Today I’ll be telling you about 10 of my favorite autumn deep-sky objects, with suggestions for viewing them. I hope you’ll enjoy this list, and I’ll share some other deep-sky favorites of mine from time to time in the future. What’s a large telescope? The notion of what’s big changes from observer to observer, butContinue reading "Autumn observing targets for large telescopes"

The post Autumn observing targets for large telescopes appeared first on Astronomy Magazine.

In one of the most ambitious mapping efforts in the history of radio astronomy, China’s Five-hundred-meter Aperture Spherical radio Telescope — better known as FAST — has delivered a cosmic atlas of staggering proportions. The FASHI team, headquartered at the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC), has announced the second data release of the FAST All Sky HI Survey, published as a cover story in the journal Science China Physics, Mechanics & Astronomy. Just two years after the survey’s first data release catalogued 41,741 galaxies, the new edition explodes that figure to 156,411 extragalactic sources detected in neutral hydrogen — nearly a fourfold leap that firmly establishes FASHI as the largest census of atomic hydrogen gas in galaxies ever compiled.

The scale of the achievement is difficult to overstate. FASHI DR2 now spans roughly 19,500 square degrees of sky, equivalent to 47.3 percent of the entire celestial sphere, and reaches out to redshifts below z = 0.09 — distances of well over a billion light years. For comparison, the previous benchmark in this field, the ALFALFA survey conducted with the now-collapsed Arecibo telescope in Puerto Rico, charted approximately 31,500 hydrogen-bearing galaxies across about 7,000 square degrees. FASHI DR2 has thus quintupled the galaxy count of its Arecibo predecessor while covering nearly three times the sky area, and it has done so with a sensitivity roughly 0.7 times deeper than ALFALFA’s, measured as a median noise level of 0.57 mJy per beam at a velocity resolution of 6.4 kilometers per second. In radio astronomy, lower numbers mean better sensitivity — and FASHI’s numbers are among the best ever achieved in a blind, wide-area survey.

The engine behind this performance is FAST itself. With its 500-meter dish nestled in the karst hills of Guizhou Province, the telescope remains the most sensitive single-dish radio instrument on Earth at the 21-centimeter wavelength of neutral hydrogen. The survey exploited a 19-beam receiver array, allowing the telescope to sweep the sky in an efficient drift-scan mode that turned routine transits of the sky above the dish into a torrent of data. Crucially, the instrument maintained exceptional stability across five full years of observing, from August 2020 through July 2025. In surveys of this kind, calibration drifts and instrumental fluctuations can quietly corrupt weak extragalactic signals; FAST’s long-term steadiness meant that faint hydrogen emission from distant galactic disks could be stacked, averaged and trusted.

Neutral hydrogen — the primordial fuel of star formation — is invisible to optical telescopes. The 21-centimeter spectral line, produced when the electron in a hydrogen atom flips its spin relative to the proton’s, provides the only direct tracer of the cold atomic gas from which galaxies build their stars. Detecting this feeble signal across cosmological distances requires both enormous collecting area and exquisite receiver stability. A blind HI survey, unlike targeted observations, simply listens to every direction on the sky and catalogs whatever whispers back, making it an unbiased inventory of gas-rich galaxies regardless of their optical brightness, dust content or morphology. That is precisely why such surveys are prized: they catch the dim, gas-dominated dwarfs and bloated hydrogen clouds that optical catalogs systematically miss.

To give the hydrogen detections physical meaning, the FASHI team cross-matched their radio catalog with the two great optical spectroscopic endeavors of the current era: the Dark Energy Spectroscopic Instrument survey and the Sloan Digital Sky Survey. This cross-identification yielded precise optical counterparts for approximately 64,000 sources, anchoring the hydrogen detections to galaxies with measured distances, stellar masses and star-formation rates. From these matches the team constructed a completeness-corrected statistical sample of more than 109,000 galaxies — a sample robust enough to push fundamental measurements into territory previously beyond reach.

The flagship result of the release is a new measurement of the HI mass function, the statistical distribution describing how many galaxies of a given hydrogen mass populate the local universe. Thanks to the sheer sample size and FAST’s depth, the team extended the reliable measurement range of this function down to hydrogen masses of about one million solar masses — roughly 10 to the power 6.2 in solar mass units — a regime where only the faintest dwarf galaxies live. These smallest gas reservoirs are the building blocks of galaxy evolution, the raw material that hierarchically assembles larger systems, and their census has long been fragmentary. Equally striking is the derived cosmic density of neutral hydrogen: Ω_HI equals (4.71 ± 0.03 statistical ± 0.40 systematic) × 10⁻⁴ in units of h₇₀³ Mpc⁻³ dex⁻¹. The statistical uncertainty is a mere 0.6 percent — the most precise determination of the universe’s atomic hydrogen budget ever made at low redshift.

Why does such a number matter? The cosmic HI density tracks how much raw fuel exists for future star formation, and comparing its evolution across cosmic time constrains how galaxies acquire and exhaust their gas. Because atomic hydrogen sits at the interface between the intergalactic medium and the star-forming interstellar medium, its abundance is a sensitive probe of gas accretion onto dark matter halos and of the feedback processes that heat or expel gas from galaxies. Baryonic matter, after all, is hidden in far more places than stars: cold gas, warm gas, plasma in and around halos. A precision measurement of the hydrogen component anchors models of where ordinary matter resides within the cosmic web of dark matter.

The applications of the new catalog ripple far beyond that single number. For galaxy evolution studies, the 156,411 detections provide direct observational constraints on gas accretion, star formation feedback and galaxy transformation — the processes that turn blue, gas-rich spirals into passive red systems. For cosmology, the HI mass function tests galaxy formation models within the prevailing dark matter framework. For large-scale structure work, the survey complements optical galaxy catalogs to trace the so-called HI cosmic web, revealing how diffuse gas threads through filaments and pools in voids — architecture that optical surveys alone can only infer. The dataset will also aid gravitational-wave and transient science: rapid localization of the host galaxies of gravitational-wave events depends on accurate galaxy catalogs, and an inventory of hundreds of thousands of nearby galaxies sharpens those host identifications. Finally, FASHI serves as a pathfinder for the Square Kilometre Array, the giant interferometric observatory under construction in Australia and South Africa, providing benchmark samples against which next-generation interferometric surveys will calibrate their systems and select their targets.

The trajectory of the project tells its own story. When FASHI DR1 appeared, it broke ALFALFA’s long-standing record for the largest blind HI survey. With DR2, the team argues, FASHI has moved from record-breaking to benchmark-setting — a transition that observers of Chinese radio astronomy describe as the shift from follower to leader in 21-centimeter science. The cover of the journal issue reinforces the mood: a warm sunset scene evoking the faint 21-centimeter photons from distant galaxies arriving at FAST after journeys of billions of light years, messengers of the cold gas that permeates the universe.

The survey’s story is also one of institutional perseverance. The work is led by NAOC in collaboration with the Guizhou Radio Astronomical Observatory, the Shanghai Astronomical Observatory, the CAS South America Center for Astronomy, Peking University and other partner institutions, with funding from the National Natural Science Foundation of China, the Chinese Academy of Sciences and the Ministry of Science and Technology of China. Sustained national investment in a single flagship instrument has now produced a dataset whose statistical power no other facility can currently match at these wavelengths.

The full FASHI DR2 source catalog has been released to the public through the FAST data portal, a decision that will likely seed hundreds of follow-up studies in the coming years, from targeted observations of hydrogen-rich dwarfs to statistical analyses of gas in galaxy groups and environmental studies of gas stripping in clusters. As the SKA era approaches, FASHI DR2 stands as both a scientific foundation and a demonstration: that a single dish, used patiently and precisely over five years, can still redraw the map of the gaseous universe. With nearly half the sky covered and a quarter of a million more square degrees potentially within reach, the question is no longer whether FAST will extend the hydrogen frontier, but how far.

Using NASA’s Chandra X-ray Observatory, scientists have discovered a new class of objects behaving unlike any they have seen before. Astronomers suggest these newly spotted objects in other galaxies may help solve not one, but two long-standing questions in astrophysics.

These mysterious objects give off unusually low-energy X-rays but intense levels of ultraviolet radiation. This discovery is featured in a paper published Wednesday in Nature Astronomy.

“We’ve never encountered a group of objects that act like this,” said Mustafa Muhibullah of the University of Alabama who led the study. “Of course, the next step was to try to figure out what these things are.”

The researchers found a total of 84 of these “hypersoft X-ray sources” – so named because they give such low-energy X-rays – in the six different galaxies they searched, using data openly available to the public in the Chandra archive. Two of the galaxies are spirals, M31 (the Andromeda galaxy) and M101 (the Pinwheel galaxy), while the other four are ellipticals. They found hypersoft X-ray sources both in regions of active star formation and areas where there are older stars.

The team spotted the sources by finding objects that appeared in Chandra images taken at the lowest X-ray energies but vanished in higher-energy images. That means these objects give off far more low-energy X-rays than high-energy ones. Because low-energy X-rays border energetic ultraviolet radiation on the electromagnetic spectrum, the researchers determined that these sources are producing large amounts of energetic ultraviolet radiation as well.

It is unclear what types of objects are responsible for these low-energy X-rays and intense ultraviolet radiation. The team thinks they most likely involve a black hole, neutron star, or white dwarf pulling material from a companion star. The material pulled from the companion star is heated up to produce X-rays before falling onto the white dwarf or neutron star, or into the black hole. Such binary systems have been seen before, but not with such bright ultraviolet radiation and low-energy X-rays.

The discovery suggests that there may be large populations of binary systems with energetic ultraviolet radiation that have been undetected until now.

“These clandestine X-ray sources are actually among the most energetic objects in galaxies, and they could be solving two cosmic mysteries at once,” said Muhibullah.

Scientists think that some white dwarf systems pulling material from companion stars may eventually explode as a supernova – known as a Type Ia – that is critical for measuring the expansion of the universe. These supernovae played a key role in discovering that this expansion is accelerating. Astronomers have been looking for the stars that turn into Type Ia supernovae for many years, so far without success.

“If we could find a way to spot these Type Ia supernova explosions before they go off, that would be really important,” said co-author Jimmy Irwin, also of the University of Alabama. “Right now, we study them after they’ve exploded, and astronomers have struggled to understand what is actually ignited.”

The other mystery these hypersoft X-ray sources might explain is what strips electrons from gas between the stars in some galaxies. This stripping of electrons is important to probe because it can affect how quickly stars form and influence the life cycles of galaxies. Hot, massive stars play a role, but they do not completely explain what is causing this stripping. The intense levels of ultraviolet radiation from the hypersoft X-ray sources may play a vital role.

Why were these hypersoft X-ray sources not found until now? In addition to the low-energy X-ray output, which is very difficult for X-ray telescopes to detect, the high-energy ultraviolet radiation is readily absorbed by helium and hydrogen gas that fills the space between the stars, creating a nearly impenetrable barrier to look through.

“By combing through the Chandra archive, we were able to eliminate what used to be a blind spot for telescopes,” said co-author Rosanne Di Stefano of the Center for Astrophysics | Harvard & Smithsonian. “That’s how we found what appears to be a new class of cosmic objects with remarkable qualities.”

NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program. The Smithsonian Astrophysical Observatory’s Chandra X-ray Center controls science operations from Cambridge, Massachusetts, and flight operations from Burlington, Massachusetts.

Read more from NASA’s Chandra X-ray Observatory

To learn more about NASA’s Chandra mission, visit:

https://www.nasa.gov/chandra

Visual Description

This release features a composite image of a spiral galaxy, M101; one of six identified galaxies housing a new class of mysterious objects that give off unusually low-energy X-rays.

In this composite image, M101 faces us directly. It has multiple arms in shades of purple, spiraling clockwise around a golden yellow core. Scattered along and between the arms are scores of tiny specks in white and purple. Most of those specks are pairs of stars, but seven of them are a mystery.

To casual observers, the unusual objects are visually indistinguishable from the other specks of light in the galaxy. An annotated version of the composite image is included in this release, with red circles around the mysterious specks for easy identification.

These mystery specks behave like no other class of object discovered before. The curious objects give off X-rays of such low energy, they in fact produce large amounts of ultraviolet radiation, as UV radiation borders X-rays on the electromagnetic spectrum. Searching images of galaxies with low-energy X-rays in the Chandra Observatory archive, scientists have found a total of 84 such objects spread across M101 and five other galaxies. They have dubbed these mysterious objects “hypersoft X-ray sources.”

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

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

The Windy Galaxy

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

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

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

Earth’s Favorite Carcinogen: Polycyclic Aromatic Hydrocarbons

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

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

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

Figure 1: Makani’s spectrum overlaid on top of the JWST filters. The peaks in the spectrum are PAH features that neatly overlap with JWST’s observing bands. [Veilleux et al. 2025]

PAHs Gone Extragalactic

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

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

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

Original astrobite edited by Chloe Klare.

About the author, Natalie Price:

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

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

Little red dots, an astronomical mystery unveiled by NASA’s James Webb Space Telescope, have perplexed researchers since their discovery—and now, according to new findings, we may be closer than ever to unraveling their mystery.

According to new research that appeared in Nature Astronomy, the enigmatic little red dots could be residents of very small, but also very dense galaxies with masses close to a billion times our Sun’s mass.

If confirmed, the new research may set the pace for unraveling how these cosmic curiosities, first detected in 2023, first came to be in the early stages of the universe’s development. Not only that, it could hint at the possibility that these “little” cosmic curiosities may also be keeping a big secret.

What Are Little Red Dots?

One of the reasons little red dots remained unknown until recent years has to do with their great distances from Earth. With the help of the Webb telescope’s powerful eye, these tiny objects were initially detected three years ago and detailed in a formal scientific paper in 2024.

The nature of these objects remains unknown, and astronomers still aren’t certain about what the exact source of the light they produce might be. In past research, some clues have pointed to the possibility of material around these objects at ultraviolet wavelengths—this would potentially offer a strong hint that these faint, distant objects are galaxies, or are at least related to them in some way.

One of the challenges astronomers have encountered is the lack of optical light measurements indicating the presence of galactic material. Because of this, questions remain about the mass that could be present in stellar bodies associated with the little red dots.

Capturing Optical Emissions from Enigmatic Objects

Now, in new research by Xuheng Ding, Lilan Yang, and colleagues, imagery captured by the James Webb Space Telescope of 217 examples of little red dots has produced new clues about their mysterious origin.

A key component of the new imagery includes the detection of optical emissions, albeit faint ones, observed beyond the primary central objects themselves. The detections are strong enough to be clearly observed, and based on the team’s models, these emission lines are indeed most likely linked to the presence of stellar mass, meaning that the little red dots are good candidates for star-forming galaxies, as many astronomers have suspected.

If correct, this would help confirm a potentially big secret about these objects, since it would also mean the little red dots reside at the centers of these galaxies—as supermassive black holes.

Anatomy of a Little Red Dot

Additional clues about these objects include new estimates for their size, clocking in with a radius of about 210 parsecs—roughly 685 light-years—on average. This means that the little red dots are far more compact than the kinds of star-generating galaxies expected to possess comparable mass, which date to around the same time in our universe’s development that the dots are believed to originate.

Based on the new research, little red dots are more than just unusual light sources in the distant universe, and their likely association with star-forming galaxies offers a significant clue toward revealing their exact nature.

Still, a few questions remain, and the team cautions that their analysis deals with the average properties based on the recent collection of Webb imagery of these cosmic features, since many of the individual characteristics of the little red dots remain too faint to characterize on an individual basis.

Going forward, the team believes additional observations that include spectrographic analysis of the light produced by little red dots will offer more clues about not only their origins, but will also help astronomers determine clearer estimates for their distances.

The recent study, “Extended components of little red dots in the rest-frame optical,” appeared in Nature on August 24, 2026.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org. Follow him on X @MicahHanks, and at micahhanks.com.

In the 1970s, astronomer Vera Rubin analyzed a series of observations of how quickly different galaxies rotated around their centers. She found that these galaxies rotated much faster than they should have, given the amount of material visible through starlight. She concluded that a large fraction of these galaxies’ masses must therefore consist of some kind of unseen material.

Scientists have since observed galaxies to more precisely measure this unseen material, which they now call dark matter. They’ve shown that dark matter moves much more slowly than the speed of light. Unlike most celestial objects, it can’t emit or block any light, meaning it doesn’t interact with the electromagnetic force. Even though it doesn’t interact with a fundamental force in the universe, dark matter constitutes over 80% of the mass of an average galaxy.

In the prevailing model of cosmology, known as ΛCDM (pronounced “Lambda-CDM”), dark matter determined where galaxies formed. The theory is that gravity pulled dark matter together more quickly than regular matter in the primordial universe. This redistribution led to regions of space with greater mass density, which collected chaotic, regular matter into pools that eventually formed galaxies, stars, and planets. 

In the present universe, dark matter and regular matter coexist in galaxies, though generally separated. Regular matter pools in the very center of galaxies, forming the more familiar galactic disks. Dark matter stays towards the edges of galaxies, enveloping the regular matter disks in what are known as dark matter halos. Because these halos make up so much of the mass of the average galaxy, researchers Guangze Sun, Fangzhou Jiang, and Jing Wang conducted a series of simulations to test how dark matter halos affect regular matter in galaxies. Specifically, they examined what characteristics of dark matter halos determine galaxy sizes. 

The force of gravity works such that more massive dark matter halos draw in more regular matter and produce larger galaxies. So these researchers fixed the total mass of the galaxies they simulated at 100 billion times that of the Sun to determine which properties of the dark matter halos would produce the widest galaxies. They selected 4 variables to test: the total density of dark matter in the galactic halo, the density of dark matter in the innermost 1% of the halo, how fast the halo spun, and the ratio of regular matter to dark matter in the entire galaxy. For each variable, they tested 3 to 4 inputs representing low, moderate, and high values against a standard reference value and ran 132 total simulations to generate enough data for statistical analyses.

To simulate the complex physics of dark matter halos, the team used the modeling software GIZMO to model gravity, magnetism, and fluid dynamics. They combined this with a second model, called FIRE-3, which modeled how stars explode at the end of their lives, how much metal these explosions produce, and how gases heat up and cool down over time. The researchers ran the dynamic simulations for 3 billion model years, then used a program they developed based on the existing Pynbody to measure the radii of the simulated galaxies. 

The team found that the faster a dark matter halo spun around a simulated galaxy, the larger the galaxy became, as shown in scenario 1 in the figure below. By contrast, in scenario 2, increasing the halo’s total dark matter concentration caused the central galaxy to shrink. Increasing only the innermost dark matter density in the halo had a smaller effect but generally reduced the galaxy’s size, as shown in scenario 3. And lastly, increasing the ratio of regular matter to dark matter in galaxies resulted in smaller galaxies that formed stars closer to their centers, as shown by a greater concentration of stars in scenario 4. 

This diagram showcases the team’s results across 4 broad scenarios. The left column depicts a default galaxy under the baseline parameters. The right column depicts how the galaxies changed when the team varied a single experimental factor, including 1) increasing how fast the dark matter halo spun, 2) increasing the total density of the dark matter halo, 3) increasing the density of dark matter in the innermost halo, and 4) increasing the ratio of regular matter to dark matter in the entire galaxy. The dark matter haloes are represented as red rings surrounding the galaxies, not to scale. Illustration by Andrew Bizal.

The team’s results suggest a previously unknown connection between the properties of dark matter halos and galactic disks. However, they cautioned that their simulations were all run on galaxies in isolation, unlike in the actual ΛCDM universe, where their interactions are complicated by galaxies tugging on one another or by material moving from one to another. To further test their results for single galaxies, the researchers plan to run follow-up experiments with halos of different sizes, as well as with galaxies with different shapes and features, such as clumps, gaps, and lopsidedness.

The post Dark matter influences the size of galaxies appeared first on Sciworthy.

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

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

Universe Breakers

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

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

The Impact of Color Gradients

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

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

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 (CC BY 4.0) 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.

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. 2026]

Is the Universe Broken?

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

Original astrobite edited by Ansh Gupta.

About the author, Margaret Verrico:

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

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

Exactly how the universe became reionized after cooling to form stars and galaxies remains an open question. A recent study investigates how a merging set of galaxies may have spurred an onslaught of ionizing photons into the intergalactic medium.

Lyman Continuum Leakers and Merging Galaxies

After its chaotic and blistering beginning, the universe cooled and began to coalesce, stars and galaxies flickering on to set the universe alight. From a few hundred million to about a billion years after the Big Bang, galaxies pumped high-energy ionizing Lyman continuum (LyC) photons into the intergalactic medium during what is known as the epoch of reionization. The exact mechanisms that produced and allowed LyC emission to escape galaxies during this era remain elusive.

Hubble Space Telescope image of the interacting galaxy group Arp 194. The blue stream connecting the galaxies is where tidally stripped gas formed millions of new stars. [NASA, ESA, and the Hubble Heritage Team (STScI/AURA)]

While observational studies have increasingly pointed to galaxy mergers as considerable drivers of LyC escape, only a small number have explicitly considered the role of mergers in the epoch of reionization due to spectral and spatial resolution limitations. Leveraging the high-resolution capabilities of JWST, Shengzhe Wang (University of Chinese Academy of Sciences; National Astronomical Observatories, Chinese Academy of Sciences) and collaborators performed detailed analyses of two LyC-leaker candidates to better understand the role of galaxy mergers in heating up the universe.

Confirming (and Refuting) Merger-Driven Leaks with JWST

Using candidate LyC leakers first identified in the LymAn Continuum Escape Survey (LACES) carried out with the Hubble Space Telescope, Wang and team obtained high-resolution JWST spectroscopy for two galaxies at redshift z = 3.1 (about 2 billion years after the Big Bang), LACES94460 and LACES104037. After carefully identifying and measuring emission lines in each candidate, the authors confirmed LACES104037 as a merger-driven LyC leaker. However, the apparent LyC emission from LACES94460 was determined to be contamination from a low-redshift object.

Hubble and JWST observations of LACES104037. The top panels show the Hubble images (first two panels) and the JWST spectral observations (last three panels). The bottom panel shows a composite three-color image marking the location of two primary interacting galaxies and the LyC emission along with the associated spectra. Click to enlarge. [Wang et al 2026]

LACES104037-LyC represents the first confirmed LyC leaker of its kind: an external clump of star formation in a tidal tail. This discovery aids in our understanding of how mergers can drive high-energy photon emission, and highlights the critical role galaxy mergers may play in the epoch of reionization. Future high-resolution observations will hopefully reveal more knots of LyC-leaking stars within merging galaxies and provide a deeper understanding of this important evolutionary phase of the universe.

Citation

“Confirmation and Refutation of Lyman Continuum Leakers at z ~ 3 with JWST NIRSpec IFU,” Shengzhe Wang et al 2026 ApJL 1006 L17. doi:10.3847/2041-8213/ae8523

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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.

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.

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.

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.

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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.

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Star formation in the nearby Andromeda Galaxy (M31) has been slowing down over the last 500 million years, according to a new study using Hubble Space Telescope data.  The study was published July 27, 2026, in The Astrophysical Journal. Lead author Tobin Wainer, an astronomer at the University of Washington, and his team used dataContinue reading "Andromeda’s star formation is slowing down"

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