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Messier 83

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: Accepted to 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.

NGC 2683 with annotations

Figure 1: The spiral galaxy NGC 2683 with its bulge and disk labeled. The inner part of the galaxy (“bulge”) contains older, redder stars; the outer part of the galaxy (“disk”) contains bright blue stars that indicate that star formation is ongoing. Here, the slit spectrograph only covers the galaxy’s bulge, leading to incorrect mass and star formation rate measurements. [ESA/Hubble & NASA with annotations by Margaret Verrico]

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.

color-color diagram for the galaxies studied

Figure 2: The location of each of the four galaxies on a color–color diagram that separates red galaxies (above/to the left of the dashed lines) from blue galaxies (below/to the right of the dashed lines). The open markers indicate the galaxy color as measured from the center of the galaxy; the filled-in markers show the color at different radii from the galaxy center. For all but one galaxy, the color at several radii is bluer than at the center, indicating that the galaxy might have younger stars at least at some radii. [Adapted from Cutler et al., in press]

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.

NGC 1068

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

Galaxies emit light not only in the visible band, but also across other wavelengths via different physical mechanisms. Finding correlations between emissions at different wavelengths is a powerful tool for astrophysicists to explore the underlying mechanisms of galaxy formation. One of the most famous examples is the infrared–radio continuum correlation, which holds across several orders of magnitude in luminosity.

The typical explanation for this correlation is that both infrared light and radio synchrotron emission share the same energetic origin: star formation. Young stars emit ultraviolet radiation, which is absorbed by interstellar dust and re-radiated in the infrared. When these stars end their lives in core-collapse supernovae, the resulting shock waves accelerate cosmic-ray electrons, which then interact with the magnetic field to produce synchrotron emission.

For this correlation to hold, cosmic-ray electrons must radiate away all their energy within the galaxy, unable to escape and carry away some of their energy. Otherwise, the amount of radio emission would depend on how quickly cosmic rays escape and would no longer correlate with the infrared emission. This assumption is called the “electron calorimeter” model, which assumes that electrons lose all their energy due to radiation without escaping the galaxy. However, observations show that the infrared–radio correlation persists even in low-density galaxies where cosmic rays can easily escape, suggesting that the calorimeter model may not be the full story. To reconcile this, previous studies proposed “conspiracy-like” scenarios in which cosmic-ray injection, transport, magnetic-field amplification, and gas density are all coupled in a finely self-regulating way to maintain the correlation across diverse galactic environments — a somewhat awkward explanation that requires multiple unrelated physical processes to conveniently work together.

With the increased sensitivity of the Fermi Large Area Telescope, high-energy gamma-ray (or γ-ray) detections add new information to this picture. A galaxy’s γ-ray emission is produced by two different sources: cosmic-ray protons interacting with the interstellar medium gas, and cosmic-ray electrons inverse-Compton scattering off of nearby starlight or photons from the cosmic microwave background. Since both processes involve cosmic rays, the total γ-ray emission should also reflect the amount of star formation and therefore correlate with infrared and radio emission.

Because all three wavelengths trace different pieces of the same cosmic-ray population, studying them together reveals how the galaxy’s total cosmic-ray energy budget is distributed across its multi-wavelength emission.

The infrared–radio correlation has been established not only at the scale of entire galaxies, but also locally, down to patches roughly 100 parsecs across. Think of it like the relationship between a city’s electricity consumption and its population: we have long known they track each other city-wide, but it turns out the same relationship holds block by block. The infrared–radio correlation works the same way: zoom into any 100-pc region within a galaxy, and the correlation is still there. Unfortunately, the resolution of γ-ray observations is still stuck at around the kiloparsec scale. It is therefore unclear whether the local infrared–radio correlation reflects a genuine local physical coupling, one that would extend to γ-rays if we had sufficient resolution, or whether it is simply a spatial average of the emission, smoothed out by insufficient resolution. The answer determines whether radio and γ-ray emission can be used as reliable tracers of star formation, or whether they are just smeared out by cosmic-ray propagation over kiloparsec scales.

In this study, the authors numerically model galactic cosmic-ray transport, building a suite of physically motivated three-dimensional models of the Milky Way. They consider two variants for each of the four key inputs: 1) the cosmic-ray source distribution, 2) the interstellar gas, 3) the amount of starlight within the galaxy (the interstellar radiation field), and 4) the galactic magnetic field. Crucially, every model is normalized to reproduce the locally observed cosmic-ray data to within 5%, so that any differences in the predicted emission come purely from the large-scale geometry of each input, rather than from variations in the total cosmic-ray budget.

After evolving cosmic-ray transport for these realistic models, the authors then compute synthetic observations at radio, infrared, and γ-ray bands from various viewing inclinations to investigate how the underlying galactic model and the viewing angle affect the correlations between these emissions.

Surprisingly, inclination plays the most important role — more so than the cosmic-ray source distribution, the radiation field, or the magnetic field. Figure 1 shows the correlation plots between the radio and γ-ray emission (left two columns) and between infrared and γ-ray emission (right two columns) when viewing the galaxy face-on (i.e., viewing the galaxy from directly above the disk). Different colors represent γ-ray emission components from different physical mechanisms, with orange showing the total γ-ray emission. For both sets of correlation plots, starting from the upper-left panel, each panel modifies one physical input and recomputes the emission. Although there is some variation, one can see that the γ-ray emission correlates with both the radio and the infrared emission over several orders of magnitude, regardless of the underlying input model.

plots of radio and infrared flux versus gamma-ray intensity

Figure 1: Radio (left two columns) and infrared (right two columns) vs. γ-ray correlation plots for a galaxy viewed face-on from 50 kpc. Each panel modifies one input — the cosmic-ray source distribution, gas, radiation field within the galaxy, or magnetic field — relative to a baseline model. Colors indicate γ-ray production mechanisms (green: proton–gas; black: inverse-Compton; orange: total). Across all variations, both the radio–γ-ray and infrared–γ-ray correlations remain quasi-linear over several orders of magnitude, showing that the correlations are robust to the choice of underlying galactic model. [Adapted from Porter et al. 2026]

In contrast, Figure 2 shows the correlation plot between radio and γ-ray emission (upper panels) and between infrared and γ-ray emission (lower panels) when viewing the galaxy edge-on (viewing the disk from the side). The radio–γ-ray correlation is preserved, while the infrared–γ-ray correlation is no longer observed. This is because, at this inclination, geometric projection separates the emission components. Infrared-emitting dust and γ-ray emission from cosmic ray–interstellar medium interactions are confined to the disk, while synchrotron radio emission and γ-ray emission from inverse-Compton scattering extend to larger distances above and below the disk. As a result, in an edge-on view, a sight line passing through the disk sees strong infrared emission, while a sight line that misses the disk sees zero infrared but still some γ-ray emission, breaking the infrared–γ-ray correlation. On the other hand, because radio emission is more extended than infrared emission, sight lines that miss the disk can still see both radio and γ-ray emission together, preserving the radio–γ-ray correlation.
relationships between infrared, radio, and gamma-ray flux for two types of galaxy models

Figure 2: Correlation plots for two galaxy models viewed edge-on from 50 kpc — simple axisymmetric (left) and complex with spiral arms (right). Top: radio vs. γ-ray; bottom: infrared vs. γ-ray. Colors indicate γ-ray production mechanisms (green: proton–gas; black: inverse-Compton; orange: total). The infrared–γ-ray correlation breaks down, while the radio–γ-ray correlation is broadened but partially preserved. [Porter et al. 2026]

However, there is one situation where the edge-on correlation comes back: when the observer is far enough away that the image becomes too blurry to resolve the galaxy’s internal structure. Figure 3 shows the same galaxy model viewed from 500 kpc, roughly the distance to Messier 31, our nearest large neighbor. At this distance, a single pixel in the image covers about 1 kpc, large enough for everything along the line of sight. The messy, broken-up pattern seen in the closer 50 kpc edge-on view (lower-right panel of Fig. 2) disappears, and a tight, nearly linear correlation comes back. This explains why nearly all distant, unresolved galaxies show tight radio–infrared–γ-ray correlations: the tightness is simply what you get when you blur everything together, not evidence that cosmic rays are actually losing all their energy locally inside those galaxies.
relationships between infrared, radio, and gamma-ray flux for simulated galaxies viewed from 500 kiloparsecs

Figure 3: Correlation plots from a complex galaxy model viewed from 500 kpc, at three different inclinations (face-on, 45°, edge-on). Top row: radio vs. γ-ray; bottom row: infrared vs. γ-ray. Colors indicate different γ-ray production mechanisms (green: proton–gas; black: inverse-Compton; orange: total). At this distance, the blurring effect of large pixels restores a tight correlation even in the edge-on view. [Porter et al. 2026]

Now we have a consistent picture of the radio–infrared–γ-ray correlation: the authors conclude that this correlation is not a direct signature of cosmic-ray calorimetry, but a geometric projection effect. In face-on systems, a single sight line integrates emission from both the disk and off-disk regions, naturally producing a linear correlation, even when cosmic rays do not lose all their energy locally to radiation (i.e., local calorimetry is absent). In edge-on systems, the correlation breaks down because geometric stratification separates the different emission components. The practical implication is significant: for unresolved galaxies, the tightness of the global correlation alone cannot be used as evidence for cosmic-ray calorimetry, and the physically meaningful information about cosmic-ray transport and escape efficiency is encoded in the scatter around the mean trend, not in the correlation itself.

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.

Arp 142

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

How to Quench a Galaxy

Look at an image of the sky taken with a sufficiently sensitive telescope, and you’ll quickly notice that galaxies tend to cluster into two main types: blue spiral galaxies, which have flat disk shapes with a central bulge, and red elliptical galaxies, which look like spherical or elliptical balls of red stars. Blue spiral galaxies can form tens to hundreds of stars every year, while elliptical galaxies have completely stopped forming stars, meaning that some process has to transform star-forming spirals into non-star-forming, or “quiescent,” elliptical galaxies. To form the current population of massive elliptical galaxies, this process had to be common about 2–3 billion years after the Big Bang at “cosmic noon,” the period when star formation in the universe peaked. The processes that “quench” star formation in massive galaxies are still being studied, and one of the best ways to study them is to find galaxies that recently quenched and look for clues about the processes that quenched them.

Post-starburst galaxies are galaxies that have rapidly quenched after a short burst of star formation. Because these galaxies quenched so quickly and so recently, it’s often possible to find signs of whatever quenched them, like the signatures of past galaxy mergers, feedback from supermassive black hole accretion, or the shutoff of gas flowing into the galaxy. Rapid quenching is pretty uncommon now, but it was a much more common way for galaxies to quench at cosmic noon. Understanding post-starburst galaxies at cosmic noon is therefore critical for understanding the formation of massive elliptical galaxies in the local universe.

The Quest for Quenched Galaxies

Today’s article uses data from the JWST Early eXtragalactic Continuum and Emission Line Survey (EXCELS) to identify post-starburst galaxies at cosmic noon and try to determine why they quenched. EXCELS is a spectroscopic survey, so the authors get a spectrum for every galaxy. The spectrum encodes information about the galaxy’s stellar population, including the mass of stars in the galaxy, the number of stars forming every year, and the history of star formation throughout the galaxy’s life.

The authors use a technique called principal component analysis to further divide the sample into young and old post-starbursts. Principal component analysis is a machine-learning technique that learns the most important features of a data set. This technique is used for “dimension reduction,” or reducing the number of data points needed to learn something about the object. A typical spectrum has hundreds or even thousands of data points, which means performing data analysis on a spectrum can be very computationally expensive. Principal component analysis takes these thousands of data points and learns broad patterns that correlate with each other. These patterns are called “supercolors” in the context of spectral data, and they encode things like the overall shape and color of the spectrum as well as the spectral shape around key features (see Figure 1 for a visualization). Since the overall shape, color, and emission/absorption line features of a spectrum come from the galaxy’s stellar population, this method can be used to identify galaxies with lots of star formation a billion years ago but very little star formation today — in other words, post-starburst galaxies.

demonstration of principal component analysis for spectra of different types of galaxies

Figure 1: An example of principal component analysis for sample star-forming (SF), quiescent (Qu), and post-starburst (PSB) galaxy spectra (left-hand side). Principal component analysis simplifies a many-dimensional data set (for example, spectra) into fewer dimensions. In this case, Super-Color 1 measures the overall color of the spectrum (shown on the left-hand side as the slope of the spectra, marked with red lines), while Super-Color 2 measures the shape of the spectrum around 4,000 angstroms (orange box). The authors use principal component analysis to identify post-starburst galaxies for further analysis (right-hand side). [Adapted from Skarbinski et al. 2026]

The authors apply principal component analysis to the galaxies in their sample and find that 11 of the galaxies in their sample are classified as post-starburst, 9 are quiescent, and 4 still have some star formation. To further analyze the stellar populations of the post-starbursts in their sample, the authors use a program called Bagpipes to fit the galaxies’ spectra. Bagpipes is a spectral energy distribution fitting software, which means that it takes the observed spectrum of a real galaxy and tries to match it to a library of different stellar spectra. By measuring the relative contribution of different kinds of stars (which all have different lifetimes), Bagpipes can compute the likely history of star formation in the galaxy (e.g., when the star formation rate peaked) as well as the present-day properties of the galaxy (things like the mass in stars versus dust and the current star formation rate). The authors use the galaxies’ star formation histories to try to find clues as to how they quenched.

How Quickly Do Post-Starbursts Quench?

First, the authors measure something called a “quenching timescale,” which they define as the length of time between when the galaxy’s star formation rate peaked and when it fell low enough that the galaxy was quenched. The quenching timescale depends on which process shut down star formation in the galaxy — feedback from black hole accretion or star formation should cause fast quenching, while galaxies that are starved of gas from the intergalactic medium should quench more slowly. The authors find that 15 of their galaxies quenched in under 500 million years, 6 took between 500 million and 1 billion years, and 3 took longer than 1 billion years to quench. The galaxies that had the highest peak star formation rates quenched the fastest, suggesting that feedback from star formation could have played a role in quenching these galaxies.

plots showing possible evolutionary tracks for two example galaxies

Figure 2: Possible evolutionary tracks in supercolor for two example galaxies as they quench. The galaxy in the top panel quenches quickly and has post-starburst supercolors for about a billion years, while the galaxy in the bottom panel quenches without ever going through the post-starburst phase. The authors use these tracks to determine how important the post-starburst phase is for quenching massive galaxies. [Skarbinski et al. 2026]

Next, the authors measure how important the post-starburst phase is to form massive quiescent galaxies. Not all galaxies that quench go through a post-starburst phase; some objects, especially those that quench slowly, will transition directly from star forming to quiescent. The authors use the star formation histories from their spectral energy distribution fits to predict how the galaxies’ supercolors changed after their star formation peaked (Figure 2) and find that six of the nine quiescent galaxies went through a post-starburst phase in the past, while the other three did not. For the objects that went through a post-starburst phase, the median time spent as a post-starburst was around 600 million years.The authors can use this measured “visibility timescale” to constrain whether the post-starburst phase is important for forming massive quiescent galaxies. If the fraction of post-starburst galaxies in a sample is high, that can be for two reasons: either a larger fraction of galaxies will eventually go through a post-starburst phase, or the post-starburst phase is very long, making it easy to find post-starburst galaxies. Using the measured timescale of 600 million years and combining with results from another article, the authors find that 40% of quiescent galaxies likely went through a post-starburst phase; for the more massive end of the sample, this fraction increases to around 73% due to the shorter visibility timescale. This suggests that the post-starburst phase is very important for forming the kind of massive quiescent galaxies we see in the local universe.

While the post-starburst phase is important, the different quenching timescales present across the sample suggest that multiple pathways existed to quench galaxies at cosmic noon, similar to what has been found in less-distant galaxies and at cosmic noon in other samples. This is also supported by the fact that four of the five galaxies with sufficient data show evidence of an actively accreting supermassive black hole that may help shut down star formation in many (but perhaps not all) massive galaxies. While the precise processes that quench massive galaxies are still uncertain, one thing is clear: JWST EXCELS at solving the mystery!

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.

Messier 17

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, Σgas, to the star formation rate surface density, ΣSFR. (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.

Schematic of the gas cycling framework

Figure 1: Schematic of the gas cycling framework used in the authors’ theoretical model for how gas is converted into stars, which takes into account a supply, removal, and depletion timescale. [Adapted from Kocjan and Semenov 2026]

Building on this picture of gas cycling, the authors turn to simulated galaxies, where the motion and evolution of the gas can be followed directly. By tracking how gas flows through the ISM in the simulations, they derive the three characteristic timescales above — τ+, τ–, and τ* — that describe how gas is supplied to, removed from, and ultimately consumed by star formation. The goal is to connect the small-scale physics of the ISM to the large-scale star formation efficiencies and scaling relations observed across galaxies.

The Timescales of the Interstellar Medium

To explore how gas and star formation are connected across different galactic environments, the authors analyze three simulated systems: a dwarf galaxy, a Milky Way–like galaxy, and a gas-rich starburst galaxy, as shown in Figure 2. Despite spanning very different physical regimes, the galaxies exhibit remarkably similar trends in the fraction of gas actively forming stars as a function of Σgas​. This suggests that the amount of star-forming gas is governed primarily by local interstellar conditions, since regions with higher gas surface densities tend to contain denser, more strongly self-gravitating gas.

galaxy simulations and star-forming gas fraction

Figure 2: Using simulations (left to right) of an isolated intermediate-mass dwarf galaxy, Milky Way–like galaxy, and gas-rich galaxy, the authors measure the star-forming gas fraction versus the gas surface density (right). [Adapted from Kocjan and Semenov 2026]

To further understand this trend, this study then applies the authors’ theoretical framework to determine the timescales of ISM gas cycling on the scales of individual star-forming regions. These include the timescales for the formation, dispersal, and local depletion of star-forming gas as described above: τ+, τ−, and τ∗, which also exhibit strong correlations with the gas surface density (see Figure 3). Specifically, the timescale for supplying gas into the star-forming state is linked to the rate at which turbulence redistributes material through the galactic disk. The depletion timescale, over which star-forming gas is turned into stars, decreases at higher Σgas​, since denser regions more efficiently collapse and form stars. By contrast, the removal timescale is comparatively short, reflecting how quickly feedback and changes in local equilibrium can disrupt star-forming clouds. In this way, the “boiling kettle” framework is able to capture the major processes driving small-scale gas evolution.
Scaling relations between the supply, depletion, and removal times

Figure 3: Scaling relations between the supply (left), depletion (middle), and removal (right) times in kiloparsec-scale regions of the simulated galaxies (shown in different colors). Based on the measurements from the simulations, the authors introduce a parameterized model for the gas cycling framework, as defined in the equations. [Adapted from Kocjan and Semenov 2026]

The cycling of gas through different phases of the ISM offers a useful framework for understanding how galaxies form stars. In the relatively well-ordered systems studied here, this balance can be described in simple terms; however, it is likely to become more complex in extreme environments where additional physical processes — such as those operating in the early stages of galaxy formation — play a significant role. Even so, these results highlight how key galactic properties can emerge naturally from the interplay between galaxy-scale dynamics, ISM turbulence, and the state of star-forming gas.

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.

Mars from Mars Global Surveyor

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.

Plot of planetary radius versus planet mass, with symbols colored by incident stellar flux.

Figure 1: A plot of confirmed exoplanets by mass and radius. Most confirmed exoplanets are larger and more massive than Earth. Mars-like planets (dots in the blue box) are rare in confirmed exoplanet detections. [Kane et al. 2026]

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.

Diagram showing Earth and Mars to scale

Figure 2: Earth and Mars to scale, showing their internal structures and atmospheres. Despite being neighbors, Mars is dramatically smaller with a thin CO2 atmosphere, similar to the planets JWST is struggling to characterize. [Kane et al. 2026]

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.

side-by-side images of Venus's surface today and an imagining of what its surface might have looked like in the past

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).

two plots of final surface temperature as a function of initial surface water mass

Figure 1: Final surface temperature after simulating 4.5 billion years of evolution as a function of initial water inventory for Earth-like planets. Each dot represents a model run with different assumptions (total carbon inventory, temperature dependence of weathering, soil age, etc.). The yellow region shows where most simulations result in uninhabitable surface temperatures from an unbalanced carbon cycle, and the cyan region shows where most simulations result in habitable surface temperatures and a balanced carbon cycle. The left and right plots show the results for two different parameterizations of the deep water cycle implemented in the model. [White-Gianella & Krissansen-Totton 2026]

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.

distant starburst galaxies

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.

UVJ color–color diagram

Figure 1: UVJ colour–colour diagram, which uses galaxy colours in the ultraviolet (U), visible (V), and near-infrared (J) to identify quenched galaxies. The five ultramassive galaxies (UMGs) studied here (filled cyan circles) lie firmly in the quenched region (QG), consistent with a lack of ongoing star formation. Crosses indicate galaxies undetected in ALMA dust emission, while other massive galaxies at similar redshifts are shown in grey for comparison. [Chang et al. 2026]

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 Mdust/Mstar < 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 Mdust/Mstar = 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.

Plot of dust-to-stellar mass ratio versus redshift

Figure 2: The ratio of dust mass to stellar mass versus redshift for massive quenched galaxies (QGs). This quantity measures how dust rich a galaxy is compared to its stellar mass. The ultramassive galaxies in this study (cyan pentagons) fall well below the dust content expected for star-forming galaxies (solid blue line), highlighting their extreme dust deficiency. [Adapted from Chang et al. 2026]

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!

disk of hot gas swirling around a black hole

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).

AT2024tvd from Hubble and JWST

Figure 1: A color image of AT2024tvd from the Hubble Space Telescope and JWST. The yellow dot inside the square marks the galaxy’s center, while the TDE is the white dot, which is visibly offset to the upper left, about 2,600 light-years away. [Adapted from Guolo et al. 2026]

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).

brightness evolution of AT2024tvd

Figure 2: The brightness of AT2024tvd measured across a wide range of wavelengths, after correcting for absorption and the galaxy’s motion. The symbols show individual measurements from different telescopes, while the shaded contours represent the best-fit disk model and its uncertainty. Left: The early X-ray data, which is well explained by emission from the hot inner accretion disk. Right: The later data covering both visible/ultraviolet light and X-rays, all consistently explained by the same disk model. [Adapted from Guolo et al. 2026]

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.

plot of black hole mass versus host galaxy mass

Figure 3: AT2024tvd (yellow star) plotted on the relationship between black hole mass and host galaxy stellar mass. Red squares show nearby galaxies with dynamically measured black hole masses plotted against galaxy bulge stellar mass, while blue squares show the relation using total galaxy stellar mass. Purple points and green diamonds represent nuclear TDE host galaxies with black hole masses inferred from different TDE modeling techniques. Yellow diamonds mark the two previously known off-nuclear TDEs. AT2024tvd stands out as a clear outlier, with a very high black hole mass compared to the upper limit on any surrounding stellar mass. [Guolo et al. 2026]

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.

Illustration of an active galactic nucleus

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.

comparison of black hole mass functions

Figure 1: A comparison of black hole mass functions from various previous works and today’s article. The red hexagons are data points computed using the AGNs observed in this work, while blue circles and yellow squares are measurements computed using AGNs observed in previous articles. The red, green, and blue shaded areas represent the expected black hole mass function from various theoretical models. [Adapted from Fei et al. 2026]

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.

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: Dynamics of Planetary Rings Under Thermal Forces
Authors: Wen-Han Zhou et al.
First Author’s Institution: The University of Tokyo
Status: Published in ApJL

If you ask anyone what their favourite planet is, the answer you’ll most likely hear is Saturn. Why? Why else than the beautiful and intricate ring system surrounding the gas giant. The other gas and ice giants in our solar system — Jupiter, Uranus, and Neptune — have ring systems themselves but none quite as striking as Saturn’s. Would it surprise you to learn that astronomers’ best models have not yet totally explained why Saturn’s rings look the way they do?

In contrast, it may not surprise you to hear that people have been trying to explain the rings for as long as we have seen them with telescopes. We now know that planetary rings are collections of relatively small particles (think micrometre up to metre sized), most likely having once been the material of a larger body that was disrupted either by collisions or tidal forces. This material, due to the gravity of its local planet, is sculpted into a flat disc where more subtle interactions then lead to substructure forming within the disc such as gaps and ringlets. Many of these substructures are explained by well-understood physics — for example gaps being carved by embedded moonlets or resonances sculpting ring edges — though there remain some outstanding problems in our understanding.

The authors of today’s article set their sights on the problematic inner edge of Saturn’s A ring (Figure 1). They describe some mechanisms — namely the collisions of micrometeoroids within the rings — by which a sharp ring edge can be maintained, but there exists a gap in the understanding of how such an edge can form in the first place. All hope is not lost, though, as today’s authors reintroduce a physical process they call the “eclipse–Yarkovsky” effect, which seems to explain these phenomena.

horizontally sliced Saturn's rings

Figure 1: A horizontally sliced image of Saturn’s rings shows the rich substructure and gaps within the various rings. Saturn, which is not shown here, would be to the left of the image. The authors of today’s article are particularly concerned with the bright and sharp inner (left side) edges of the A and B rings. Click to enlarge. [NASA/JPL/Space Science Institute]

Billions of Rocket-Powered Bumper Cars

The idea behind this process revolves entirely around light. When sunlight hits a particle within a planetary ring, non-absorbed sunlight imparts a little “bump” onto a particle, ever so slightly altering its trajectory. At the same time, some of the sunlight is absorbed into the particle, which heats it up; light is soon re-emitted via thermal radiation, which, again, can slightly change the trajectory of these tiny particles. From a point on the surface of each ring particle (see rightmost side of Figure 2), the photons from this thermal radiation are all emitted in random directions; however, across the whole surface of the particle there is a net force! These tiny particles are spinning, and so the side just near “sunset” is hottest and emitting the most thermal photons. In this way, the pressure from the sunlight plus the thermal radiation of each particle imparts a net torque on the ring itself which changes the angular momentum of the ring.

diagram showing sunlight falling on Saturn and the planet casting a shadow on its rings

Figure 2: Starlight is the main source of light onto the particles that make up a planetary ring (right panel) though reflected light from the planet’s surface hits it too. This light “heats up” the ring and forms an asymmetry as the ring is eclipsed by the planet, which produces a net force. [Zhou et al. 2026]

What we described just then is essentially solar radiation pressure plus the Yarkovsky effect. It turns out that the net force from this process typically averages out to zero as the ring particles orbit around the planet. This assumes, though, that the sunlight is constantly shining on the ring. Stunning imagery of Saturn tells us that this isn’t the case, so what happens when we take into account the shadow cast on the ring from the planet? Today’s authors find that the net effect induces a positive change in angular momentum of the ring particles, which they call the eclipse–Yarkovsky effect.

After today’s authors detailed all of the math involved in this process (and there is a lot), they put it into practice to try to help explain Saturn’s curious rings. Including the eclipse–Yarkovsky effect together with other known effects that drive ring evolution allowed them to reproduce the optical depth profile (how thick the ring looks) of Saturn’s A ring better than ever before (notably the sharp inner edge in Figure 3). On top of this, the effect provides another avenue for moonlet formation in the outer edges of ring systems as the positive torque from the effect drives material out toward and away from the Roche limit.

plot of optical thickness versus radius

Figure 3: The authors try to explain the current structure of Saturn’s A ring by initialising it with a Gaussian profile of optical thickness versus radius (black dot-dashed line) and evolving it for 81 million years under different effects. When viscous effects are included (blue dashed line), the ring spreads out, but it’s only when the eclipse–Yarkovsky (EY) effect is included (red line) that the model closely matches the observed data (grey line). [Zhou et al. 2026]

Undeniably useful for Saturn, the eclipse–Yarkovsky effect may also explain some other conundra in the solar system. Mars may have once had rings, which are thought to have eventually clumped together to form the inner moon Phobos. This idea is problematic, though, in that current models suggest there should still be some residual ring system around Mars even after Phobos formed. Enter the eclipse–Yarkovsky effect: being 100 times stronger for Mars than Saturn (due to its more intense incident sunlight), the effect may have driven out and dispersed that residual ring altogether. While the authors are currently looking into this possibility, the reintroduction of the eclipse–Yarkovsky effect has already shown great promise for our most beautiful ringed planetary neighbour.

Original astrobite edited by Wasi Naqvi.

About the author, Ryan White:

I am a first-year PhD student at Macquarie University in Australia, working mainly on binary/multiple systems with massive stars (Wolf–Rayets in particular!). Outside of study, I’m probably drinking coffee, baking, reading, or going for a run. You can also find me procrastinating on Bluesky @astroryan.bsky.social.

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