate, but a Supermassive Black Hole

From their fit, the authors found a black hole mass of about one million solar masses. Black holes above about 100,000 solar masses are considered supermassive, while those below that threshold but above about 100 solar masses are called intermediate-mass black holes. So, this puts AT2024tvd in the supermassive category. This matters because the two previously known off-nuclear TDEs, called 3XMM J2150-05 and EP240222a, were both powered by intermediate-mass black holes. Those black holes were found inside small, dense collections of stars called ultra-compact dwarf galaxies that were orbiting larger host galaxies.

AT2024tvd is different. When the authors looked at deep images of the location where the TDE happened, they found no star cluster or small galaxy there. Whatever group of stars once surrounded this black hole has been almost entirely pulled apart by the gravity of the much larger parent galaxy. The ratio of the black hole’s mass to the mass of any remaining stars around it is extreme: greater than 3%, which is far above what we normally see. This is the signature of a “wandering” supermassive black hole, one that was brought in during a past galaxy merger and has been slowly sinking toward the center of its new host ever since, losing its surrounding stars along the way.

The authors compared AT2024tvd to other TDEs using established relationships between accretion disk properties and black hole mass (Figure 3). In terms of its disk temperature, luminosity, and inferred mass, AT2024tvd behaves like a typical nuclear TDE powered by a supermassive black hole. However, when placed on the black hole mass versus host stellar mass relation, it stands out as a strong outlier. The black hole mass is far too large for the small amount of surrounding stellar mass detected at its location.

The Big Picture

This discovery matters for several reasons. It shows that off-nuclear TDEs are not limited to intermediate-mass black holes sitting in small satellite galaxies. Some are powered by fully supermassive black holes that have been displaced from their original galactic centers. It also shows that TDE modeling, when done carefully during the plateau phase of the light curve (when the emission is dominated by well-understood accretion disk physics), can provide reliable black hole masses on its own, without assuming any relationship between the black hole and its host galaxy. This is particularly important for wandering black holes, where those relationships do not apply.

Looking ahead, upcoming surveys like Vera C. Rubin Observatory’s Legacy Survey of Space and Time are expected to find many more off-nuclear TDEs. Combined with X-ray follow-up from telescopes like XMM-Newton, these events could become our main tool for mapping out the population of displaced black holes in the nearby universe. One disrupted star at a time, we are starting to find black holes that theory told us should exist but that had, until now, stayed hidden.

Original astrobite edited by Kelsie Taylor and Veronika Dornan.

About the author, Serat Saad:

Serat is a first-year PhD student in astronomy at The Ohio State University. His research is on stellar and galactic dynamics, where he uses observational data to understand gravity. He also has interests in active galactic nuclei and tidal disruption events.

The post A Wandering Supermassive Black Hole Eating a Star appeared first on AAS Nova.

Editor’s Note: Astrobites is a graduate-student-run organization that digests astrophysical literature for undergraduate students. As part of the partnership between the AAS and astrobites, we occasionally repost astrobites content here at AAS Nova. We hope you enjoy this post from astrobites; the original can be viewed at astrobites.org.

Title: A GLIMPSE of Intermediate Mass Black Holes in the Epoch of Reionization: Witnessing the Descendants of Direct Collapse?
Authors: Qinyue Fei et al.
First Author’s Institution: University of Toronto
Status: Published in ApJ

In the late 2010s, astronomers were getting hints of something strange going on in the early universe. Using the most powerful telescopes on the ground, we were able to find black holes in the centers of some of the most distant galaxies. The light from these active galactic nuclei (AGNs) was emitted over 12 billion years ago, revealing conditions close to the beginning of cosmic time.

To an astronomer, that’s the mundane part. I’m only half joking — thousands of AGNs had already been identified in the previous half century. However, the most distant of these sources were startling because their central black holes appeared much more massive than expected. To be specific, we know how much mass black holes can start with, and we can write down equations to predict how quickly they should grow. The AGNs we observed seemed to violate these predictions. This raised some serious questions and was highlighted as a major point of inquiry to focus on going into the 2020s.

Then JWST launched, and the mystery grew deeper. No matter where we looked, enormous black holes kept cropping up in astonishing numbers. Astronomers who model the universe with equations and computer simulations have tried to explain these results, but this has been a serious challenge. Some researchers have broken the emergency glass and reached for a highly theoretical tool: the direct-collapse black hole. Unlike the normal pathway for forming black holes (the deaths of massive stars), this model suggests that an enormous cloud of gas could directly collapse into a black hole without forming a star at all. The resulting black holes could have as much mass as a hundred thousand Suns put together — orders of magnitude more than a “normal” black hole. Although this pathway can explain the presence of huge black holes so early in the universe’s history, we’ve never directly seen this happen, so for now it remains just a theoretical possibility.

However, if direct-collapse black holes were widespread at early times, there is one indirect signature we might be able to detect: a large population of intermediate-mass black holes with masses between a hundred thousand and a million times that of the Sun. If we can find such a population, we’ll have strong evidence for the direct-collapse model. Let’s go looking!

Intermediate Mass, Impossible Difficulty

Identifying intermediate-mass black holes requires a tricky combination of spectral resolution and sensitivity. Light from an object must be examined very finely in order to detect the telltale features of an intermediate-mass black hole. However, these sources are already quite faint, so spreading their emission this thin means that any potential signal is very likely to get lost in the noise.

The authors of today’s article use an unprecedented set of observations to leap over this hurdle and directly probe intermediate-mass black holes. They not only use the most sensitive spectroscopic instrument (NIRSpec) on the most powerful space telescope (JWST), but they take advantage of a phenomenon known as gravitational lensing.

According to Einstein’s general theory of relativity, gravity is the result of matter curving spacetime. Areas with more concentrated matter will experience stronger curvature. When an object traveling in a completely straight line moves through curved space, its path appears bent. This same effect happens to light. Intriguingly, a large concentration of matter can redirect diverging rays of light to a focus, acting like an enormous magnifying glass.

Astronomers take advantage of gravitational lensing by pointing telescopes at massive clusters of galaxies. The intense gravity of these regions can magnify sources in the background by tens or even hundreds of times. Galaxies that would be impossibly faint to see under normal conditions can thus be observed if they are strongly lensed, making this a powerful technique to examine otherwise hidden objects.

Putting Faint AGNs Under a Lens

The Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) program took extremely powerful observations of a galaxy cluster called Abell S1063 using JWST. The long exposure time, combined with the lensing power of the cluster, allows extremely faint sources in the background to be seen in unprecedented detail. The researchers scoured the data to search for signs of active black hole growth in the observed galaxies. Specifically, they carefully studied the emission lines in the spectrum of each object.

Normally, emission lines appear narrow. However, gas surrounding a massive black hole circulates at rapid speeds. A phenomenon known as the Doppler effect then comes into play. This is the same effect responsible for the characteristic rise and fall in the pitch of a siren from an emergency vehicle passing by. Emission from gas moving toward our line of sight appears bluer than it otherwise would, and gas moving away from us appears redshifted. The net effect is that a single emission line is widened, and we see characteristic “broad wings.” The width of the emission line can be used to estimate the mass of the central black hole, where broader wings imply a more massive black hole.

The researchers uncover 10 AGNs that display broadened Balmer series lines. Strikingly, they estimate that these galaxies have central black holes with masses as low as 400,000 times that of the Sun. While that might sound like a lot, it’s practically nothing compared to the monstrous 100-million-solar-mass black holes that JWST consistently turns up in other studies. Detecting such lightweight black holes, pushing into the realm of intermediate-mass black holes, is only possible due to the incredible sensitivity of these observations.

The researchers also compute what’s known as the black hole mass function. This is a measure of how many black holes exist at each mass. In other words, the black hole mass function measures how common lightweight black holes are compared to heavy ones. The mass function computed in this work and several other points of comparison are shown in Figure 1.

Because the AGNs analyzed in today’s article host central black holes with much lower masses than any that have been observed in the past (at this early time in the universe), the authors are able to probe a completely new region of the black hole mass function. As seen in Figure 1, the leftmost red hexagon (lowest-mass data point) deviates from the roughly straight line traced by the other data points. This suggests that there are substantially more black holes with a few hundred thousand solar masses than previously expected.

Amazingly, this is exactly the measurement you would get if direct-collapse black holes (the hypothetical kind of massive black holes mentioned earlier in this article) were common in the early universe. If you scatter huge seeds across a large field, then you shouldn’t be surprised to see a sea of barely larger plants a few months later. Likewise, since these black holes start off so massive, they only need to grow a tiny bit to reach the threshold of that first red hexagon.

So is this proof that the early universe was filled with direct-collapse black holes, with this study probing the tip of an astonishing iceberg? It’s too early to tell for sure. While this is certainly an exciting result that sheds light on a previously unexplored population of objects, one can only do so much with a sample size of 10. More follow-up observations will be needed to measure the black hole mass function more precisely. Still, it’s amazing to think how much has changed in just a few years. JWST continues to provide unprecedented insight into some of the most breathtaking questions in all of astronomy.

Original astrobite edited by Tori Bonidie.

About the author, Ansh Gupta:

I’m an astronomy graduate student at the University of Texas at Austin working with Steven Finkelstein. I use data from JWST to study the formation and growth of the first galaxies and black holes in the universe. In my spare time, I enjoy playing piano, reading, and making YouTube videos.

The post Black Holes Reveal Their Seeds appeared first on AAS Nova.

Editor’s Note: This week we’re at the 248th AAS meeting in Pasadena, CA. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com for daily summaries, or follow @astrobites.bsky.social on Bluesky for more coverage. The usual posting schedule for AAS Nova will resume on 22 June

Table of Contents:

• Plenary Lecture: Ian Roederer, The Ultraviolet Path to the Origin of the Elements

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Plenary Lecture: Ian Roederer, The Ultraviolet Path to the Origin of the Elements (by Susanna Kohler)

When the universe first formed, the only elements present were the three lightest — hydrogen, helium, and lithium. Today, there are 118 confirmed chemical elements that make up the periodic table. So where did the heavier elements come from, when were they first created, and what can they tell us about the first stars that formed in the universe? In the final plenary of AAS 248, Ian Roederer (North Carolina State University) walked us through how ultraviolet observations are the key to understanding the origin of the elements.

We know that hydrogen, helium, and lithium were cycled through the earliest stars in the universe, where fusion and other processes produced the first heavy elements that were spit back out into the cosmos when these stars ended their lives as supernovae. According to Roederer, within just the first two or three generations of stars, every element with stable or long-lived isotopes had already been created. For the heaviest of these elements, the dominant source is likely a process called r-process nucleosynthesis, in which a rapid burst of neutrons bombards atomic nuclei, resulting in the quick capture of the neutrons to form very massive nuclei that subsequently decay to stable isotopes.

The conditions needed for r-process nucleosynthesis are too extreme to reproduce in a laboratory on Earth, so we rely on models and observations of the cosmos to better understand it. We know that to get a rapid burst of neutrons, you need some sort of explosive event — and, in fact, the first example of an r-process event caught in the act was GW170817, the first detected neutron star–neutron star merger.

But remember all those news headlines about how binary neutron star mergers produce gold and platinum? Roederer points out that these were all inferred results — we didn’t actually observe direct evidence of gold and platinum in the aftermath of this explosion. That’s because it’s very difficult to detect the spectroscopic emission lines of heavy elements. To do so effectively, Roederer argues, requires high-resolution ultraviolet (UV) spectroscopy. He showed that in a cool star, high-resolution UV spectra provide a 50% increase in the number of elements you can detect. The heaviest of elements are only detectable with spectra in the UV range.

Roederer walked us through some of the insights we can gain into the production of the heaviest nuclei found in nature when we use a combination of UV and optical spectroscopy. He also gave examples of what we can learn about even some of the lighter elements like phosphorus or boron when we have high-resolution UV spectroscopy to reveal the presence of these elements in low-metallicity stars.

Roederer ended his talk with a look ahead at the future of UV spectroscopy and the path to the universe’s first stars. Right now, the only mission we have that produces high-resolution UV spectroscopy is Hubble. NASA’s next big flagship mission with a high-resolution spectrograph, the Habitable Worlds Observatory (HWO), will be transformational in the ability to detect — or verify the absence of — heavy elements in stellar atmospheres. An observatory like HWO will provide our best shot at identifying low-mass, metal-free stars that survive from the first generation of stars in our universe. As Roederer concluded, “The future of studying the origin of the elements is bright in the UV.”

You can read Astrobites’s interview with Ian Roederer here.

The post AAS 248: Day 4 appeared first on AAS Nova.

Editor’s Note: This week we’re at the 248th AAS meeting in Pasadena, CA. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com for daily summaries, or follow @astrobites.bsky.social on Bluesky for more coverage. The usual posting schedule for AAS Nova will resume on 22 June.

Table of Contents:

• Plenary Lecture: Eliza Kempton, Common Worlds, Uncommon Complexity: Sub-Neptune and Super-Earth Atmospheres in the JWST Era
• Press Conference: Stars Reshaping Galaxies: Clusters, Feedback, and Explosive Aftermaths
• Plenary Lecture: George Helou, The Cosmic Infrared Window: Photons, Technology, and People
• Plenary Lecture: Carolyn Kuranz, Creating Astrophysical Conditions at High Energy Density Facilities
• Plenary Lecture: Cara Battersby, Our Galaxy’s Dynamic Center

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Plenary Lecture: Eliza Kempton, Common Worlds, Uncommon Complexity: Sub-Neptune and Super-Earth Atmospheres in the JWST Era (by Lexi Gault)

Eliza Kempton began her talk addressing the undergraduates in the room, showing the meeting website from AAS 200, which she attended as an undergraduate in 2002. Searching this page, there were no presentations on exoplanets — a stark difference from the boom of exoplanet science that we see 24 years later at AAS 248, marking how significantly a field can transform from the start of your career.

Situating us outside of the solar system, Kempton showed the quintessential mass–period plot of all 6,000+ detected exoplanets thus far. Strikingly, we have discovered new types of planets that are not seen in our own solar system, but uncovering exoplanet populations is heavily skewed due to detection limits. While it is much easier to find large planets closely orbiting their stars, occurrence rate studies have shown that small planets on close-in orbits are much more common and typically come in two sizes: gas-rich sub-Neptunes and rocky super-Earths. However, actually deciphering their compositions relies on detailed studies of their atmospheres, which is the primary research work Kempton dove into.

How do we study exoplanet atmospheres? Thankfully, JWST provides the wavelength range and collecting area necessary for observing the minute details in exoplanet atmospheres. Beginning with sub-Neptunes, Kempton laid out four important takeaways from recent work coming from her team:

1. Sub-Neptune atmospheres composed of heavier elements and molecules are common.
2. Clouds and hazes are pervasive for planets that get a moderate amount of light from their host star. Early JWST observations showed signatures that may point to soot formation in sub-Neptune inner atmospheres; this soot can rise, making the upper atmosphere we observe hazy.
3. Sub-Neptune atmospheres may provide a window into their interiors. Observed atmosphere compositions may indicate no solid planetary surface, but rather magma, water, and/or gas that can mix more readily with the upper atmosphere.
4. Tying atmospheric observations to formation scenarios is still a challenge. Constraints on planets’ bulk composition has not provided clear answers.

Moving down to super-Earths, Kempton introduced the idea of the cosmic shoreline — the empirical relationship between planetary escape velocity and stellar irradiation shows a stark dividing line between planets with and without atmospheres in our solar system. Does this concept of a cosmic shoreline exist for exoplanets? Kempton highlighted three takeaways for the study of super-Earth atmospheres:

1. Spectroscopy of super-Earth atmospheres presents degeneracies in determining their characteristics. A featureless spectrum could be due to the presence of heavy molecules and clouds, or it could just mean the planet has no atmosphere.
2. Thermal emission is a powerful tool for identifying candidate atmospheres. The eclipse depth of a transiting planet is directly related to the day-side temperature of the planet. Planets with and without atmospheres will have different day-side temperatures — hotter without an atmosphere, cooler with an atmosphere.
3. Many (but not all) transiting super-Earths are bare rocks. 

Clearly, JWST has allowed astronomers to make significant progress in understanding exoplanet composition and subsequently their possible formation and evolutionary histories. Kempton ended with reminding us where we came from 24 years ago — the field of exoplanet science has boomed, and we can wonder how much we’ll continue to uncover in the next 24 years.

Return to Table of Contents.

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Press Conference: Stars Reshaping Galaxies: Clusters, Feedback, and Explosive Aftermaths (Briefing video) (by Niloofar Sharei)

This press conference brought together four very different views of star-forming environments: the most distant galaxy cluster caught strongly lensing a background galaxy, the dense young star clusters tucked inside the rings of nearby galaxies, the extreme feedback physics inside the closest luminous infrared galaxy merger, and the first known pair of supernova remnants born from a binary star system.

A New Era of Galaxy Cluster Analysis at Cosmic Noon: JWST’s View of a Distant Galaxy Cluster

Kyle Finner (IPAC/Caltech), presented the discovery of strong gravitational lensing in XLSSC 122, the most distant known galaxy cluster, seen as it was about 10.3 billion years ago. When the team got their JWST imaging back, they noticed bright blue arcs around the brightest cluster galaxy, a clear signature of strong lensing from a background galaxy sitting in just the right place behind the cluster. Using those arcs, the team measured the radial dark matter profile and found that XLSSC 122 is much more centrally concentrated than cosmological simulations predict. To check whether the cluster itself is unusual, they combined the JWST result with multiwavelength data. All of those tracers show an elongated structure pointing the same way as the dark matter, which is a strong sign that XLSSC 122 is a merging cluster. But mergers usually lower concentration rather than boost it, so the question of why this early cluster looks so concentrated is still open. The team plans to follow up with more strong-lensing measurements. | Press release

Hidden Gems in the Hearts of Nearby Galaxies: Evolution of Young Massive Star Clusters in Circumnuclear Rings

Sajia Shahrin Neha (University of Kentucky) presented a study of young massive clusters in the circumnuclear rings of two nearby galaxies: NGC 3351, a normal spiral, and NGC 1097, which hosts an active galactic nucleus. Young massive clusters are extreme star-forming systems that pack hundreds of thousands to millions of stars into a small region, and the ring-shaped star-forming zones around these galactic nuclei provide exactly the kind of dense gas and dust reservoir these clusters need. Because the clusters are heavily obscured, optical, ultraviolet, and infrared light cannot pass through cleanly, so Neha and collaborators turned to radio continuum imaging with the Atacama Large Millimeter/submillimeter Array and the Very Large Array, which can see through the dust. Their team identified 53 young massive cluster sources across the two rings. They also placed them on an evolutionary sequence: heavily embedded “starless” clumps, newborn clusters that are just starting to become visible, and older “exposed” clusters that have already burned through much of their gas. All evolutionary stages are present in both galaxies, regardless of whether the host has an active nucleus. | NRAO press release | U. Kentucky press release

Dusty and Over-Pressured: Measuring Stellar Feedback in the Closest Luminous Infrared Galaxy

Deb/Debosmita Pathak (The Ohio State University/IPAC) presented new measurements of pre-supernova stellar feedback in NGC 3256, the closest luminous infrared galaxy and a merger between two roughly Milky Way–mass galaxies. NGC 3256 forms stars about 60 times faster than the Milky Way, which makes it a useful nearby laboratory for the intense, clumpy star formation that dominated the early universe. Using JWST plus multiwavelength data, the team measured feedback pressures around roughly 1,700 young star clusters and compared them with what we see in quieter, normal star-forming galaxies. The results break the standard picture in two ways. First, the feedback and interstellar medium pressures in NGC 3256 are about a hundred times higher than what is typical in normal star-forming galaxies. Second, the dominant feedback mode is different: instead of warm ionized gas pressure doing most of the work, ultraviolet and infrared radiation pressure on dust dominates in these much dustier environments. Even at these much higher pressures, most clusters are still over pressured relative to their surroundings, meaning radiation is strong enough to drive expansion against a dense ambient interstellar medium. | NRAO press release | OSU press release

Unveiling the First Binary-System Supernovae

Miltiadis Michailidis (Kavli Institute for Particle Astrophysics and Cosmology (KIPAC)/Stanford University/SLAC National Accelerator Laboratory) closed the session with the discovery of what appears to be the first known supernova remnant pair born from a binary star system. The starting point is IC 443, one of the best-studied supernova remnants and one of the brightest gamma-ray sources in the sky, thanks to its interaction with a dense nearby molecular cloud. Recent eROSITA observations showed that the faint X-ray “shell” long noticed next to IC 443 is in fact a second, overlapping supernova remnant. The question was whether the two are physically related or just a chance alignment on the sky. They used 15 years of Fermi gamma-ray data and modeled and subtracted IC 443’s contribution and uncovered a distinct, hidden gamma-ray source associated with the new remnant.

The gamma-ray emission is strongly spatially segregated: its northern boundary, where it overlaps the same molecular cloud that IC 443 illuminates, is dominated by accelerated protons colliding with dense gas, while its southern boundary is dominated by accelerated electrons. The team also showed that the optical filament along the northern boundary is a radiative shock that has cooled past the point of efficient particle acceleration. Combined with the fact that both remnants sit at the same distance (about 6,000 light-years), and that a population-synthesis run with a million massive binaries reproduces the observed separation and age difference, the team argues that the two remnants are the remains of a true binary pair. | Press release

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Plenary Lecture: George Helou, The Cosmic Infrared Window: Photons, Technology, and People (by Kerry Hensley)

George Helou (Caltech) gave a sweeping overview of infrared astronomy over the past 40 years, including the impressive role of the Infrared Analysis and Processing Center, or IPAC, which is celebrating its 40th anniversary.

The infrared portion of the electromagnetic spectrum is broad, encompassing wavelengths from 1 to 1,000 microns. In that wavelength range, there’s a lot going on in our universe: redshifted emission from distant galaxies, stars revealed by the drop in dust extinction, rotational and fine structure lines, molecular transitions, and fully half of the radiation density in the universe.

There have been many successful ground- and space-based infrared missions and surveys over the past four decades, starting with the Infrared Astronomical Satellite (IRAS), which proved the potential of infrared astronomy from space and set a new standard for making data accessible and well documented. Next, the Infrared Space Observatory (ISO) revealed the full span of infrared emission from objects like galaxies and gave researchers their first glimpse of H₂ lines. The Two Micron All Sky Survey (2MASS) produced the first fully digital survey of the sky at high angular resolution, creating a dataset that is still widely used. The Spitzer Space Telescope, the Wide-field Infrared Survey Explorer (WISE), the Herschel Space Observatory, and the Planck satellite followed, bringing a slew of new and unexpected discoveries, and pushing the field of infrared astronomy to longer wavelengths.

Helou introduced a few science areas in which infrared missions have contributed greatly. First was the study of polycyclic aromatic hydrocarbons (PAHs), which are a class of molecules containing interconnected rings of carbon atoms. Thanks to infrared instruments, astronomers showed that various unassigned spectral features arose due to PAHs and that these molecules were important for the heating and cooling of the interstellar medium. As infrared instruments became more capable, observations showed how the fraction of carbon that is locked up in PAHs changes with metallicity and revealed novel species like deuterated hydrocarbon nanoparticles.

In the field of exoplanets, IRAS found evidence of excess infrared emission around Vega, hinting at a disk of debris that was later confirmed. (IPAC was instrumental in this discovery; the star Vega was being used for calibration, and the calibration wouldn’t converge because of the unexpected infrared excess.) Later, ISO used spectroscopy to investigate stellar debris disks and showed that they were similar in properties to solar system comets. Though Spitzer wasn’t initially expected to be used for exoplanet studies, the telescope made the first detection of photons from the surface of a planet and helped to discover several planets in the TRAPPIST-1 system. (Here, IPAC played an integral role again; instrument teams and flight engineers worked to ready the spacecraft for exoplanet observations by modifying the heaters on the spacecraft for pointing stability and finding “sweet spots” for photometric stability.) Today, JWST continues to advance the field with sensitive measurements of exoplanet atmospheres.

Finally, Helou touched on the work that IPAC has done to archive data. IPAC maintains the NASA/IPAC Extragalactic Database, or NED, which receives at least three queries per second. It’s also responsible for the Infrared Science Archive, data from which is used in more than 50% of research articles. IPAC also manages the NASA Exoplanet Archive, showing that the center isn’t limited to infrared astronomy!

As for the future of IPAC and infrared astronomy, it’s clear that there is much left to explore and lots of exciting work to be done; the Roman Space Telescope is scheduled to launch in a few months, and within a few years the Near-Earth Object Surveyor will follow. The Ultraviolet Explorer, for which IPAC will provide the UVEX Science Data Center, is slated to launch in 2030, and the PRobe far-Infrared Mission for Astrophysics (PRIMA) is under final review.

Read Astrobites’s interview with George Helou.

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Plenary Lecture: Carolyn Kuranz, Creating Astrophysical Conditions at High Energy Density Facilities (by Lucas Brown)

While astronomy and astrophysics are often considered primarily observational sciences, with most targets of interest being millions to trillions of miles away from us — Carolyn Kuranz’s plenary talk helped to challenge this idea, demonstrating how we can sometimes bring the heavens down to Earth with high energy density (HED) facilities. The basic idea behind Kuranz’s work, and the field of HED laboratory astrophysics more broadly, is that some extreme astrophysical environments, particularly plasmas, can indeed be studied in the lab by producing smaller-scale analogs — so long as we choose the right types of experiments.

Kuranz structured her talk around three big questions she commonly gets asked about her field: firstly, why would astronomers want to do laboratory experiments? Then, what defines the “high energy density” in HED astrophysics? And finally, how do we actually do these experiments in practice? Starting with the why, Kuranz notes that studying systems in astrophysics is inherently difficult as a result of the fact that they are far away, and theoretical models can suffer from a lack of data or missing physics. Experiments can bridge the gap here, providing the ability to collect more data on specific systems of interest, complementing both theoretical models and observations.

While this philosophy can apply to a variety of laboratory astrophysics areas, such as laboratory-based studies of the chemistry of planetary atmospheres, Kuranz focused specifically on the emerging role of HED laboratory astrophysics. Typically, high energy density refers to systems with more than 1 million atmospheres of pressure and temperatures above 1 million K. Generally, this means we’re in plasma territory. In some ways, it’s only natural that studies of these sorts of environments would eventually help astrophysicists, given that around 99.9% of the matter in the universe is in plasma form. Despite this, the laboratory astrophysics side of HED physics has really only emerged over the past 30 years, with its origins being traced to work out of Lawrence Livermore National Lab, the current site of the National Ignition Facility (NIF).

Today, the NIF is only one of a few dozen facilities in which HED laboratory astrophysics experiments are conducted. This is because the type of equipment needed to reproduce environments analogous to various astrophysical systems can vary greatly. This gets to Kuranz’s third question of how these experiments are conducted. The process begins by answering a set of questions, including: do the astrophysical and proposed laboratory environments obey the same governing equations (conservation of mass, momentum, etc.)? Is one able to re-scale the physical parameters in each system (timescale, size, density, pressure, etc.) in a way that retains relevant dynamics like viscosity, Reynolds number, and so on? Once these details have been nailed down, a researcher can assess what types of facilities they may need. For example, the NIF, which uses symmetrical arrangements of lasers to compress and heat up millimeter-scale pellets containing gases that can undergo nuclear fusion, provides a fairly natural environment for studying processes in the cores of stars, which are also hot, dense, and neutron-rich environments where fusion is actively occurring.

As for the future, things look promising for HED laboratory astrophysics — since the NIF achieved “ignition” in 2022, producing a net gain of energy from one of these pellet-crushings for the first time, a huge amount of interest and investment has been flowing into facilities studying HED physics. These facilities range from private companies working to achieve sustained nuclear fusion power generation to longstanding federal research centers working on understanding nuclear weapons and stockpile management, but they all provide astronomers with the opportunity to do what was once unthinkable: bring the universe’s most extreme environments down to Earth.

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Plenary Lecture: Cara Battersby, Our Galaxy’s Dynamic Center (by Niloofar Sharei)

Cara Battersby (University of Connecticut) used her plenary to walk us through her group’s ongoing effort to understand one of the most extreme environments in our own galaxy: the central molecular zone (CMZ), which is a dense, turbulent gas reservoir at the heart of the Milky Way. Her talk pulled together large surveys, new high-resolution zoom simulations, and machine-learning methods designed to bridge between the two.

She opened with the PRobe far-Infrared Mission for Astrophysics (PRIMA), the far-infrared probe mission concept she is a co-investigator on. This mission concept was developed in response to NASA’s recent billion-dollar probe mission call, and selection is expected later this year. PRIMA covers roughly 1.8–25 microns with imaging and spectroscopy, and galactic ecosystems make up about a quarter of its science case.

Battersby paused mid-talk to speak directly to early-career people in the audience about work-life balance, including bringing her daughter and sister with her to the meeting, and the SPARK program at UConn that supports women and underrepresented groups in physics. Her message: astronomy should be done from a place of joy, and visible work-life balance is something the community should be more open about.

Battersby framed the Milky Way as the natural laboratory for star formation across a huge range of environments, and briefly reviewed the ALMAGAL survey, which observed roughly 1,000 high-mass star-forming regions across the disk. But her main focus was the CMZ, which she argued is essentially a different kind of galaxy embedded inside our own. About 80% of all the dense molecular gas in the Milky Way sits in the CMZ, and its gas properties — density, turbulence, temperature, and pressure — are extreme compared to the rest of the disk. Because nearby starburst and high-redshift environments share many of these properties, the CMZ is the closest place we can actually resolve individual star-forming cores under conditions that resemble the early universe.

She showed that gas in the CMZ is extremely filamentary across all scales, and that the relationship between dense gas and star formation rate is unusual: when individual clouds in the CMZ actually form stars, they form them much like clouds elsewhere in the galaxy, but a large fraction of the dense gas in the CMZ never engages in star formation at all. Battersby argued that this is likely because so much of that gas is being shaped by flows toward the black hole and the strong turbulence and dynamics of the central environment, and it never cools and collapses into stars.

She closed with the simulation and machine-learning side of the program. Her group runs zoom simulations that can follow individual star-forming regions and can resolve down to CMZ scales while still capturing the full ~5 kpc bar driving gas inward. To extract more from these simulations than just side-by-side image comparisons, the team built a GPU-accelerated radiative transfer code that runs roughly 10,000 times faster than existing codes. They generate hundreds of thousands of synthetic observations and use them to train an encoder–decoder convolutional neural network that learns to predict the top-down structure of a region from the side-on view we actually observe. Applied to the CMZ, the network produces 3D reconstructions that are physically plausible and broadly consistent across independent training runs.

Read Astrobites’s interview with Cara Battersby.

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The post AAS 248: Day 3 appeared first on AAS Nova.

Editor’s Note: This week we’re at the 248th AAS meeting in Pasadena, CA. Along with a team of authors from Astrobites, we will be writing updates on selected events at the meeting and posting each day. Follow along here or at astrobites.com for daily summaries, or follow @astrobites.bsky.social on Bluesky for more coverage. The usual posting schedule for AAS Nova will resume on 22 June.

Table of Contents:

• Helen B. Warner Prize Lecture: Kyle Kremer, Globular Clusters: Astronomical Factories of Gravitational-wave and Electromagnetic Transients
• Press Conference: Our Changing Galaxy and the Skies We See
• High Energy Astrophysics Division Early Career Prize Lecture: Carolyn Kierans, Developing Next-Generation MeV Telescopes
• Press Conference: Fire and Ice in Planetary Systems Near and Far
• Solar Physics Division George Ellery Hale Prize Lecture: Yi-Ming Wang, The Sun as a Laboratory for Astrophysical Magnetic Fields
• Plenary Lecture: David Jewitt, The Interstellar Interlopers

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Helen B. Warner Prize Lecture: Kyle Kremer, Globular Clusters: Astronomical Factories of Gravitational-Wave and Electromagnetic Transients (by Lucas Brown)

Kyler Kremer (University of California, San Diego) kicked off day two of AAS 248 with a “dynamic” talk on the complex interactions and environments that exist within globular clusters — the most massive category of stellar clusters, which can contain upwards of one billion stars. Because of the extreme stellar density of these environments, they are thought to be good factories of systems ranging from accreting white dwarfs to millisecond pulsars, which are often spun up to high speeds via accretion from companion stars. Such accretion, whether onto pulsars or other compact objects, can often generate bright X-rays, and as Kremer noted early on in his talk, it was in fact these X-ray sources that provided some of the earliest evidence of the abundance of exotic systems within globular clusters. By the early 2000s, hundreds of X-ray sources had been identified in globular clusters.

Along with these brightly emitting compact binary systems, we expect globular clusters to be home to a vast population of darker, more elusive objects — black holes. Kremer noted that because about one out of every thousand stars is thought to be massive enough to evolve into a black hole, older globular clusters are thought to produce tens or hundreds of thousands of black holes over time. These black holes are then expected to gradually sink towards the centers of the clusters through dynamical friction, where they will increasingly find themselves in the company of other black holes, or black hole binaries. As these massive objects swing by each other, they can occasionally merge or eject black holes out of clusters entirely. This leads to a complex evolutionary history, leaving many open questions in regards to how many black holes should persist in globular clusters today and with what properties.

Luckily, strong evidence for the presence of black holes in globular clusters began to come in around the early 2000s as a result of the direct measurements of stellar orbits around an invisible, at least 4-solar-mass object in NGC 3201. Since this discovery, multiple others have been made via similar techniques. The role of black holes in shaping our observation of globular clusters may go well beyond these isolated systems, however. Theoretical work has also shown that the presence of black hole binaries in the centers of globular clusters may contribute to dynamical heating of the overall cluster. This suggests that globular clusters that have not undergone core collapse, which represents over 80% of clusters, should retain significant black hole populations today.

While individual black holes can often be hard to spot, black hole binaries are becoming increasingly easy to hear, thanks to the advent of gravitational wave astronomy. At facilities like LIGO, Virgo, and KAGRA, merging black hole binaries are now regularly detected, creating a population-level catalog of black hole binary properties. While many of these systems can be well explained by black holes formed in the “isolated” channel, meaning originating from a preexisting binary star system, some mergers show signs of a “dynamical” formation history, which can arise in the sort of dense environments globular clusters provide. As one example, there seems to be more black holes in the 40-solar-mass and above range than many models predicted. At these high masses, more potential progenitor stars actually become too large to form black holes thanks to a phenomenon called pair instability. However, such massive black holes could be formed from the repeated mergers of smaller black holes — a hierarchical process that simulations have consistently shown can be highly efficient in globular clusters, especially the most massive ones. On top of this, Kremer noted that individual gravitational wave signals have recently shown other telltale signs of hierarchically formed binary black holes, such as asymmetric masses and spin–orbit misalignment.

Finally, Kremer turned to another class of exotic astrophysical objects to explore other ways globular clusters can generate interesting phenomena: pulsars. Pulsars, like black holes, form from the collapse of massive stars. This rapidly rotating sub-category of neutron stars beams radiation from their poles, which gradually slows them down and depletes the strength of their radio emission over millions or billions of years. Because of this, one would expect older globular clusters to have a slowly rotating and mostly invisible population of pulsars. Instead, we see many rapidly rotating millisecond pulsars in these environments, suggesting that the dense cluster environment allows for many of these systems to be spun back up through accretion from other stars. In contrast, we also see young globular clusters which contain slowly rotating pulsars — another puzzle, yet one that may again be explained by the dense environment. Some researchers believe that these pulsars are formed via the direct merger of multiple white dwarfs, rather than from the collapse of a massive star. Such systems may also be the source of some fast radio bursts; the first fast radio burst associated with a globular cluster was discovered in 2020. It seems like globular clusters may be the gift that just keeps on giving!

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Press Conference: Our Changing Galaxy and the Skies We See (Briefing video) (by Niloofar Sharei)

This press conference walked through four very different ways our view of the Milky Way and our skies are changing right now — from extreme gas physics at the Galactic Center, to the hidden stellar populations of a cluster, to the rapid flickers of dying stars, and finally, to a very practical question: what are dark night skies actually worth to the public?

Resolving the Supersonic-to-Subsonic Gas Transition in the Galactic Center for the First Time

Rojita Buddhacharya (Center for Astrophysics | Harvard & Smithsonian; Liverpool John Moores University) presented new results from the central molecular zone (CMZ), the dense gas reservoir at the center of our galaxy about 25,000 light-years from Earth. The CMZ is one of the most extreme environments we know of: gas there is roughly a hundred times denser, hotter, and more turbulent than what we see in nearby molecular clouds. Despite holding huge amounts of star-forming fuel, the CMZ forms stars at a surprisingly inefficient rate, and one of the puzzles is what conditions actually let any of that gas collapse into stars.

Buddhacharya mapped how gas transitions from highly turbulent, supersonic motion to calmer, subsonic motion across a region in the CMZ. This transition is well known in nearby clouds, where it usually precedes gravitational collapse, but it had never been clearly resolved in the CMZ before. Their team found a “calm island” of subsonic gas embedded in the otherwise turbulent sea. This is the first observation of its kind in the galactic center.

The Multi-Age Stellar Populations of Terzan 5 as Revealed by JWST

R. Michael Rich (University of California, Los Angeles) presented new JWST and Hubble Space Telescope observations of Terzan 5, an unusual stellar system near the center of the Milky Way that looks like a globular cluster but doesn’t act like one. Earlier work had already shown that Terzan 5 hosts at least two distinct stellar populations with very different ages and chemical abundances, which is unusual for a typical globular cluster.

Because Terzan 5 sits behind a thick screen of foreground stars and dust, separating real cluster members from contamination is one of the hardest parts of the analysis. The team combined JWST and Hubble imaging to measure stellar proper motions, then used a vector point diagram to isolate the stars that move together with the cluster. They identified two additional stellar populations beyond the two already known. Rich argued that Terzan 5 may be a “fossil fragment,” possibly the leftover core of a small galaxy or early bulge structure that was accreted long ago.

Sub-Day Periodic Variability of Two Central Stars of Planetary Nebulae

Gaoyang Du (Dalian University of Technology) reported the discovery of new short-period signals in the central stars of two planetary nebulae. These central stars are the hot, exposed cores left behind by Sun-like stars at the end of their lives, and finding rapid brightness variations in them tells us about their interior structure and pulsation physics.

Short-period signals well under a day are very hard to recover from the ground because Earth’s rotation introduces strong one-day aliasing, so the team turned to the Transiting Exoplanet Survey Satellite, which observes continuously from space. Using a neural-network pipeline designed to handle the crowded, contaminated backgrounds these stars sit in, they recovered the previously known variability in NGC 1501 and detected a new signal at roughly half an hour. Follow-up spectroscopy will be needed to pin down whether the signals come from pulsations, rotation, or something else, but the result opens a new window on the rapid variability of dying-star cores.

Artificial Light at Night Significantly Degrades the Value of Public Lands

Jordan Smith (Utah State University) closed the press conference with a very different kind of measurement: the economic value of dark night skies. While astronomers usually treat light pollution as an observing problem, Smith and his collaborators are asking what the public is actually losing and what people would pay to keep dark skies on public lands. The team surveyed 634 visitors across International Dark Sky-certified parks, asking detailed questions about visitation behavior, motivations, and willingness to pay. Combining those surveys with satellite-based sky-brightness data, they estimated how the actual darkness of a site translates into the value visitors place on a trip. Based on the survey results, they found that visitors were willing to pay $45 more per visit to go to a park with skies one level darker on the Bortle scale. That sounds small per visitor, but scaled across all visits to all dark-sky public lands, it represents a very large economic value tied to a public good that current policy frameworks rarely account for.

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High Energy Astrophysics Division Early Career Prize Lecture: Carolyn Kierans, Developing Next-Generation MeV Telescopes (by Niloofar Sharei)

Carolyn Kierans (NASA Goddard Space Flight Center) was awarded the AAS High Energy Astrophysics Division Early Career Prize for her work on developing the next generation of megaelectronvolt (MeV) gamma-ray telescopes. Her talk laid out why the MeV band is so valuable (it contains nuclear emission lines like the 511 keV positron-annihilation line that traces high-energy processes in our galaxy) and why it has been so technologically difficult to observe well, since photons at these energies do not focus easily and the dominant interaction in the detector is Compton scattering. She walked through her work using balloon-borne Compton telescopes to mature the technology, the tools her team has developed in recent years to improve event reconstruction, and the broader roadmap for the next generation of MeV instruments.

Read Astrobites’s interview with Carolyn Kierans.

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Press Conference: Fire and Ice in Planetary Systems Near and Far (Briefing video) (by Kerry Hensley)

The afternoon press conference was convened by Astrobites’s own Niloofar Sharei, this meeting’s Astrobites Media Intern. First up in the parade of planetary presentations was Neal Turner (NASA’s Jet Propulsion Laboratory/Caltech). Turner’s focus was Io, a moon of Jupiter and the most volcanically active body in the solar system. Turner’s team put forward a new claim: that the long-lived volcanoes that speckle Io’s surface spew out volcanic ash in their plumes, producing enough of this material to rapidly erase impact craters.

The team modeled Io’s plumes in order to explain the moon’s lack of impact craters, understand how certain plumes have persisted on Io for more than 45 years, and motivate a possible mission concept that includes a plume flythrough and sample return. They modeled the formation of ash as magma rises in a volcanic vent as well as the trajectory of the ash once it’s ejected into the plume, aiming to 1) match the plume observations from the Galileo spacecraft and 2) understand how an ash-filled plume might look different from one filled only with sulfur dioxide. While Turner found that Galileo data couldn’t discern between the two scenarios, ash makes a dramatic difference at the sub-millimeter wavelengths accessible to the Atacama Large Millimeter/submillimeter Array, which the team hopes to use to test this prediction. Finally, returning to the development of the mission concept, Turner proposed using a probe to first fly through the shadow of a plume to assess the particle size before sending the spacecraft through to collect a sample.

Up next, Tunhui Xie (University of California, Los Angeles) turned to Io’s neighbor, Europa. Europa and the other large icy moons of Jupiter are of interest to researchers because they might have subsurface oceans. Observations of Europa from ~30 years ago show that the moon has strange radar properties: its radar albedo is large, and the return signal retains the strong circular polarization of the outgoing signal.

To investigate the moon’s radar weirdness further, Xie’s team observed Europa from 2011 to 2024 and showed that the moon’s albedo (fraction of light reflected from its surface) is roughly constant with longitude. There is some evidence of differences between Europa’s hemispheres, which could be a sign of spiky ice structures called penitentes protruding from the frosty surface; Europa’s trailing hemisphere might be subjected to more particles from Jupiter’s magnetosphere, which could suppress the growth of penitentes in that region and cause a difference in the albedo. Xie also addressed the polarization behavior of the radar signal, showing that this is likely due to the coherent backscatter opposition effect. Finally, Xie showed how observations helped to place an upper bound of 32 meters for the penetration depth of X-band radar photons into Europa’s icy shell.

Bryce Bolin (Eureka Scientific) reported on the third known visitor from another star system: 3I/ATLAS. Using the Multi-Object Spectrograph for Infrared Exploration on the Keck I telescope, Bolin’s team obtained some of the first infrared images of the interstellar visitor and detected its fuzzy coma — the cloud of gas and dust that surrounds the solid body of the comet — shortly after its discovery in July 2025. In August, observations revealed that the comet had started to grow a tail. As the team monitored 3I/ATLAS during its journey through our solar system, they saw significant changes. The comet was initially very red, likely due to space weathering from the galactic environment, but it became greener as the comet became active and refreshed its surface.

In some of the earliest spectroscopy taken by a large-aperture telescope, Bolin’s team found cometary volatiles like CN, but not C₂ or C₃, which are common in comets, giving them a green color. These missing species emerged a couple of months later, which coincided with the comet taking on a greener appearance. As the comet retreated from its encounter with the Sun, these emission features faded. This presentation by Bolin showed how early and continuing monitoring of an interstellar object by a large-aperture telescope can reveal how these objects are affected by their journeys through our solar system.

Moving out of the solar system entirely, Tiffany Kataria (NASA JPL/Caltech) turned the discussion toward exoplanets. Kataria discussed HD 80606b, a 4.1-Jupiter-mass planet on an extremely eccentric (e = 0.93) orbit with a period of 111 days around a solar-type star. Kataria used JWST to observe HD 80606b’s phase curve, which revealed different parts of the planet as it travels along its orbit. Each time the planet makes its extremely close approach to its host star, it gets “flash roasted” and experiences a rapid increase in temperature. This rapid heating event is complex, depending on factors like the planet’s atmospheric chemistry and winds. The JWST data revealed that HD 80606b’s temperature skyrockets an incredible 1,100℉ over the course of its orbit. There’s still much more to learn from the rich spectroscopic dataset from JWST, so keep an eye out for more findings about this flash-roasted exoplanet in the future!

Finally, Aurora Kesseli (IPAC/Caltech) discussed another toasty exoplanet called CoRoT-2b, which is a hot Jupiter on a 1.7-day orbit around its star. Because hot Jupiters orbit their hosts so closely, astronomers expect all of these extreme exoplanets to be tidally locked, with one side of the planet perpetually facing the star and the other side in perpetual darkness. However, CoRoT-2b might be breaking that trend. In 2018, researchers reported that CoRoT-2b had winds that blew in the opposite direction from all other known hot Jupiters. They drew this conclusion from the location of the hottest point on CoRoT-2b, which appeared just after the planet ducked behind its star, rather than just before — unlike all other hot Jupiters studied so far. At the time, researchers hypothesized that the unusual location of the hotspot was due to clouds on one side of the planet, a magnetically induced westward wind, or a lack of tidal locking.

Using new data from the Very Large Telescope and Gemini South, Kesseli found that the planet’s strange properties can be explained best by a lack of tidal locking. CoRoT-2b appears to rotate at 2.24 km/s — far slower than the 4.37 km/s expected if the planet were tidally locked. There are no theories yet that explain why CoRoT-2b wouldn’t be tidally locked, so it remains a mystery for now!

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Solar Physics Division George Ellery Hale Prize Lecture: Yi-Ming Wang, The Sun as a Laboratory for Astrophysical Magnetic Fields (by Kerry Hensley)

Yi-Ming Wang (Naval Research Laboratory) gave the AAS Solar Physics Division’s George Ellery Hale Prize Lecture for “sustained, foundational contributions to understanding solar magnetism, the solar cycle, and the solar wind.” Wang is perhaps most known for the Wang–Sheeley–Arge solar wind model, which is widely used in space weather forecasting.

Wang began by musing on how his winding career path brought him to solar physics. Initially a theorist who modeled neutron stars, Wang was later hired to do radio astronomy but arrived for the position later than expected, so he was roped into working on solar physics instead — where he learned many things about magnetic fields that he wished he’d known when working on neutron stars!

In this talk, Wang gave an overview of three aspects of the Sun related to its magnetic field: quasi-rigid rotation of coronal holes, the source of coronal heating, and the speed of the solar wind. First up, coronal holes: these are dark areas in the solar corona where magnetic field lines stream out into space, which solar physicists call “open” field lines. (“Closed” field lines curve back to the solar surface.) Wang described how we know that coronal holes are regions of open field lines by extrapolating the photospheric magnetic field. (The photosphere is the visible “surface” of the Sun.) Because coronal holes rotate quasi-rigidly (compared to the photosphere, which rotates differentially), magnetic field lines must be continually converted from closed to open and vice versa at the boundaries of these holes.

Next, Wang discussed the source of coronal heating. The solar corona is famously extremely hot, running at a few million degrees compared to the ~6000K solar photosphere. For decades, the Parker nanoflare model has been the leading explanation for how the corona gets so hot. In this model, the places where coronal loops touch down in the solar photosphere are continually wiggled around by random motions of the photosphere, which tangles up the magnetic field lines in the coronal loop. When the current sheets in this system dissipate, the corona is heated. Instead, Wang favors an observationally based model in which the heating comes from magnetic reconnection with underlying small-scale fields.

Finally, Wang briefly touched on the connection between the Sun’s magnetic field and the speed of the solar wind. The solar wind has been measured at Earth since the 1960s, and these measurements have shown that the mass and energy flux density of the solar wind scales with the strength of the footprint magnetic field strength, but the observed solar wind speed is independent of the footprint field strength. Instead, the speed scales inversely with the rate of magnetic flux-tube expansion in the corona. Thus, the solar wind speed at Earth can be calculated from the rate of flux-tube expansion along open magnetic field lines using maps of the photospheric magnetic field.

In the end, Wang’s work on the solar magnetic field did connect back to his earlier work in high-energy astrophysics, with relevance to period changes in binary pulsar systems and other areas. Magnetic fields really do connect all parts of astrophysics!

Read Astrobites’s interview with Yi-Ming Wang.

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Plenary Lecture: David Jewitt, The Interstellar Interlopers (by Niloofar Sharei)

David Jewitt focused his plenary talk on a rare kind of small body: the three confirmed interstellar objects that have passed through the solar system so far. He opened with the Oort cloud as a framing device. Comets in the Oort cloud did not form there — they formed near the giant planets and were scattered out by gravitational interactions. The same process leaks comets to interstellar space very efficiently: only about 1–10% are trapped into a star’s own Oort cloud. Across the galaxy, that implies a free-floating population on the order of 10²⁴–10²⁵ objects. Seeing some of them pass through our solar system is exactly what we should expect.

The bulk of the talk walked through the three known interlopers. ʻOumuamua (1I/2017 U1) is the strangest: a tiny inert body about 80 meters across, with an extreme light curve consistent with either a 6:1 elongated shape or a flattened disk, and a non-gravitational acceleration of about 0.1% of solar gravity despite no detectable outgassing. Jewitt walked through the proposed explanations: exotic ices, ultra-low-density fluffballs, and sail-like sheets, and showed why each runs into a physical problem. 2I/Borisov (2019), by contrast, looks like a perfectly normal comet, with a ~400–500-meter nucleus, a coma, a tail, and a solar-system-like composition. 3I/ATLAS is the largest at roughly a kilometer, and JWST infrared spectroscopy has revealed CO₂ emission about an order of magnitude richer than typical solar system comets. This is the first detailed look at the volatile inventory of an interstellar visitor.

Jewitt then turned to what the three objects tell us as a population. The strongest constraint comes from ʻOumuamua, precisely because its discovery was such a near miss: catching one in that brief visibility window implies roughly 1,000 similar objects crossing the solar system per year. The three objects look very different in size, activity, and composition, which is consistent with sampling a broad galactic population, and their velocity dispersion lines up with the disk-heating picture, in which older galactic populations are kinematically hotter. The leading origin story is the same one he set up at the start: scattering from the protoplanetary disks of giant planets around other stars.

Read Astrobites’s interview with David Jewitt.

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