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.

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.

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.

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: 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.

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 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.

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]

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: Searching Within Galaxies for the Earliest Signs of Quenching With Spatially Resolved Star Formation Histories in UVCANDELS
Authors: Charlotte Olsen et al.
First Author’s Institution: New York City College of Technology
Status: Published in ApJ

How and Why Do Galaxies Stop Forming Stars?

Galaxies serve as important laboratories for many subfields of astrophysics. As such, astrophysicists are interested in the full galactic life cycle, from how galaxies are born to the end of their formation. Whereas young galaxies in the distant universe are actively forming stars, many nearby galaxies are much quieter, with little or no ongoing star formation. Astronomers like to describe these galaxies as “red and dead” because their stars are older and therefore appear redder in color. The process by which galaxies shut down their star formation is known as quenching, and it causes galaxies to become quiescent. Understanding the origin of quiescent galaxies is a fundamental question in galaxy evolution. Studying this process is challenging, however, because star formation is intertwined with many other factors that shape galaxies, including their environments and supermassive black holes.

Star formation occurs on scales that are much smaller and over timescales much shorter than the global properties and overall lifetime of a galaxy. By observing star formation on these smaller scales over time, astronomers can gain a clearer picture of how star formation ceases in a galaxy, and they can determine whether this process is driven from the inside out — starting with the supermassive black hole in the galactic center — or from the outside in, influenced by environmental effects. In today’s article, the authors use the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey with high-resolution ultraviolet coverage (UVCANDELS) to examine eight “golden” galaxies, shown in Figure 1. They then reconstruct the star formation histories within small regions of each galaxy to detect the “earliest signs of quenching.”

Figure 1: Images of the eight galaxies in the “Golden Sample” of galaxies studied in this work (right). These galaxies were chosen because of their clear detection, and they show a range of edge-on to face-on views as well as shapes. The authors divided each galaxy into small, resolved regions in which they measured the star formation (example shown on left). [Adapted from Olsen et al. 2026]

Characterizing the Star Formation Histories

To trace a galaxy’s star formation over cosmic time, today’s authors model its integrated light — which encodes the many episodes of past star formation — using spectral energy distribution (SED) fitting. By comparing a galaxy’s light across many wavelengths to computer models, SED fitting can estimate the ages of its stars and when they formed. This approach allows astronomers to study how a galaxy’s star formation rate evolves as a function of its stellar mass, known as the SFR – M_stellar relation. Note that the article considers the star formation rate and stellar mass in individual regions of each galaxy, so it analyzes these properties as surface densities, Σ_SFR and Σ_Mstellar, instead of the total values.

Using SED fitting, the authors determine the expected star formation rate and stellar mass of the regions from 250 million to 1 billion years before the time of observation. Figure 2 shows the line of best fit for the SFR – M_stellar relation of the regions for each galaxy, before and at the time of observation. The SFR – M_stellar relations are very similar across the sample 1 billion years before the observations; however, by the time of observation, the overall star formation rates have decreased slightly, indicating that the galaxies are beginning to quench. They also exhibit increased variation suggesting that each galaxy is quenching in a different way.

Figure 2: The best-fit SFR – M_stellar relation of the regions for each galaxy, represented by the different colored lines, 1 billion years (1 Gyr) before and at the time of observation (left and right panels, respectively). At 1 billion years before the observation, the galaxies appear to have the same relation, but their variation at the time of observation suggests that the galaxies have begun quenching in different ways. [Olsen et al. 2026]

Figure 3: The specific star formation rate of the regions as a function of their distance from the center for three of the galaxies in the sample. Overall, the star formation rate decreases from 1 billion years before the observation (red) to the time of observation (blue). However, the regions where the star formation decreases most significantly (indicated with the yellow boxes) vary from galaxy to galaxy. [Olsen et al. 2026]

Detecting the Earliest Signs of Quenching

It should be noted that all of the galaxies in this study are still forming stars. However, they are on the verge of having their star formation suppressed, offering valuable insight into how quenching begins and how diverse the process can be. New telescopes are pushing the boundaries of what we can observe. On one hand, JWST can capture galaxies at exceptionally high resolution; on the other, the Vera C. Rubin Observatory and the Nancy Grace Roman Space Telescope will deliver vast datasets containing billions of galaxies. Future investigations leveraging these facilities could reveal more about the physical processes that quench galaxies on small scales, shedding light on the mechanisms that drive galaxy evolution.

Original astrobite edited by Catherine Slaughter.

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 Observing the End of Star Formation in Galaxies 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: Characterizing The Star Cluster Populations in Stephan’s Quintet Using HST and JWST Observations
Authors: P. Aromal et al.
First Author’s Institution: University of Western Ontario
Status: Published in ApJ

On 12 July 2022, the first images taken with JWST were released to the public. All of the astronomers in my department gathered together to watch the images be revealed in real time. It was exciting for everyone, from graduate students getting to see a glimpse into the future possibilities of their fields, to retired professors getting to see the fruits of their decades-long labor in advocating for the telescope to be built.

One image that was showcased was of Stephan’s Quintet (Figure 1), an actively interacting galaxy group. We were all immediately impressed by the clarity of the star-forming regions in the dense gas between the galaxies in the image. Now, more than three years later, the authors of today’s article lay out a comprehensive study of the star clusters in those same regions, taking advantage of JWST’s multi-wavelength imaging capabilities.

Figure 1: The JWST imaging of Stephan’s Quintet with member galaxies labeled (including NGC 7320C, which is outside of the field of view). Gray arrows indicate the direction the associated galaxy is moving. The inlaid image shows the distribution of young star clusters in relation to the six main tidal features in the group. [Inset image: Adapted from Aromal et al. 2025; Background image: NASA, ESA, CSA, STScI]

HST + JWST = OMG!

While JWST’s depth and resolution are exciting for those studying high-redshift galaxies, for local-universe astronomers JWST really shines when used in combination with the Hubble Space Telescope (HST). The star clusters in Stephan’s Quintet were already cataloged in 2015 using HST, but the filters used in the imaging only captured optical and very-near-infrared wavelengths. If you want to get enough data to be able to accurately estimate the ages of these star clusters, especially accounting for reddening caused by surrounding dust, you need to push your imaging further into the infrared.

Unlike HST, JWST can take images in two wavelength filters at once, meaning you can get more data with roughly the same observing time. Here the authors imaged the same star clusters that HST looked at with five new JWST filters, all at longer infrared wavelengths than HST. Together, the authors had flux measurements at 10 different wavelengths across the optical and infrared spectrum for each star cluster, meaning they could perform spectral energy distribution fitting.

Go Ahead, Guess My Age

The best way to estimate an unresolved star cluster’s age is to get its full spectrum of light and fit spectroscopic models with varying ages to it until the model matches the observations. But, if you have individual brightness measurements at many different, spread-out wavelengths, you can still fit spectral models to the measurements and make a best guess despite the gaps in your full spectrum. Here the authors compare their multi-wavelength data to Code Investigating GALaxy Emission (CIGALE) spectral energy distribution models, allowing both the cluster ages and amount of dust extinction to vary.

This is where the long-wavelength JWST imaging is most necessary, because it is sometimes difficult to determine a cluster’s age with this method. For instance, is the light a cluster is giving off mostly red because its stars are very old, or is it because its stars are actually bluer and younger and the cluster just appears red due to foreground dust?

Infrared imaging can “see through” any dust and break this age–extinction degeneracy. For most of the clusters, the authors found that the original HST-only age estimates were accurate, but for 121 clusters in the sample (about 8% of the total), the JWST imaging made a significant change in the age estimates, shifting them to younger ages.

Where Do the Hip, Young Star Clusters Hang Out?

Once the authors had their updated ages for the clusters, they then mapped out where the clusters were located in Stephan’s Quintet and how these clusters traced the known tidal structures in the group. Tidal structures are structures, usually consisting of gas, dust, and stars, that are formed from the tidal forces galaxies exert on one another as they interact and merge. The authors found that throughout all tidal regions of Stephan’s Quintet, there were many young, low-mass star clusters, all about 3–5 million years old. This timescale lines up with previous studies’ estimates of when NGC 7318B was thought to first fall into Stephan’s Quintet, compressing the gas in the group and creating tidal shocks that would trigger star formation.

Additionally, the authors’ star cluster age distribution for the group (see Figure 2) has a second, broader peak of higher-mass clusters with ages around 200 million years. This would correspond to when the most recent encounter between NGC 7320C (which has now passed through the group and is out of frame in Figure 1) and NGC 7319 is estimated to have occurred.

Figure 2: A histogram of the ages of all the star clusters in Stephan’s Quintet. The blue dashed line shows the estimates using only the HST data, and the orange solid line shows the estimates using both HST and JWST together. Notice how a lot of the clusters around 10⁸ years old turned out to be less than 10⁷ years old. [Adapted from Aromal et al. 2025]

Taken together, the updated age measurements of the star clusters in Stephan’s Quintet provide not only a study of star formation history, but also of the galaxy–galaxy interaction history in the system!

Looking Ahead

While the authors have improved the age estimates of the clusters in Stephan’s Quintet, there are still limitations to their analysis. Their estimated star formation rate for the group is much lower than that estimated with other tracers, such as H-alpha emission, meaning this study may still be missing a fraction of very young star clusters. The most likely culprits are embedded clusters that haven’t had enough time to expel the surrounding gas from the massive clouds they formed within.

Future work will attempt to identify these embedded clusters using near-infrared JWST imaging, since in this study the authors focused only on the previously HST-identified clusters. In addition, combining their JWST data with high-resolution radio observations taken with the Atacama Large Millimeter/submillimeter Array will allow them to study the gas in the group more closely and understand how it influences the star formation.

This work highlights JWST’s excellent application to star cluster observations, building on the data we already have from decades of HST use, and it looks like we’re only just getting started!

Original astrobite edited by Skylar Grayson.

About the author, Veronika Dornan:

Veronika is a postdoctoral research associate at the University of Edinburgh. Her research is in observations of globular star clusters and how they can be used to study the evolution of their host galaxies.

The post Astronomers Hate Them! This Star Formation Ingredient Makes Clusters Look 300 Million Years Older 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: Stars Born in the Wind: M82’s Outflow and Halo Star Formation
Authors: Vaishnav V. Rao et al.
First Author’s Institution: University of Michigan
Status: Published in ApJ

Outflows and Star Formation

Some galaxies, known as starburst galaxies, form stars at exceptionally high rates. High star formation rates mean, you guessed it, lots of new stars! The most massive stars live comparatively short lives and can die in a brilliant cosmic explosion known as a supernova. So, when you have a starburst galaxy, you get lots of young stars, a fraction of which produce supernovae. While these stellar explosions occur on relatively small scales, they can collectively drive galactic-scale expulsions of gas and dust known as outflows.

Outflows expel gas laden with metals out of the galaxy, playing a pivotal role in the evolution of galaxies. They can also drive additional star formation in the areas surrounding the galaxy, which is exactly what today’s authors are interested in. Messier 82 (M82), a quintessential local starburst galaxy, is the focus of today’s article (see Figure 1). M82’s proximity makes its spectacular outflows a prime testing ground for studying the impact of outflows on a galaxy and its surroundings.

Figure 1: An image of the local starburst galaxy M82, taken by the Subaru Telescope. The Hubble Space Telescope field encompassing the Southern Arcs is highlighted as a green box. [Rao et al. 2025]

The Southern Arcs of M82

The main focus of today’s article are arc-like groups of stars called the Southern Arcs that are located near M82’s southern outflow. Using photometry from the Hubble Space Telescope, today’s authors derive star formation histories for the Southern Arcs, with the goal of understanding the impact that M82’s outflows have had on star formation in its halo. Figure 1 shows the Southern Arcs region of M82 highlighted in green alongside an image of M82.

Star Formation Histories

If you’ve ever taken an astronomy class, you’re probably familiar with the Hertzsprung–Russell (HR) diagram. HR diagrams are one of the most powerful tools available to astronomers, as they encode a ton of information regarding populations of stars and their formation. In practice, astronomers can construct HR diagrams using resolved stellar populations. That is, if you can resolve individual stars in a galaxy, you can construct a color–magnitude diagram, which is essentially the HR diagram you may be familiar with, but uses observed properties as a proxy for temperature (color) and luminosity (magnitude).

Today’s authors use the color–magnitude diagrams of the Southern Arc to derive star formation histories for the region. Star formation histories describe the star formation rate as a function of time, providing an insight into when and how stellar populations formed. To derive the Southern Arc star formation histories, the authors use the MATCH color–magnitude diagram fitting code, which determines the combination of stellar populations that reproduce the observed color–magnitude diagram, accounting for observational biases along the way. Figure 2 shows the star formation histories obtained using three different stellar evolution models. The authors find that about 85% of the stellar mass in the Southern Arc field formed sometime between 70 and 150 million years ago before star formation slowed down. About 30 million years ago, star formation picked up again, producing the rest of the stellar mass in the Southern Arc field.

Figure 2: The star formation history (left) and cumulative star formation history (right) of the Southern Arc region. Different stellar evolutionary models are marked in different colors. The star formation history shows the star formation rate as a function of time, while the cumulative star formation history shows the buildup of the stellar mass as a fraction of the total mass observed now. [Rao et al. 2025]

So, What’s the Deal with the Southern Arcs?

The authors explore two mechanisms that could explain the star formation histories. In the first scenario, M82’s outflows trigger star formation when impacting the cooler circumgalactic gas. When the outflow shocks collide with the cooler gas, it causes it to collapse and form stars. In the second scenario, star formation is occurring within the outflows themselves. Figure 3 is a schematic of the two proposed mechanisms.

Figure 3: A schematic outlining the two possible mechanisms for the formation of the Southern Arcs. The left panel shows the scenario in which shocks produced by the outflow collide with gas in the circumgalactic medium, triggering star formation. The right panel shows the scenario in which star formation is triggered within the outflow itself. [Rao et al. 2025]

Original astrobite edited by Jessie Thwaites.

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 Outflows, Shocks, and Star Formation in Messier 82 appeared first on AAS Nova.