Baryons at the Edge: SRG/eROSITA Survey Detects “Missing” Cosmic Gas at the Outskirts of Galaxy Clusters

X-rays from the large-scale environment around a galaxy cluster in the IllustrisTNG simulation show how the cluster’s outer atmosphere, beyond r200m, connects to other halos through cosmic filaments. © Xiaoyuan Zhang / MPE, based on the IllustrisTNG simulations

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A team of astronomers at the Max Planck Institute for Extraterrestrial Physics (MPE), has detected hot gas extending beyond galaxy clusters using data from the eROSITA All-Sky Survey. This finding reveals the distribution of hot gas in the outskirts, indicating that galaxy clusters are actively accreting material from the cosmic web. This study shows that these regions host the baryonic matter that is missing from the galaxy cluster center, enhancing our understanding of galaxy cluster growth and the surrounding intergalactic environment.

Using data from the SRG/eROSITA All-Sky Survey, the team of international researchers have now achieved a key advance in tracing ordinary matter in the Universe. They detected hot, shock-heated gas extending far beyond the previously studied boundaries of galaxy clusters, offering a new perspective on how these vast cosmic structures grow by drawing in material from the surrounding intergalactic medium.

The study, led by scientists at the Max Planck Institute for Extraterrestrial Physics (MPE), focuses on the outermost regions of galaxy clusters – areas that have been particularly difficult to observe until now. The results reveal how hot gas is distributed in and around these distant outskirts, offering insights into the environments surrounding some of the most massive structures in the Universe.

Bridging the Gap Between Clusters and the Cosmos

Galaxy clusters are among the largest gravitationally bound systems in the Universe, containing hundreds to thousands of galaxies embedded within vast halos of dark matter and filled with hot, diffuse plasma. Yet, the transition between a cluster and the surrounding cosmic web – the network of gas filaments connecting large-scale structures – has long remained uncertain.

Over the past five decades, X-ray space telescopes have shown that galaxy clusters host hot thermal atmospheres with temperatures of tens of millions of degrees and spatial extents of several million light-years. However, the true size of these atmospheres has been unclear because their X-ray brightness drops sharply at large distances from the cluster center.

By “stacking” X-ray data from 680 galaxy clusters, the team amplified the faint glow of gas in these remote regions. They detected a statistically significant X-ray signal extending out to 4.5 megaparsecs (about 14 million light-years) – well beyond the virial radius, which is generally considered the cluster’s edge.

“The survey’s observation depth for a single object is shallow, but it covers the entire western Galactic hemisphere. By selecting 680 galaxy clusters in the nearby Universe, we obtained an extremely high signal-to-noise surface brightness profile through stacking,” explains lead author Xiaoyuan Zhang, postdoctoral researcher at MPE.

Significant stacked X-ray emission

Animation showing the improvement in the signal-to-noise ratio as more galaxy clusters are added to the stacking. Both the noise level in the stacked image (left) and the surface brightness profile uncertainty (right) decrease with increasing stacking sample size.

“Historically, observations have focused mainly on cluster centers because signals from the outskirts are weak. It is extremely exciting that we can now probe the very edges of clusters – regions that can tell us much about the fundamental physics of gas and dark matter,” adds co-author Benedikt Diemer, Assistant Professor at the University of Maryland.

Using the IllustrisTNG cosmological simulations, developed by researchers at the Max Planck Institute for Astrophysics, the team showed that gas around galaxy clusters is not distributed evenly. It is much denser along cosmic filaments – the large-scale structures connecting matter across the Universe – than in the low-density voids between them. This indicates that galaxy clusters are actively accreting material from the cosmic web through these filamentary channels.

MPE research group leader Esra Bulbul, second author of the study, adds: “Astronomers have long searched for the Universe’s ‘missing baryons’ – the normal matter that should exist but has been difficult to detect. Our results show that, in the far outskirts of galaxy clusters, the amount of gas reaches about 90 percent of what we expect based on the Universe’s average matter density. This suggests that much of the ‘missing’ matter is indeed present, hidden in these vast, hot, and turbulent outer regions. This helps us understand not only how clusters grow but also the physics of the gas that fills the cosmos.”

This study highlights that, in addition to its strong source-detection capabilities, the eROSITA All-Sky Survey also enables the exploration of extremely faint emission – down to below one percent of the sky background – through stacking techniques.

Source: Max Planck Institute for Extraterrestrial Physics/News

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eROSITA

The eROSITA instrument (extended ROentgen Survey with an Imaging Telescope Array) is the primary telescope aboard the Spektr RG (SRG) mission. It was designed to perform the most sensitive all-sky X-ray survey to date, mapping millions of active galactic nuclei and galaxy clusters to study the evolution of the large-scale structure of the Universe and the nature of dark energy.

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Contacts:

Dr. Xiaoyuan Zhang
Postdoc Highenergy Group
Tel.: +49 89 30000-3807
Email: xzhang@mpe.mpg.de
Max Planck Institute for Extraterrestrial Physics

Dr. Esra Bulbul
Head of galaxy clusters group
Tel: +49 89 30000-3502
Email: ebulbul@...
Max Planck Institute for Extraterrestrial Physics

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Original Publication

X. Zhang, E. Bulbul, B. Diemer, Y. E. Bahar, J. Comparat, V. Ghirardini, A. Liu, ,N. Malavasi, T. Mistele, M. Ramos-Ceja, J. S. Sanders, Y. Zhang, E. Artis, Z. Ding, L. Fiorino, M. Kluge, A. Merloni, K. Nandra, and S. Zelmer

The SRG/eROSITA All-Sky Survey Detection of shock-heated gas beyond the halo boundary into the accretion region.
A&A

Source | DOI

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Further Information

eROSITA website of the MPE
ERC Project DarkQuest
Webpages of the ERC funded project led by Esra Bulbul

eROSITA relaxes cosmological tension

February 14, 2024

Results from the first X-ray sky survey resolve the previous inconsistency between competing measurements of the structure of the Universe

Unveiling the 'Ghost' Baryonic Matter

November 19, 2024

A team of scientists from the Max Planck Institute for Extraterrestrial Physics (MPE) has shed light on one of the most elusive components of the universe: the warm-hot intergalactic medium (WHIM).

The X-ray sky opens to the world

January 31, 2024

First eROSITA sky-survey data release makes public the largest ever catalogue of high-energy cosmic sources

June 19, 2020

Cosmic dance of the ‘Space Clover’

April 30, 2024

A group led by MPE has, for the first time, detected X-ray gas at the location of the cloverleaf ORC, an odd radio circle (ORC). The origin of ORCs is unknown; in the case of the cloverleaf ORC, the combined data from different wavelengths indicate that the emission is due to a merger of two small galaxy groups.

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Hubble Survey Sets Up Roman’s Future Look Near Milky Way’s Center

About this image: This near-infrared image from the ground-based VISTA VVV Survey shows the galactic bulge near Sagittarius A* (pronounced “A star”), the black hole at the Milky Way’s center. The region, outlined in white, shows five stacked fields of view from NASA’s Nancy Grace Roman Space Telescope that will be observed as part of its Galactic Bulge Time-Domain Survey, one of its three core community surveys. (Roman will also observe a sixth field at the galactic center that is not shown here.) Prior to Roman’s launch, a team of researchers sought to use Hubble to capture the same regions in preparation for potential microlensing events.

These events cause the light from a more distant object to warp as a mass precisely aligns in front of that object. These masses, therefore, act like lenses, bending the light from objects behind them like background stars. In this case, the glow from the densely packed stars within the galactic bulge would be the distant light source. Having these Hubble observations allows us to capture the moments before these microlensing events happen, providing astronomers a way to clearly characterize objects (stars, planets, and even stellar-mass black holes) that cause microlensing by passing in front of stars within the galactic bulge.

The colored lines representing the Hubble survey area are stylized and represent a large number of individual pointings.

Credits Image: NASA, Alyssa Pagan (STScI) - Acknowledgment: VISTA, Dante Minniti (UNAB), Ignacio Toledo (ALMA), Martin Kornmesser (ESO) //imag

Microlensing Event at OGLE-2013-BLG-0341 (Hubble Image)

A follow-up observation by NASA’s Hubble Space Telescope shows a field containing a microlensing event that was captured by the Optical Gravitational Lensing Experiment (OGLE) in 2013. This provides an example of how a Hubble image could be used to analyze future microlensing events spotted by NASA’s Nancy Grace Roman Space Telescope.

In gravitational microlensing, the gravity of a foreground object acts as a lens, magnifying and distorting the light of a background star when the two objects align in the sky. Credits Image: NASA, ESA, Sean Terry (UMD), Jay Anderson (STScI) - Image Processing: Alyssa Pagan (STScI)

Microlensing Infographic

This graphic illustrates a microlensing event, which occurs when the light from a distant object warps as a mass, such as a star (depicted here) or a stellar-mass black hole, precisely aligns in front of that object. In this image, a red, foreground star intervenes between the telescope, acting as the “lens,” bending, and magnifying the light of the yellow background star. Unlike some gravitational lensing events, which occur at the scale of galaxies or galaxy clusters, microlensing events occur on a much smaller scale, such as that of individual stars. The lensing effect is, therefore, much smaller.

This image also provides a representation of what the background star would look like to a telescope in a microlensing event. Because of the curvature of space around the background star (represented by the white arrows that curve around it in the image), the background star appears to increase in brightness as the event begins before decreasing in apparent brightness as it falls out of alignment. The graph at bottom plots the apparent brightness of the background star over time. Credits Illustration: NASA, STScI, Joyce Kang (STScI)

Zoom Into the Milky Way's Galactic Bulge - Hubble/Roman Survey Regions

This video shows a zoom into the Milky Way’s galactic bulge near the galactic center. As it zooms in, the view changes from the near-infrared 2MASS survey to the VISTA VVV survey (both ground-based). At the conclusion of the zoom, part of the region of the galactic bulge that will be surveyed by Roman’s Galactic Bulge Time-Domain Survey is highlighted with five stacked fields of view. (Roman will also observe a sixth field at the galactic center that is not shown here.)

Prior to Roman’s launch, a team of researchers are using NASA’s Hubble Space Telescope to observe the same regions to enable better analysis of microlensing events detected by Roman. The colored lines representing the Hubble survey area are stylized and represent a large number of individual pointings. The video also labels Sagittarius A* (pronounced “A star”), the black hole at the Milky Way’s center. Credits Video: NASA, Alyssa Pagan (STScI) - Acknowledgment: VISTA, Caltech, Caltech/IPAC, Sean Terry (UMD), Jay Anderson (STScI), Dante Minniti (UNAB), Ignacio Toledo (ALMA), Martin Kornmesser (ESO), 2MASS

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The Milky Way’s galactic bulge, the bulbous region that surrounds the galactic center, contains a dense collection of stars, planets, and other free-floating objects. This region has been studied for decades with numerous ground-based and space-based telescopes, including NASA’s Hubble and James Webb space telescopes. Soon, NASA’s Nancy Grace Roman Space Telescope will be the first to make studying the galactic bulge a part of its core science objectives, building on the data collected from all observatories before it. Roman’s field of view will cover more area at a far faster cadence than previous space telescopes, allowing it to survey millions of stars and find thousands of new exoplanets.

To support Roman in characterizing numerous stars and planets, astronomers sought to use Hubble to observe many of the same areas of the galactic bulge that Roman will observe in its core Galactic Bulge Time-Domain Survey. By comparing Hubble data taken months or years earlier to new Roman data, astronomers will be better able to interpret Roman’s forthcoming observations. The Roman telescope team is targeting as soon as early September 2026 for launch.

“A top priority of our Hubble survey is to cover as much sky area as possible,” said Sean Terry, project lead and assistant research scientist from the University of Maryland, College Park and NASA’s Goddard Space Flight Center in Greenbelt.

A paper about the team’s work published May 11, 2026 in the Astrophysical Journal.

‘Small’ lenses, large discoveries

Many planetary systems within the Milky Way evolve much like our solar system did, beginning with the collapse of a cosmic gas cloud, the growth of a star, and the formation of surrounding planets. However, in some systems, different events can result in a planet being ejected from the system where it formed. Hundreds of these “rogue planets” will be detected by Roman’s Galactic Bulge Time-Domain Survey, in addition to previously unseen, isolated neutron stars, and even black holes with masses similar to our Sun.

This survey consists of six 72-day observing seasons during which Roman will take a snapshot every 12 minutes of a large portion of the bulge (approximately 1.7 square degrees of the region, or the area of 8.5 full moons). While it will detect a variety of targets, the survey is optimized to look for a specific type of event known as microlensing.

Microlensing events, a type of gravitational lensing event, occur when the light from a more distant object is warped by the mass of a closer object along the line of sight. These events occur on a much smaller scale than larger lensing events (on the order of individual stars instead of galaxies or galaxy clusters) and allow us to search for exoplanets between us and the densely packed stars within the galactic bulge.

“The great thing about microlensing is that we’ll be able to do a complete census of objects as small as Mars that are moving between us and these fields in the bulge, no matter what it is,” said co-author Jay Anderson of the Space Telescope Science Institute in Baltimore.

For Roman, from Hubble

When a telescope observes a lensing object, such as a bright star, aligning with a star in the galactic bulge, it can be difficult for astronomers to decipher which of the two the starlight comes from. Therefore, timing is a key consideration. If astronomers can identify light sources separately before a microlensing event occurs, it becomes far easier to disentangle them.

To collect this pre-Roman data, astronomers used the Hubble Space Telescope to conduct a large-scale survey, which began in the spring of 2025, covering much of the same area that Roman will observe in the Galactic Bulge Time-Domain Survey. The size of this program is even larger than two previous surveys (each around 0.5 square degrees) that led to Hubble’s largest mosaic, that of our neighboring Andromeda galaxy, which took over 10 years to assemble.

“The main goal of these observations is to be able to identify objects that participate in lensing events during the Roman survey, catching them before they undergo the lensing event,” said Anderson. “When, in a couple of years, an event happens during Roman's long stare at the field, we can go back and say, ‘This was a red star, this was a blue star, and the event happened when the red star went in front of the blue star.’”

The data from Hubble also will help shape the analysis of the lensing objects themselves. The microlensing event itself measures only a ratio of the masses of a host star and its planet. With data from stars before or after their microlensing events, however, scientists would be able to measure the stars’ individual masses, echoing the way Hubble previously determined the mass of a star and its planet in the Milky Way. This method turns a more opaque measurement of the relationship between a star and its planet into one far more certain.

“Instead of estimating a mass ratio of a planet that's orbiting a star, we can say that we're confident it's a Saturn-mass planet orbiting a star that's 0.8 solar masses, for example,” Terry said. “So with the help of precursor imaging from Hubble you can hope to get direct measurements of the masses as opposed to indirect mass ratios.”

Next leap in magnitude

While exoplanet discovery is a large part of Roman’s Galactic Bulge Time-Domain Survey, observing such a large area with Hubble also can help identify areas of extinction, dense pockets of dust and gas that absorb or scatter light, allowing us to create maps detailing where we can see stars and where we can’t.

Hubble’s survey also has provided the crucial beginning of a brand-new catalog of stars, which will help astronomers characterize the host stars of exoplanets discovered by Roman. The research team predicts Roman will add to Hubble’s star catalog by an order of magnitude.

“This Hubble survey will build a catalog of 20 to 30 million point sources,” said Terry. “But, by the end of the Galactic Bulge Time-Domain Survey, Roman may measure about 200 to 300 million, and it will produce, essentially, some of the deepest images ever taken of any part of the sky.”

The data from the most recent Hubble survey is available in the Mikulski Archive for Space Telescopes.

The Hubble Space Telescope has been operating for over three decades and continues to make ground-breaking discoveries that shape our fundamental understanding of the universe. Hubble is a project of international cooperation between NASA and ESA (European Space Agency). NASA Goddard manages the telescope and mission operations. Lockheed Martin Space, based in Denver, also supports mission operations at Goddard. The Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy, conducts Hubble science operations for NASA.

The Nancy Grace Roman Space Telescope is managed at NASA Goddard with participation by NASA's Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute; and a science team comprising scientists from various research institutions. The primary industrial partners are BAE Systems, Inc. in Boulder, Colorado; L3Harris Technologies in Melbourne, Florida; and Teledyne Scientific & Imaging in Thousand Oaks, California.

Source: Space Telescope Science Institute (STScI)/News

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About This Release

Credits

Media Contact:

Matthew Brown
Space Telescope Science Institute, Baltimore

Christine Pulliam
Space Telescope Science Institute, Baltimore

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Related Links and Documents

• The science paper by S. Terry et al.
  

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NASA's Roman Poised to Transform Hunt for Elusive Neutron Stars

Isolated Neutron Star (Artist's Concept)

This artist’s concept shows an isolated neutron star as an ultra-dense stellar remnant, packing more mass than the Sun into a city-sized sphere and radiating energy as it slowly cools in the depths of space.

NASA’s upcoming Nancy Grace Roman Space Telescope will search for, and could measure the mass of, isolated neutron stars using astrometric microlensing. Credits Illustration: NASA, STScI, Ralf Crawford (STScI)

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Astronomers have long known that neutron stars, the crushed cores left behind after massive stars explode, should be scattered throughout the Milky Way galaxy. However, most of them are effectively invisible. A new study published in Astronomy and Astrophysics suggests NASA’s upcoming Nancy Grace Roman Space Telescope could spot them anyway.

Using detailed simulations of the Milky Way and Roman’s future observations, researchers showed the flagship observatory may be able to identify and characterize dozens of isolated neutron stars through a subtle effect called gravitational microlensing.

“Most neutron stars are relatively dim and on their own,” said Zofia Kaczmarek of Heidelberg University in Germany, who led the study. “They are incredibly hard to spot without some sort of help.”

Finding what’s invisible

Neutron stars pack more mass than the Sun into a sphere about the size of a city. Studying them helps us understand how stars live, die, and spread heavy elements throughout the universe. They also provide a chance to study what happens under the most extreme conditions (pressures and densities) imaginable.

Yet, unless they are pulsars that beam in radio wavelengths or glow in X-rays, they can remain hidden from even the most powerful telescopes.

Roman can search for them in a different way. When a massive object like a neutron star moves in front of a distant background star, its intense gravity warps spacetime and deflects the background star’s light. This microlensing effect briefly makes the background star brighter and appear offset from its true position in the sky.

While many telescopes can detect the temporary brightening, Roman can measure both the brightening (photometry) and the tiny positional shift (astrometry) of the lensed star with exceptional precision.

Because neutron stars are relatively massive, they produce a larger astrometric signal than lighter objects, allowing missions like Roman to not only detect them, but also weigh them in some cases, something that is nearly impossible with photometry alone.

“What’s really cool about using microlensing is that you can get direct mass measurements,” said paper co-author Peter McGill of Lawrence Livermore National Laboratory. “Photometry tells us that something passed in front of the star, but it’s the amount the star’s position shifts that tells us how massive that object is. By measuring that tiny deflection on the sky, we can directly weigh something that is otherwise unseen.”

Roman’s measurements could help astronomers determine whether there is a true gap between the masses of neutron stars and black holes and how fast neutron stars are moving.

Scientists are particularly interested in understanding the powerful “kicks” neutron stars receive when they are born in supernova explosions. These kicks can send them racing through the galaxy at hundreds of miles per second.

Huge surveys, high chance of payoff

The research team will utilize Roman’s future Galactic Bulge Time Domain Survey, which will monitor millions of stars at a time in vast images of the sky, taken at a high frequency.

“We’re going to get to work as soon as the data start coming in,” said McGill. “Even in the first months after commissioning, we expect to start identifying promising events.”

Even a relatively small number of confirmed detections could significantly improve models of stellar explosions and extreme matter.

“We don’t know the mass distribution of neutron stars, black holes, or where one ends and the other begins with any certainty,” McGill said. “Roman will really be a breakthrough in that.”

Although only a few thousand neutron stars have been detected so far, mostly as pulsars, scientists estimate there could be tens of millions to hundreds of millions in the Milky Way. Additionally, to date, researchers have only been able to measure the masses of neutron stars in binary pairings.

“We’re seeing a small sample that’s not representative of the big picture,” Kaczmarek said. “Even a single mass measurement would be very powerful. If we found just one isolated neutron star, it would already be incredibly stimulating to our research.”

Looking ahead

The study also highlights a creative use of the mission’s capabilities. While Roman’s survey is designed primarily to find exoplanets using photometric microlensing, its powerful astrometric capabilities open the door to entirely new discoveries with astrometric microlensing.

“This wasn’t part of the original plan,” said McGill. “But it turns out Roman’s astrometric capability is really good at detecting neutron stars and black holes, so we can add a whole new kind of science to Roman’s surveys.”

If the predictions hold true, the mission could provide the first large sample of isolated neutron stars discovered through their gravity alone, revealing a hidden population that has remained out of reach until now. Roman is expected to transform the study of microlensing and the hidden populations of objects in our galaxy, from rogue exoplanets to stellar remnants like neutron stars.

The Nancy Grace Roman Space Telescope is managed at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, with participation by NASA’s Jet Propulsion Laboratory in Southern California; Caltech/IPAC in Pasadena, California; the Space Telescope Science Institute in Baltimore; and a science team comprising scientists from various research institutions. The primary industrial partners are BAE Systems Inc. in Boulder, Colorado; L3Harris Technologies in Rochester, New York; and Teledyne Scientific & Imaging in Thousand Oaks, California.

Source: Space Telescope Science Institute (STScI)/News

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About This Release

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Media Contact:

Hannah Braun
Space Telescope Science Institute, Baltimore

Christine Pulliam
Space Telescope Science Institute, Baltimore

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Is the Large Magellanic Cloud a First-Time Visitor?

Artist's image of the LMC and Milky Way and their associated coronas.
Credit: NASA, ESA, Leah Hustak (STScI)

Our most massive satellite galaxy, the Large Magellanic Cloud (LMC), has been the center of a heated debate in the astrophysics community over the last few years. That debate centers on whether this is the LMC’s first or second “pass” by the Milky Way itself - and it has huge implications for the evolution of our galaxy given the disruption such a large grouping of stars has. A new paper from Scott Lucchini, Jiwon Jesse Han, Sapna Mishra, and Andrew J. Fox and his co-authors, currently available in pre-print on arXiv, provides what they claim to be definitive evidence that this is, in fact, the first time LMC has encountered the Milky Way. To understand the debate, it’s best to look at its history. For decades, there was an ongoing debate about the orbital path of the LMC. The discussion centered around a collisionless N-body dynamics model that tracked stars and their gravity. But back in 2024, physicist Eugene Vasiliev released a stunning paper that presented an argument that the LMC might have first passed the Milky Way 6-8 billion years ago at a distance of roughly 100 kiloparsecs.

Upon release of that paper, the debate was reignited. Vasiliev posited that, if the Milky Way’s dark energy halo was anisotropic (meaning the velocities of dark matter particles are skewed in certain directions), the current speed and position of the LMC would align perfectly with a “second pass” orbit. Dr. Lucchini and his co-authors are firmly on the other side of that argument.

Large Magellanic Cloud (LMC)
Anton Petrov discusses the Large Magellanic cloud and what it means for the future of our own galaxy.
Credit - Anton Petrov YouTube Channel

They released two papers directly tackling the idea. First was paper tracing trajectories of “hypervelocity stars” that had been previously ejected by the LMC’s central black hole. They found that the stellar dynamics of these fast-moving stellar objects aligned with both a first pass and second pass model. In other words, it did nothing to settle the argument.

So they began looking for a second, more definitive option. That option presented itself through an unexpected avenue - hydrodynamics. Using a software simulation package known as GIZMO, they combined rigid, analytical dark matter models of both the LMC and Milky way with “live” gas particles representing the mediums surrounding the two galaxies. Once they ran the simulations, they used another software package called Trident to generate mocked up data that would be expected in the ultraviolet spectroscopic observations of the simulated gas.

After they had their simulated data, they began to compare it to observational data - specifically Carbon IV and Hydrogen II absorption data from background quasars, located past the LMC itself. The results were conclusive - the simulation beautifully reproduced the observed velocity and column density profiles of the modern LMC. Just as conclusively, the model of a second-pass does not fit as well. Specifically, the LMC’s time spent “swimming” through the Milky Way’s gas in this scenario results in a much smaller “corona” - the massive halo of warm, ionizing gas surrounding the galaxy.

Video describing how the LMC could survive a collision with the Milky Way’s halo.
Credit - European Space Agency YouTube Channel

While those results seem very cut and dry, there are a few simplifications the authors took in the interest of saving computing capacity. The Small Magellanic Cloud (SMC) was completely excluded from the simulation, and it actually contributes a majority of the neutral gas in the Magellanic Streams that both galaxies trail. Ignoring this could significantly alter the gas profile, the authors note. Also, the simulations massively simplified the Corona itself, using a warm-hot, single-phase model instead of the massively complicated multi-phase reality - largely in a nod to saving computational power.

Ultimately, these two papers together offer a brilliant tie-breaker in this debate. However, they weren’t the only ones contributing to the debate. A few weeks before these two papers were released, an independent team utilizing the Subaru Hyper Suprime-Cam published a paper that showcased stars sitting around 30kpc out in the Milky Way’s halo. This tidal debris aligns well with Vasiliev’s second-passage model, and is recent enough that the other side of the debate hasn’t yet had time to process counter arguments.

In other words, it's still not clear whether or not this is our first rodeo with the Large Magellanic Cloud. Hopefully upcoming missions, such as NASA’s Aspera mission, will allow us to look directly at the morphology and distribution of the Magellanic gas more closely. But until then, the debate will continue in the pages of academic journals.

Source: Universe Today

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Learn More:

S. Lucchini et al. - The LMC Corona Favors a First Passage

S. Lucchini & J.J. Han - Threading the Magellanic Needle: Hypervelocity Stars Trace the Past Location of the LMC

UT - Our Galaxy Has a Hot Side and Now We Know Why

UT - The Large Magellanic Cloud Survived its Closest Approach to the Milky Way

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Andy Tomaswick

Andy has been interested in space exploration ever since reading Pale Blue Dot in middle school. An engineer by training, he likes to focus on the practical challenges of space exploration, whether that's getting rid of perchlorates on Mars or making ultra-smooth mirrors to capture ever clearer data. When not writing or engineering things he can be found entertaining his four children, six cats, and two dogs, or running in circles to stay in shape.

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Starry spiral in a familiar neighbourhood

A spiral galaxy seen close up and tilted at an angle, so that its disc fills the view from corner to corner. Its disc is yellow near to the centre and pale blue farther out, showing cooler and hotter stars, respectively. Thin brown clouds of dust, glowing pink spots of star formation, and sparkling blue patches filled with star clusters swirl through the galaxy. Behind it, small orange dots are very distant galaxies. Credit: ESA/Hubble & NASA, D. Thilker and the PHANGS-HST Team Hi-res image (7.64 MB)
Licence: CC BY 4.0 INT or ESA Standard Licence (content can be used under either licence)

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In this new image by the NASA/ESA Hubble Space Telescope, a spiral galaxy glittering with star clusters is the centre of attention. NGC 3137 is located 53 million light-years away in the constellation Antlia (The Air Pump). As a nearby spiral galaxy, this target offers astronomers an excellent opportunity to study the cycle of stellar birth and death, as well as giving researchers a glimpse of a galactic system similar to our own.

NGC 3137 is of particular interest to astronomers because it travels through space with a group of galaxies that is thought to be similar to the Local Group, the galaxy group that contains the Milky Way. Similar to the Local Group, the NGC 3175 group contains two large spiral galaxies: NGC 3137 and NGC 3175, which Hubble has also observed. In the Local Group, the largest members are the Milky Way galaxy and Andromeda, another spiral galaxy. In addition to two large spiral galaxies, both groups also contain a number of smaller dwarf galaxies, although it’s not yet known how many of these tiny companions the NGC 3175 group has; researchers have found more than 500 dwarf galaxy candidates. By studying this nearby galaxy group, astronomers can learn about the dynamics of our own galactic home.

NGC 3137 is revealed in fantastic detail by Hubble. This image is crafted from observations in six different colour bands, creating a view that highlights several facets of this beautiful spiral. The galaxy’s centre, which is encircled by a network of fine, dusty clouds, hosts a black hole estimated to be 60 million times more massive than the Sun. NGC 3137 is highly inclined from our point of view, giving a unique perspective on its loose, feathery spiral structure. A couple of photobombing Milky Way stars and a smattering of far more distant background galaxies complete the image.

As stunning as each of these features may be, it’s the galaxy’s brilliant star clusters that steal the show. The galaxy is peppered with dense clusters of bright blue stars and glowing red gas clouds, which signal the presence of hot, young stars still encased in their birth nebulae.

Unsurprisingly, these star clusters are exactly what has drawn Hubble’s keen eye. Researchers are using Hubble to carry out an observing programme (#17502; PI: D. Thilker) focusing on star clusters in 55 nearby galaxies. These observations give an in-depth view of stellar life in spiral galaxies, from the young stars still in the process of forming to the ancient stellar populations that grew up in the early years of their galactic hosts.

Source: ESA/Science Exploration

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Links

• Release on esahubble.org
• Pan video: NGC 3137

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Astronomers Explore the Surface Composition of a Nearby Super-Earth

This high-resolution photo of the planet Mercury probably resembles the rocky exoplanet LHS 3844 b. Results from JWST observations favour an airless rocky planet with a dark, basalt-like surface, likely space-weathered by irradiation and meteorite impacts. © NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington (cropped)

Infrared spectrum of LHS 3844 b’s hot dayside derived from the brightness contrast to its host star in ppm (parts per million = 0.0001%) at different wavelengths. The observational data obtained from the James Webb and Spitzer Space Telescopes are consistent with mantle or lava rock, whereas they rule out an Earth-like crust. © Sebastian Zieba et al./MPIA

A close-up view of an astronaut’s boot print in the fine-powdered lunar regolith, during the Apollo 11 extravehicular activity (EVA) on the Moon. Similar conditions may exist on the exoplanet LHS 3844 b due to prolonged space weathering by stellar irradiation and meteorite impacts. © NASA

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To the point:

• JWST Observations: The James Webb Space Telescope analysed the rocky exoplanet LHS 3844 b, revealing a dark, hot surface without an atmosphere.


• Surface Composition: The analysis indicates the planet's surface is likely composed of basalt or mantle rock, ruling out a composition similar to Earth's silicate-rich crust.


• Geological Activity: The findings suggest that LHS 3844 b may have undergone prolonged geological inactivity, as no signs of volcanic gases were detected.

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Using MIRI (Mid Infrared Instrument) on board the James Webb Space Telescope (JWST), a team of researchers led by former MPIA (Max Planck Institute for Astronomy, Heidelberg, Germany) PhD student Sebastian Zieba (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA) and Laura Kreidberg, MPIA Director and study PI (principal investigator), analysed the surface composition of the rocky exoplanet LHS 3844 b. Beyond characterizing exoplanetary atmospheres, this kind of deciphering the geological properties of planets orbiting distant stars is the next step in unveiling their nature. The results of this investigation are now published in the journal Nature Astronomy. A dark and airless rocky super-Earth

A dark and airless rocky super-Earth

LHS 3844 b is a rocky planet 30% bigger than Earth and orbits a cool red dwarf star once within roughly 11 hours. Whirling just three stellar diameters above the host star’s surface, the planet is tidally locked to its orbit. This means one rotation takes just as long as one revolution. As a result, the same hemisphere of LHS 3844 b always faces its star, producing a constant dayside with an average temperature of about 1000 Kelvin (approximately 725 Degrees Celsius or 1340 Degrees Fahrenheit). The LHS 3844 system is only 48.5 light-years (14.9 parsecs) away from Earth.

“Thanks to the amazing sensitivity of JWST, we can detect light coming directly from the surface of this distant rocky planet. We see a dark, hot, barren rock, devoid of any atmosphere. Laura Kreidberg, MPIA”

With its dark surface, LHS 3844 b may resemble a larger version of the Moon or the planet Mercury. This conclusion is based on analysing the infrared radiation received from the planet’s hot dayside. However, when measuring this radiation, we cannot see the planet directly; instead, we register the repeating change in brightness we receive from the star and the orbiting planet combined.

MIRI divided a portion of the planet’s infrared emission, ranging from 5 to 12 micrometres, into smaller wavelength sections and measured the brightness per wavelength bin. This is what astronomers call a spectrum, a rainbow-like distribution of the light’s components. Another data point, obtained from observations with the Spitzer Space Telescope and published a few years ago, augmented the analysis.

Constraining geological activity

Instead, the dark surface points to a composition reminiscent of terrestrial or lunar basalt, or of Earth’s mantle material. However, the astronomers attempted an even more detailed characterization.

A statistical analysis of how well this spectrum fits various mineral mixtures and configurations revealed that extended solid areas of basalt or magmatic rock best match the observations. They are rich in magnesium and iron and can include olivine. Crushed material, such as rocks or gravel, also fits fairly well, whereas grains or powders are inconsistent with the observations due to their brighter appearance, at least at first glance.

Without a protective atmosphere, planets are subjected to space weathering, predominantly driven by hard, energetic radiation from the host star and impacts from meteorites of various sizes.

“It turns out, these processes not only slowly dissolve hard rocks into regolith, a layer of fine grains or powder as found on the Moon,” explains Zieba. “They also darken the layer by adding iron and carbon, making the regolith’s properties more consistent with the observations.”

What can we deduce about an exoplanet's rocky surface?

Instead, the dark surface points to a composition reminiscent of terrestrial or lunar basalt, or of Earth’s mantle material. However, the astronomers attempted an even more detailed characterization.

A statistical analysis of how well this spectrum fits various mineral mixtures and configurations revealed that extended solid areas of basalt or magmatic rock best match the observations. They are rich in magnesium and iron and can include olivine. Crushed material, such as rocks or gravel, also fits fairly well, whereas grains or powders are inconsistent with the observations due to their brighter appearance, at least at first glance.

Without a protective atmosphere, planets are subjected to space weathering, predominantly driven by hard, energetic radiation from the host star and impacts from meteorites of various sizes.

“It turns out, these processes not only slowly dissolve hard rocks into regolith, a layer of fine grains or powder as found on the Moon,” explains Zieba. “They also darken the layer by adding iron and carbon, making the regolith’s properties more consistent with the observations.”

Geologically fresh or weathered? Two possible scenarios

This assessment left the astronomers with two scenarios for the planet’s surface that match the data equally well. One involves a surface dominated by dark, solid rock composed of basaltic or magmatic minerals. Compared to geological timescales, space weathering alters its properties quickly. Therefore, the astronomers conclude that, in this scenario, the surface should be relatively fresh, produced by recent geological activity, such as widespread volcanism.

The second scenario also proposes a dark surface, comparable to the Moon or Mercury. Still, it accounts for prolonged space weathering, which leads to extended regions covered by a darkened regolith layer, a fine powder also present on the Moon, as evidenced by the iconic photos of the astronauts’ footprints. This alternative relies on longer periods of geological inactivity, thereby requiring conditions opposite to the first scenario.

Attempts to resolve the ambiguity

These two alternatives differ in the degree of recent geological activity required. On Earth and other active objects in the Solar System, a typical phenomenon during such activity is outgassing. Sulphur dioxide (SO2) is a gas commonly connected to volcanism. If present on LHS 3844 b in reasonable amounts, MIRI should have detected it. Still, it found nothing. Therefore, a recent period of activity seems unlikely, which leads the astronomers to favour the second scenario. If correct, LHS 3844 b may truly look much like Mercury indeed.

In order to test their idea, Zieba, Kreidberg, and their colleagues are already pursuing a more direct approach. They have obtained additional JWST observations, which should enable them to discern surface conditions by exploiting small differences in how solid slabs and powders emit or reflect light. The distribution of emission angles depends on surface roughness, which affects the amount of radiation received at a given viewing angle. This concept is successfully applied to characterizing asteroids in the Solar System. “We are confident the same technique will allow us to clarify the nature of LHS 3844 b’s crust and, in the future, other rocky exoplanets,” concludes Kreidberg.

Additional information

Laura Kreidberg is the only MPIA astronomer involved in this study.

Other researchers were: Sebastian Zieba (Center for Astrophysics | Harvard & Smithsonian, Cambridge, USA), Brandon P. Coy (Department of the Geophysical Sciences, University of Chicago, USA), Aaron Bello-Arufe (Jet Propulsion Laboratory, California Institute of Technology, Pasadena, USA [JPL]), Kimberly Paragas (Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, USA), Xintong Lyu (Peking University, Beijing, China), Renyu Hu (The Pennsylvania State University, University Park, USA and JPL), Aishwarya Iyer (NASA Goddard Space Flight Center, Greenbelt, USA), Kay Wohlfarth (Technische Universität Dortmund, Germany)

The JWST observations used in this study were conducted as part of GO program #1846 (PI: Laura Kreidberg, co-PI: Renyu Hu) titled “A Search for Signatures of Volcanism and Geodynamics on the Hot Rocky Exoplanet LHS 3844 b.”

The MIRI consortium comprises the ESA (European Space Agency) member states: Belgium, Denmark, France, Germany, Ireland, the Netherlands, Spain, Sweden, Switzerland, and the United Kingdom. National science organisations fund the consortium’s work – in Germany, the Max Planck Society (MPG) and the German Aerospace Center (DLR). Participating German institutions include the Max Planck Institute for Astronomy in Heidelberg, the University of Cologne, and Hensoldt AG in Oberkochen, formerly Carl Zeiss Optronics.

The James Webb Space Telescope is the world’s leading observatory for space research. It is an international programme led by NASA and its partners ESA and CSA (Canadian Space Agency)..
The Spitzer Space Telescope was operated by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA.

Similar to how exoplanetary atmosphere research has benefited from climate science, this emerging field of exoplanetary geology draws on Earth-based geologic knowledge. Zieba, Kreidberg, and their collaborators ran models and accessed template libraries of rocks and minerals known from Earth, the Moon, and Mars to see what infrared signatures they would produce under the conditions on LHS 3844 b. Comparing observation-based data with these computations confidently ruled out a composition comparable to Earth’s crust, typically silicate-rich rocks such as granite.

Although this result is not very surprising – even in the Solar System, Earth is the only planet with such a crust – it may reveal details on LHS 3844 b’s geological history. Earth-like silicate-rich crusts are thought to form through a prolonged refinement process that requires tectonic activity and typically relies on water as a lubricant. The rocky material repeatedly melts and solidifies as it is mixed with mantle material, leaving the lighter minerals on the surface.

“Since LHS 3844 b lacks such a silicate crust, one may conclude that Earth-like plate tectonics does not apply to this planet, or it is ineffective,” says Sebastian Zieba. “This planet likely only contains little water.”

Source: Max Planck Institute for Astronomy/News

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Contacs:

Dr. Markus Nielbock
Press and outreach officer
Tel: +49 6221 528-134
Email: pr@mpia.de
MPIA press team
Max Planck Institute for Astronomy, Heidelberg, Germany

Prof. Dr. Laura Kreidberg
Director
Tel: +49 6221 528-215
Email: kreidberg@mpia.de
Laura Kreidberg / MPIA
Max Planck Institute for Astronomy, Heidelberg, Germany

Dr. Sebastian Zieba
NASA Sagan Fellow
Email: sebastian.zieba@cfa.harvard.edu
Sebastian Zieba / Harvard
Center for Astrophysics | Harvard & Smithsonian, Cambridge, MA, USA

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Original publication

Sebastian Zieba, Laura Kreidberg, et al.
The dark and featureless surface of rocky exoplanet LHS 3844 b from JWST mid-infrared spectroscopy
Nature Astronomy (2026)

Source | DOI

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Links

JWST/MIRI observing program information">JWST/MIRI observing program information

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Downloads

mpia-pm_lhs3844b_2026_fig2_en_large 2.9 MB

mpia-pm_lhs3844b_lunar_footprint 9.44 MB

mpia-pm_lhs3844b_mercury_nasa 3.27 MB

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Related Articles

From Heidelberg into outer space

December 20, 2021

The James Webb Space Telescope launches into space with equipment developed and built at the Max Planck Institute for Astronomy.

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Milky Way supermassive black hole archeology

NuSTAR image of the Galactic center region. Sgr A* is the position of supermassive black hole at the center of the Milky Way galaxy. Green dashed ellipses show the areas of giant molecular clouds. The distance from the Bridge to Sgr A* is 200 light years. Credit: Mori 2015.  Download Image

At the center of our Milky Way galaxy is a black hole with a mass more than a million times the mass of the Sun, called Sgr A*. This has been directly confirmed by detailed radio imaging of material close to the black hole by the event horizon telescope as well as the motions of stars in the center of the galaxy effected by the gravitational pull of this supermassive black hole. The low luminosity of Sgr A* indicates that the system is in a relatively quiescent state compared to Active Galactic Nuclei (AGN) in other galaxies which may harbor more massive black holes. However, this has not always been the case, and evidence of higher luminosity in the past is indicated by the increasing X-ray brightness of giant molecular clouds near Sgr A*. Over the past four years observations by NuSTAR of regions close to the center of our galaxy have confirmed that X-ray emission from one of these clouds, called “The Bridge” has been increasing and is likely due to reflection of X-rays from a past Sgr A* outburst approximately 200 years ago. NuSTAR observations last week of The Bridge will add to the detailed investigation of the full profile of this Sgr A* illumination event. Characterizing past Sgr A* outbursts is a necessary step towards understanding the physical mechanisms that triggered major outbursts from a quiescent supermassive black hole, possibly similar to a tidal disruption event seen in other AGN. Observations of The Bridge will continue in 2027, for a proposal selected to be part of cycle 12 of the NuSTAR General Observer program.

Source: NASA's Nuclear Spectroscopic Telescope Array (NuSTAR)/Weekly Highlights

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Outer Solar System Object Has an Atmosphere But Shouldn’t

Artist’s conception of this research showing an imagined time sequence as a star passes behind a TNO with an atmosphere. Credit: NAOJ. Image (961KB)

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A team of professional and amateur Japanese astronomers foundidence for a thin atmosphere around a small body in the outer Solarystem. The object is so small that it should not have a sustainableatmosphere, raising questions about when and how the atmosphere formd. Future observations to better characterize the atmosphere will help solve these mysteries.

In the cold reaches of the outer Solar System lie thousands of small objects known as trans-Neptunian objects (TNOs) because they lie outside the orbit of Neptune. A thin atmosphere has been observed around Pluto, the most famous TNO, but studies of other TNOs have yielded negative results. Most TNOs are so cold, and their surface gravity so weak, that they are not expected to retain atmospheres.

But astronomers like to expect the unexpected, so they took advantage of a lucky “natural experiment” to look for an atmosphere around a TNO known as (612533) 2002 XV₉₃. This object, abbreviated as 2002 XV₉₃, has a diameter of approximately 500 km. For reference, Pluto’s diameter is 2,377 km. The orbit of 2002 XV₉₃ is such that, as seen from Japan, it passed directly in front of a star on January 10, 2024. As the star disappears behind 2002 XV₉₃, it might gradually fade, indicating that the light is being attenuated as it passes through a thin atmosphere; or it might suddenly wink out as it slips behind the solid surface of the TNO.

A team of professional and amateur astronomers, led by Ko Arimatsu at NAOJ Ishigakijima Astronomical Observatory, observed the star as 2002 XV₉₃ passed in front of it from multiple sites in Japan. The obtained data are consistent with attenuation by an atmosphere.

Calculations show that the atmosphere found around 2002 XV₉₃ is expected to last less than 1000 years unless it is replenished. So it must have been created or replenished recently. Observations by the James Webb Space Telescope show no signs of frozen gases on the surface of 2002 XV₉₃ that might sublimate to form an atmosphere. One possibility is that some event brought frozen or liquid gases from deep inside the TNO to the surface. Another possibility is that a comet crashed into 2002 XV₉₃, releasing gas that formed a temporary atmosphere. Further observations are needed to distinguish between these two scenarios.

Conceptual video for Arimatsu et al. (2026)
Conceptual video showing how the light from a star changes when it passes behind an object with an atmosphere.
Credit: NAOJ

Source: National Astronomical Observatory of Japan (NAOJ)/News

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Release Information

Researcher(s) Involved in this Release

Ko Arimatsu (Ishigakijima Astronomical Observatory, National Astronomical Observatory of Japan)

Jun-ichi Watanabe (Kyoto Sangyo University)

Coordinated Release Organization(s)

National Astronomical Observatory of Japan, NINS

Faculty of Science, The University of Tokyo

Kyoto University

Kyoto Sangyo University

Paper(s)

Ko Arimatsu et al. “Detection of an atmosphere on a trans-Neptunian object beyond Pluto”, in Nature Astronomy, DOI: 10.1038/s41550-026-02846-1

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ALMA Reveals How Planet-Forming Disks Take Shape Press Releases ALMA Reveals How Planet-Forming Disks Take Shape

A conceptual visualization of ENDTRANZ, the transition zone at the envelope–disk boundary, which is shown as a red colored, belt-like annulus where the gas motion gradually transitions from the infalling envelope to the Keplerian rotation within the protoplanetary disk surrounding a young star. This is an AI-generated illustration based on a two-dimensional spatial map of the specific angular momentum in the equatorial plane, as obtained from the numerical simulations. The specific angular momentum map offers an intuitive lens to ‘see’ ENDTRANZ, making its dynamics more apparent than in the rotational velocity map. (Image Credit: Indrani Das/ASIAA)

The figure shows the radial variation of rotational velocity and specific angular momentum with distance from the star, in astronomical units (au), on the left- and right-hand axes, respectively, as obtained from the global collapse simulations. The orange-colored region represents the ENDTRANZ of a young stellar system. The vertical dashed and dotted lines represent the outer and inner boundaries of ENDTRANZ. (Image Credit: Indrani Das/ASIAA.)

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New study identifies a long-sought transition zone where infalling gas becomes a rotating disk.

Every planet — including everyone in the Solar System — was born inside a rotating disk of gas and dust swirling around a young star. Astronomers have long understood that these disks exist and that planets take shape within them. What they couldn't explain was how the raw material gets there in the first place. Now, a new study led by Indrani Das of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA) has found the missing piece: a distinct transition zone where chaotic, infalling gas gradually settles into the orderly rotation of a planet-forming disk. The team named it ENDTRANZ — the Envelope Disk Transition Zone — and detected it for the first time in an actual young stellar system using the Atacama Large Millimeter/submillimeter Array (ALMA).

From chaos to order

Young stars are surrounded by a vast shroud of gas and dust called an envelope. Gravity pulls this material inward, feeding both the growing star and the disk around it. But the infalling gas moves differently than the disk — more slowly and chaotically — and the point at which one becomes the other had never been clearly observed.

Earlier theoretical models assumed the switch was sharp, almost instantaneous. The new study shows it isn't. Using numerical simulations with the FEOSAD code, the team tracked how a collapsing cloud core evolves into a star-disk system — and found that the transition unfolds gradually across a finite region, leaving a tell-tale signature: a characteristic "jump" in the distribution of specific angular momentum, a measure of how gas rotates as a function of its distance from the star.

"The existence of ENDTRANZ naturally results from the redistribution of mass and angular momentum during the formation of disks around young stars. This process ultimately governs how infalling material from the envelope, which rotates more slowly than the Keplerian speed, spreads out to form the disk and gradually settles into ordered Keplerian rotation," explained Das.

ALMA finds the fingerprint

To test whether ENDTRANZ exists in nature, the team turned to L1527 IRS, a young protostar about 450 light-years away in the Taurus molecular cloud. Using data from the ALMA Large Program eDisk (Embedded Disks in Planet Formation), they found exactly the same angular momentum signature that the simulations had predicted — spanning a zone roughly 16 astronomical units wide, or about 16 times the distance from Earth to the Sun.

"This ENDTRANZ tracer essentially manifests from the gradual transition in the rotational velocity, which offers a diagnostic framework for understanding the physical processes at play that drive the disk evolution," said Shantanu Basu, Interim Director of the Canadian Institute for Theoretical Astrophysics and co-author of the study.

ALMA's extraordinary resolution was essential to making this detection possible, resolving the structure at the precise interface between the envelope and the disk — a regime that had previously been beyond reach.

"A careful inspection and comparison of the radial dependence of specific angular momentum between the observational data and the simulations helped identify the evidence of ENDTRANZ in L1527 IRS," said Nagayoshi Ohashi, principal investigator of the ALMA eDisk Large Program and co-author of the study.

A new window on planet formation

The discovery establishes ENDTRANZ as a fundamental feature of how stars and planetary systems assemble — and opens the door to searching for the same signature in other young systems across the galaxy.

"In many ways, we believe this is just the beginning!" Das said.

Source: ALMA (Atacama Large Millimeter/submillimeter Array)/News

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Additional Information

The study appears as “Modeling the Break in the Specific Angular Momentum within the Envelope-Disk Transition Zone” by I. Das et al. in the Astrophysical Journal.

This article is based on the original press release by the National Astronomical Observatory of Japan (NAOJ), an ALMA partner on behalf of East Asia.

The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of the European Southern Observatory (ESO), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Republic of Chile. ALMA is funded by ESO on behalf of its Member States, by NSF in cooperation with the National Research Council of Canada (NRC) and the National Science and Technology Council (NSTC) in Taiwan, and by NINS in cooperation with the Academia Sinica (AS) in Taiwan and the Korea Astronomy and Space Science Institute (KASI).

ALMA construction and operations are led by ESO on behalf of its Member States; by the National Radio Astronomy Observatory (NRAO), managed by Associated Universities, Inc. (AUI), on behalf of North America; and by the National Astronomical Observatory of Japan (NAOJ) on behalf of East Asia. The Joint ALMA Observatory (JAO) provides the unified leadership and management of ALMA's construction, commissioning, and operation.

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Radiative Transfer Shapes Hydrogen Lines in Little Red Dots

Schematic illustration of resonance scattering in a hydrogen atom. Interactions with electrons in the ground state (1s–2p) are called Lyman-α (green), whereas excited electrons on n=2 contribute to the Balmer series (Hα and Hβ, red and blue). The next higher excitation level is then called the Paschen series (yellow). © MPA

Due to distinctive features in the spectra of the 'Little Red Dots', a new class of objects spotted by the James Webb Space Telescope, it was thought that these were distant galaxies with massive black holes at their centres. However, new research suggests that the light from these galaxies is shaped not only by the motion of gas near the central black hole, but also by the effects of radiation. MPA scientists have modelled three key processes – resonance, Raman, and Thomson scattering – and found that these, acting together, can explain the formation of hydrogen emission lines in the Little Red Dots.

Little Red Dots (LRDs) are among the most surprising discoveries of the James Webb Space Telescope. These compact, reddish sources appear in the early universe, within the first billion years of cosmic history, and exhibit unusual hydrogen spectra. Their light shows broad hydrogen emission lines, Balmer absorption features, and a pronounced break between ultraviolet and optical wavelengths. At first glance, these properties seem to point to active galactic nuclei, where broad hydrogen lines are typically interpreted as signatures of rapidly moving gas surrounding a supermassive black hole.

Yet this interpretation creates a major puzzle. If the widths of these hydrogen lines are directly interpreted as tracers of gas motion around a black hole, many Little Red Dots appear to host black holes that are unexpectedly massive compared to their young host galaxies. Such enormous black holes would challenge current ideas of how quickly black holes and galaxies could have formed and grown in the early universe. This tension raises an important question: do these spectral features truly provide a direct measure of black hole mass, or are they significantly shaped by the dense environments through which the radiation propagates?

This work explores a new possibility. Rather than assuming that hydrogen line widths primarily trace gas dynamics near a black hole, it investigates how radiative transfer through dense surrounding gas can fundamentally alter the observed spectrum. The presence of Balmer absorption and strong spectral breaks already hints that light in these systems may undergo substantial scattering and reprocessing. If so, some of the broad and complex hydrogen features in Little Red Dots may arise not only from fast-moving gas, but also from the way photons interact with thick, hydrogen-rich environments before escaping.

Understanding how radiative transfer shapes these spectral signatures therefore offers more than an alternative explanation for broad lines: it provides a new tool for probing the physical conditions, structure, and nature of Little Red Dots themselves, revealing how gas, radiation, and black hole growth interact in some of the earliest galaxies. Our focus is on three key processes:

1. Resonance scattering, where photons interact with hydrogen atoms in the excited n=2 state.
2. Raman scattering, where ultraviolet photons are converted into optical emission through inelastic scattering by atomic hydrogen.
3. Thomson scattering, where photons scatter off free electrons. Each process contributes differently to the observed spectral features.

Resonance scattering: shaping line profiles and ratios

Resonance scattering plays a crucial role when hydrogen atoms populate the n=2 or Balmer state, as indicated by Balmer absorption features and strong Balmer breaks. In this regime, Balmer photons can undergo multiple scatterings before escaping, which significantly modifies the emerging line profiles. These repeated interactions can produce asymmetric line shapes, particularly in the presence of gas motions such as outflows.

Notably, the radiative transfer of Hα and Hβ differs due to the atomic structure of hydrogen. While Hα photons predominantly remain in the same transition, Hβ photons can be converted into other lines, such as Paschen-α and Hα, through cascades involving the n=3 state. Consequently, Hβ photons are efficiently depleted in optically thick gas, while more Hα photons are produced. This leads to enhanced Hα emission and naturally increases the Hα/Hβ flux ratio beyond its intrinsic value.

Left: schematic illustration of Raman scattering of far-ultraviolet photons and the energy levels involved in neutral hydrogen. An UV photon excites the atom near the n=3 or n=4 state (green). If the electron drops down again to the ground state, it emits a Rayleigh photon (blue). If it drops down to an intermediate energy level, it emits a Raman photon (yellow or red). Right: The width of the emission line around Hα and Hβ depends on the column density (coloured lines), with the Hα wings being approximately three times broader than the Hβ wings for the same column density.© MPA

Raman scattering: generating broad wings

Raman scattering introduces a distinct spectral signature. Ultraviolet (UV) photons near the hydrogen Lyman series can be inelastically scattered by neutral hydrogen into optical wavelengths, producing broad wings around emission lines and showing systematic differences between certain hydrogen transitions. In particular, Raman scattering predicts that the wings of Hα should be significantly broader than those of Hβ.

Although broad emission lines are a defining feature of the Little Red Dots, such strong differences between lines are not always observed. This suggests that, although Raman scattering may contribute to the observed spectra, it is unlikely to be the dominant origin of the broad emission features. than those of Hβ.

Thomson-scattered line profiles for different electron temperatures. The line width increases with electron temperature.
© MPA

Thomson scattering: similar broad wings in hydrogen emission lines

Among the processes considered, Thomson scattering by free electrons provides a particularly compelling explanation for the broad components observed. Since electrons move thermally, the scattering introduces a symmetric broadening that depends on the electron temperature rather than on the motion of the bulk gas. Under typical conditions, this naturally produces line widths of around 1000 km/s, which is consistent with observations of the Little Red Dots. than those of Hβ.

The resulting profiles often exhibit exponential wings — a distinctive feature of electron scattering that has also been identified in other astrophysical environments. Importantly, this mechanism affects all emission lines in a similar way, which is consistent with the observed spectra. than those of Hβ.

Simulated spectra of the Hα, Hβ and Paα lines (red, blue and yellow) in a model combining an inner ionised region producing Thomson scattering (green) and an outer neutral region producing resonance scattering (grey). The resulting profiles illustrate how multiple scattering processes shape the observed line features together. © MPA

Implications for interpreting the Little Red Dots

The combined effects of resonance, Raman and Thomson scattering demonstrate that the Little Red Dots' diverse spectral features can naturally arise from radiative transfer in dense gas. Broad wings, absorption features and differences between hydrogen lines do not necessarily require extreme gas velocities or a classical broad-line region.

This has important consequences. If line widths are interpreted purely as indicators of gas motion, the mass of black holes may be significantly overestimated. Instead, the spectra of Little Red Dots encode the physical properties of their surrounding gas, such as density, temperature and ionisation state, through radiative processes.

These results provide a new framework for interpreting the spectra of Little Red Dots and similar systems in the early universe, offering a new perspective on early galaxy evolution. Rather than being straightforward indicators of black hole dynamics, hydrogen emission lines can reflect the complex interplay between radiation and dense gas.

Understanding this interplay is essential for correctly inferring the physical properties of galaxies and black holes at high redshifts and for developing a consistent model of their co-evolution during the first billion years of cosmic history. Current work focuses on analysing observed line profiles and using these models to decode the physical conditions imprinted in their shapes.

Source:

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Contact:

Dr. Seok-Jun Chang
Chang, Seok-Jun
Postdoc
2245
sjchang@mpa-garching.mpg.de

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Original Publication

Chang, Seok-Jun; Gronke, Max; Matthee, Jorryt; Mason, Charlotte
Impact of resonance, Raman, and Thomson scattering on hydrogen line formation in Little Red Dots
MNRAS, 545, 4, id.staf2131, 21 pp

Source | DOI

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