Physicists solve puzzle about ancient galaxy found by Webb telescope

Last September, the James Webb Space Telescope, or JWST, discovered JWST-ER1g, a massive ancient galaxy that formed when the universe was just a quarter of its current age. Surprisingly, an Einstein ring is associated with this galaxy. That's because JWST-ER1g acts as a lens and bends light from a distant source, which then appears as a ring -- a phenomenon called strong gravitational lensing, predicted in Einstein's theory of general relativity.

The total mass enclosed within the Einstein radius -- the radius of the Einstein ring -- has two components: stellar and dark matter components.

"If we subtract the stellar mass from the total mass, we get the dark matter mass within the Einstein radius," said Hai-Bo Yu, a professor of physics and astronomy at the University of California, Riverside, whose team has published new work about JWST-ER1g in the journal The Astrophysical Journal Letters. "But the value for the dark matter mass seems higher than expected. This is puzzling. In our paper, we offer an explanation."

A dark matter halo is the halo of invisible matter that permeates and surrounds a galaxy like JWST-ER1g. Although dark matter has never been detected in laboratories, physicists are confident dark matter, which makes up 85% of the universe's matter, exists.

"When ordinary matter -- pristine gas and stars -- collapses and condenses into the dark matter halo of JWST-ER1g, it may be compressing the halo, leading to a high density," said Demao Kong, a second-year graduate student at UCR, who led the analysis. "Our numerical studies show that this mechanism can explain the high dark matter density of JWST-ER1g -- more dark matter mass in the same volume, resulting in higher density."

According to Daneng Yang, a postdoctoral researcher at UCR and co-author on the paper, JWST-ER1g, formed 3.4 billion years after the Big Bang, provides "a great chance to learn about dark matter."

"This strong lensing object is unique because it has a perfect Einstein ring, from which we can obtain valuable information about the total mass within the Einstein radius, a critical step for testing dark matter properties," he said.

Launched on Christmas Day in 2021, NASA's JWST is an orbiting infrared observatory. Also called Webb, it is designed to answer questions about the universe. It is the largest, most complex, and powerful space telescope ever built.

"JWST provides an unprecedented opportunity for us to observe ancient galaxies formed when the universe was young," Yu said. "We expect to see more surprises from JWST and learn more about dark matter soon."

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Dwarf galaxies use 10-million-year quiet period to churn out stars

If you look at massive galaxies teeming with stars, you might be forgiven in thinking they are star factories, churning out brilliant balls of gas. But actually, less evolved dwarf galaxies have bigger regions of star factories, with higher rates of star formation.

Now, University of Michigan researchers have discovered the reason underlying this: These galaxies enjoy a 10-million-year delay in blowing out the gas cluttering up their environments.

Star-forming regions are able to hang on to their gas and dust, allowing more stars to coalesce and evolve.

In these relatively pristine dwarf galaxies, massive stars -- stars about 20 to 200 times the mass of our sun -- collapse into black holes instead of exploding as supernovae.

But in more evolved, polluted galaxies, like our Milky Way, they are more likely to explode, thereby generating a collective superwind.

Gas and dust get blasted out of the galaxy, and star formation quickly stops.

Their findings are published in the Astrophysical Journal.

"As stars go supernova, they pollute their environment by producing and releasing metals," said Michelle Jecmen, study first author and an undergraduate researcher.

"We argue that at low metallicity -- galaxy environments that are relatively unpolluted -- there is a 10-million-year delay in the start of strong superwinds, which, in turn, results in higher star formation."

The U-M researchers point to what's called the Hubble tuning fork, a diagram that depicts the way astronomer Edwin Hubble classified galaxies.

In the handle of the tuning fork are the largest galaxies. Huge, round and brimming with stars, these galaxies have already turned all of their gas into stars.

Along the tines of the tuning fork are spiral galaxies that do have gas and star-forming regions along their compact arms.

At the end of the tuning fork's tines are the least evolved, smallest galaxies.

"But these dwarf galaxies have just these really mondo star-forming regions," said U-M astronomer Sally Oey, senior author of the study.

"There have been some ideas around why that is, but Michelle's finding offers a very nice explanation: These galaxies have trouble stopping their star formation because they don't blow away their gas."

Additionally, this 10-million-year period of quiet offers astronomers the opportunity to peer at scenarios similar to the cosmic dawn, a period of time just after the Big Bang, Jecmen said.

In pristine dwarf galaxies, gas clumps together and forms gaps through which radiation can escape.

This previously known phenomenon is called the "picket fence" model, with UV radiation escaping between slats in the fence.

The delay explains why gas would have had time to clump together.

Ultraviolet radiation is important because it ionizes hydrogen -- a process that also occurred right after the Big Bang, causing the universe to go from opaque to transparent.

"And so looking at low-metallicity dwarf galaxies with lots of UV radiation is somewhat similar to looking all the way back to the cosmic dawn," Jecmen said.

"Understanding the time near the Big Bang is so interesting. It's foundational to our knowledge. It's something that happened so long ago -- it's so fascinating that we can see sort of similar situations in galaxies that exist today."

A second study, published in the Astrophysical Journal Letters and led by Oey, used the Hubble Space Telescope to look at Mrk 71, a region in a nearby dwarf galaxy about 10 million light years away.

In Mrk 71, the team found observational evidence of Jecmen's scenario.

Using a new technique with the Hubble Space Telescope, the team employed a filter set that looks at the light of triply ionized carbon.

In more evolved galaxies with lots of supernova explosions, those explosions heat gas in a star cluster to very high temperatures -- to millions of degrees Kelvin, Oey said.

As this hot superwind expands, it blasts the rest of the gas out of the star clusters.

But in low metallicity environments such as Mrk 71, where stars aren't blowing up, energy within the region is radiated away.

It doesn't have the chance to form a superwind.

The team's filters picked up a diffuse glow of the ionized carbon throughout Mrk 71, demonstrating that the energy is radiating away.

Therefore, there is no hot superwind, instead allowing dense gas to remain throughout the environment.

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The puzzle of the galaxy with no dark matter

A team of scientists, led by the researcher at the IAC and the University of La Laguna (ULL) Sebastién Comerón, has found that the galaxy NGC 1277 does not contain dark matter.This is the first time that a massive galaxy (it has a mass several times that of the Milky Way) does not show evidence for this invisible component of the universe. "This result does not fit in with the currently accepted cosmological models, which include dark matter" explains Comerón.

In the current standard model cosmology massive galaxies contain substantial quantities of dark matter, a type of matter which does not interact in the same way as normal matter; the only evidence for its existence is the strong gravitational pull which it exerts on the stars and the gas nearby, and this interacton is observable.

NGC 1277 is considered a prototype "relic galaxy" which means a galaxy which has had no interactions with its neighbours. Galaxies of this type are very rare, and they are considered the remnants of giant galaxies which formed in the early days of the universe.

"The importance of relic galaxies in helping us to understand how the first galaxies formed was the reason we decided to observe NGC 1277 with an integral field spectrograph" explains Comerón. "From the spectra we made kinematic maps which enabled us to work out the distribution of mass within the galaxy out to a radius of some 20,000 light years" he adds.

The team discovered that the mass distribution in NGC 1277 was just the distribution of the stars, and from this they inferred that within the radius observed there cannot be more than 5% of dark matter, although the observations are consistent with the complete absence of this component.

However, present cosmological models predict that a galaxy with the mass of NGC 1277 should have at least 10 % of their mass in the form of dark matter, with a maximum of 70 % in this form. "This discrepancy between the observations and what we would expect is a puzzle, and maybe even a challenge for the standard model" notes Ignacio Trujillo, a researcher at the IAC and the ULL, who participated in the study.

The article suggests two possible explanations for the lack of dark matter in NGC 1277. "One is that the gravitational interaction with the surrounding medium within the galaxy cluster in which this galaxy is situated has stripped out the dark matter" comments Anna Ferré-Mateu, a researcher at the IAC and the ULL who also participated in the study. "The other is that the dark matter was driven out of the system when the galaxy formed by the merging of protogalactic fragments, which gave rise to the relic galaxy."

For the authors of the study neither of these explanations is fully satisfactory "so the puzzle of how a massive galaxy can form without dark matter remains a puzzle" insists Comerón. In order to continue researching the mystery the team plans to make new observations with the WEAVE instrument on the William Herschel Telescope (WHT) at the Roque de los Muchachos Observatory, in the Canary Island of La Palma.

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Starlight and the first black holes: researchers detect the host galaxies of quasars in the early universe

New images from the James Webb Space Telescope have revealed, for the first time, starlight from two massive galaxies hosting actively growing black holes -- quasars -- seen less than a billion years after the Big Bang. A new study in Nature this week finds the black holes have masses close to a billion times that of the Sun, and the host galaxy masses are almost one hundred times larger, a ratio similar to what is found in the more recent universe. A powerful combination of the Subaru Telescope and the JWST has paved a new path to study the distant universe.

The existence of such massive black holes in the distant universe has created more questions than answers for astrophysicists. How could these black holes grow to be so large when the universe was so young? Even more puzzling, observations in the local universe show a clear relation between the mass of supermassive black holes and the much larger galaxies in which they reside. The galaxies and the black holes have completely different sizes, so which came first: the black holes or the galaxies? This is a "chicken-or-egg" problem on a cosmic scale.

An international team of researchers, led by Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) Project Researcher Xuheng Ding and Professor John Silverman, and Peking University Kavli Institute for Astronomy and Astrophysics (PKU-KIAA) Kavli Astrophysics Fellow Masafusa Onoue have started to answer this question with the James Webb Space Telescope (JWST), launched in December 2021. Studying the relation between host galaxies and black holes in the early universe allows scientists to watch their formation, and see how they are related to one another.

Quasars are luminous, while their host galaxies are faint, which has made it challenging for researchers to detect the dim light of the galaxy in the glare of the quasar, especially at great distances. Before the JWST, the Hubble Space Telescope was able to detect host galaxies of luminous quasars when the universe was just under 3 billion years old, but no younger.

The superb sensitivity and the ultra-sharp images of the JWST at infrared wavelengths finally allowed researchers to push these studies to the time when the quasars and galaxies first formed. Just a few months after JWST started regular operations, the team observed two quasars, HSC J2236+0032 and HSC J2255+0251, at redshifts 6.40 and 6.34 when the universe was approximately 860 million years old. These two quasars were discovered in a deep survey program of the 8.2m-Subaru Telescope on the summit of Maunakea in Hawai'i. The relatively low luminosities of these quasars made them prime targets for measurement of the host galaxy properties, and the successful detection of the hosts represents the earliest epoch to date at which starlight has been detected in a quasar.

The images of the two quasars were taken at infrared wavelengths of 3.56 and 1.50 micron with JWST's NIRCam instrument, and the host galaxies became apparent after carefully modeling and subtracting glare from the accreting black holes. The stellar signature of the host galaxy was also seen in a spectrum taken by JWST's NIRSPEC for J2236+0032, further supporting the detection of the host galaxy.

Analyses of the host galaxy photometry found that these two quasar host galaxies are massive, measuring 130 and 34 billion times the mass of the Sun, respectively. Measuring the speed of the turbulent gas in the vicinity of the quasars from the NIRSPEC spectra suggests that the black holes that power them are also massive, measuring 1.4 and 0.2 billion times the mass of the Sun. The ratio of the black hole mass to host galaxy mass is similar to those of galaxies in the more recent past, suggesting that the relationship between black holes and their hosts was already in place 860 million years after the Big Bang.

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James Webb Space Telescope images challenge theories of how universe evolved

The James Webb Space Telescope (JWST) appears to be finding multiple galaxies that grew too massive too soon after the Big Bang, if the standard model of cosmology is to be believed.

In a study published in Nature Astronomy, Mike Boylan-Kolchin, an associate professor of astronomy at The University of Texas at Austin, finds that six of the earliest and most massive galaxy candidates observed by JWST so far stand to contradict the prevailing thinking in cosmology. That's because other researchers estimate that each galaxy is seen from between 500 and 700 million years after the Big Bang, yet measures more than 10 billion times as massive as our sun. One of the galaxies even appears to be more massive than the Milky Way, despite that our own galaxy had billions of more years to form and grow.

"If the masses are right, then we are in uncharted territory," Boylan-Kolchin said. "We'll require something very new about galaxy formation or a modification to cosmology. One of the most extreme possibilities is that the universe was expanding faster shortly after the Big Bang than we predict, which might require new forces and particles."

For galaxies to form so fast at such a size, they also would need to be converting nearly 100% of their available gas into stars.

"We typically see a maximum of 10% of gas converted into stars," Boylan-Kolchin said. "So while 100% conversion of gas into stars is technically right at the edge of what is theoretically possible, it's really the case that this would require something to be very different from what we expect."

For all of the breathless excitement it evokes, JWST has presented astronomers with an unsettling dilemma. If the masses and time since the Big Bang are confirmed for these galaxies, fundamental changes to the reigning model of cosmology -- what's called the dark energy + cold dark matter (ΛCDM) paradigm, which has guided cosmology since the late 1990s -- could be needed. If there are other, faster ways to form galaxies than ΛCDM allows, or if more matter actually was available for forming stars and galaxies in the early universe than was previously understood, astronomers would need to shift their prevailing thinking.

The six galaxies' times and masses are initial estimates and will need follow-up confirmation with spectroscopy -- a method that splits the light into a spectrum and analyzes the brightness of different colors. Such analysis might suggest that central supermassive black holes, which could heat up the surrounding gas, may be making the galaxies brighter so that they look more massive than they really are. Or perhaps the galaxies are actually seen at a time much later than originally estimated due to dust that causes the color of the light from the galaxy to shift redder, giving the illusion of being more lightyears away and, thus, further back in time.

The galaxy data came from the Cosmic Evolution Early Release Science Survey (CEERS), a multi-institution JWST initiative led by UT Austin astronomer Steven Finkelstein.

Another ongoing collaborative JWST project, COSMOS-Web, co-led by UT Austin's Caitlin Casey, may be involved with spectroscopy and shedding more light on the findings to help resolve the dilemma. COSMOS-Web is covering an area roughly 50 times larger than CEERS and is expected to discover thousands of galaxies.

"It will be ideal for discovering the rarest, most massive galaxies at early times, which will tell us how the biggest galaxies and black holes in the early universe arose so quickly," Boylan-Kolchin said.

Read more at Science Daily

James Webb spots super old, massive galaxies that shouldn't exist

In a new study, an international team of astrophysicists has discovered several mysterious objects hiding in images from the James Webb Space Telescope: six potential galaxies that emerged so early in the universe’s history and are so massive they should not be possible under current cosmological theory.

Each of the candidate galaxies may have existed at the dawn of the universe roughly 500 to 700 million years after the Big Bang, or more than 13 billion years ago. They’re also gigantic, containing almost as many stars as the modern-day Milky Way Galaxy.

“It’s bananas,” said Erica Nelson, co-author of the new research and assistant professor of astrophysics at the University of Colorado Boulder. “You just don’t expect the early universe to be able to organize itself that quickly. These galaxies should not have had time to form.”
        
Nelson and her colleagues, including first author Ivo Labbé of the Swinburne University of Technology in Australia, published their results Feb. 22 in the journal Nature.

The latest finds aren’t the earliest galaxies observed by James Webb, which launched in December 2021 and is the most powerful telescope ever sent into space. Last year, another team of scientists spotted four galaxies that likely coalesced from gas around 350 million years after the Big Bang. Those objects, however, were downright shrimpy compared to the new galaxies, containing many times less mass from stars.

The researchers still need more data to confirm that these galaxies are as big as they look, and date as far back in time. Their preliminary observations, however, offer a tantalizing taste of how James Webb could rewrite astronomy textbooks.

“Another possibility is that these things are a different kind of weird object, such as faint quasars, which would be just as interesting,” Nelson said.

Fuzzy dots

There’s a lot of excitement going around: Last year, Nelson and her colleagues, who hail from the United States, Australia, Denmark and Spain, formed an ad hoc team to investigate the data James Webb was sending back to Earth.

Their recent findings stem from the telescope’s Cosmic Evolution Early Release Science (CEERS) Survey. These images look deep into a patch of sky close to the Big Dipper—a relatively boring, at least at first glance, region of space that the Hubble Space Telescope first observed in the 1990s.

Nelson was peering at a postage stamp-sized section of one image when she spotted something strange: a few “fuzzy dots” of light that looked way too bright to be real.

“They were so red and so bright,” Nelson said. “We weren’t expecting to see them.”

She explained that in astronomy, red light usually equals old light. The universe, Nelson said, has been expanding since the dawn of time. As it expands, galaxies and other celestial objects move farther apart, and the light they emit stretches out—think of it like the cosmic equivalent of saltwater taffy. The more the light stretches, the redder it looks to human instruments. (Light from objects coming closer to Earth, in contrast, looks bluer).

The team ran calculations and discovered that their old galaxies were also huge, harboring tens to hundreds of billions of sun-sized stars worth of mass, on par with the Milky Way.

These primordial galaxies, however, probably didn’t have much in common with our own.  

 “The Milky Way forms about one to two new star every year,” Nelson said. “Some of these galaxies would have to be forming hundreds of new stars a year for the entire history of the universe.”

Nelson and her colleagues want to use James Webb to collect a lot more information about these mysterious objects, but they’ve seen enough already to pique their curiosity. For a start, calculations suggest there shouldn’t have been enough normal matter—the kind that makes up planets and human bodies—at that time to form so many stars so quickly.

“If even one of these galaxies is real, it will push against the limits of our understanding of cosmology,” Nelson said.

Seeing back in time

For Nelson, the new findings are a culmination of a journey that began when she was in elementary school. When she was 10, she wrote a report about Hubble, a telescope that launched in 1990 and is still active today. Nelson was hooked.

“It takes time for light to go from a galaxy to us, which means that you're looking back in time when you're looking at these objects,” she said. “I found that concept so mind blowing that I decided at that instant that this was what I wanted to do with my life.”

The fast pace of discovery with James Webb is a lot like those early days of Hubble, Nelson said. At the time, many scientists believed that galaxies didn’t begin forming until billions of years after the Big Bang. But researchers soon discovered that the early universe was much more complex and exciting than they could have imagined.

“Even though we learned our lesson already from Hubble, we still didn’t expect James Webb to see such mature galaxies existing so far back in time,” Nelson said. “I’m so excited.”

Read more at Science Daily

Discovery of massive early galaxies defies prior understanding of the universe

Six massive galaxies discovered in the early universe are upending what scientists previously understood about the origins of galaxies in the universe.

"These objects are way more massive? than anyone expected," said Joel Leja, assistant professor of astronomy and astrophysics at Penn State, who modeled light from these galaxies. "We expected only to find tiny, young, baby galaxies at this point in time, but we've discovered galaxies as mature as our own in what was previously understood to be the dawn of the universe."

Using the first dataset released from NASA's James Webb Space Telescope, the international team of scientists discovered objects as mature as the Milky Way when the universe was only 3% of its current age, about 500-700 million years after the Big Bang. The telescope is equipped with infrared-sensing instruments capable of detecting light that was emitted by the most ancient stars and galaxies. Essentially, the telescope allows scientists to see back in time roughly 13.5 billion years, near the beginning of the universe as we know it, Leja explained.

"This is our first glimpse back this far, so it's important that we keep an open mind about what we are seeing," Leja said. "While the data indicates they are likely galaxies, I think there is a real possibility that a few of these objects turn out to be obscured supermassive black holes. Regardless, the amount of mass we discovered means that the known mass in stars at this period of our universe is up to 100 times greater than we had previously thought. Even if we cut the sample in half, this is still an astounding change."

In a paper published today (Feb. 22) in Nature, the researchers show evidence that the six galaxies are far more massive than anyone expected and call into question what scientists previously understood about galaxy formation at the very beginning of the universe.

"The revelation that massive galaxy formation began extremely early in the history of the universe upends what many of us had thought was settled science," said Leja. "We've been informally calling these objects 'universe breakers' -- and they have been living up to their name so far."

Leja explained that the galaxies the team discovered are so massive that they are in tension with 99% percent of models for cosmology. Accounting for such a high amount of mass would require either altering the models for cosmology or revising the scientific understanding of galaxy formation in the early universe -- that galaxies started as small clouds of stars and dust that gradually grew larger over time. Either scenario requires a fundamental shift in our understanding of how the universe came to be, he added.

"We looked into the very early universe for the first time and had no idea what we were going to find," Leja said. "It turns out we found something so unexpected it actually creates problems for science. It calls the whole picture of early galaxy formation into question."

On July 12, NASA released the first full-color images and spectroscopic data from the James Webb Space Telescope. The largest infrared telescope in space, Webb was designed to see the genesis of the cosmos, its high resolution allowing it to view objects too old, distant or faint for the Hubble Space Telescope.

"When we got the data, everyone just started diving in and these massive things popped out really fast," Leja said. "We started doing the modeling and tried to figure out what they were, because they were so big and bright. My first thought was we had made a mistake and we would just find it and move on with our lives. But we have yet to find that mistake, despite a lot of trying."

Leja explained that one way to confirm the team's finding and alleviate any remaining concerns would be to take a spectrum image of the massive galaxies. That would provide the team data on the true distances, and also the gasses and other elements that made up the galaxies. The team could then use the data to model a clearer of picture of what the galaxies looked like, and how massive they truly were.

"A spectrum will immediately tell us whether or not these things are real," Leja said. "It will show us how big they are, how far away they are. What's funny is we have all these things we hope to learn from James Webb and this was nowhere near the top of the list. We've found something we never thought to ask the universe -- and it happened way faster than I thought, but here we are."

Read more at Science Daily

Hubble finds early, massive galaxies running on empty

When the universe was about 3 billion years old, just 20% of its current age, it experienced the most prolific period of star birth in its history. But when NASA's Hubble Space Telescope and the Atacama Large Millimeter/submillimeter Array (ALMA) in northern Chile gazed toward cosmic objects in this period, they found something odd: six early, massive, "dead" galaxies that had run out of the cold hydrogen gas needed to make stars.

Without more fuel for star formation, these galaxies were literally running on empty. The findings are published in the journal Nature.

"At this point in our universe, all galaxies should be forming lots of stars. It's the peak epoch of star formation," explained lead author Kate Whitaker, assistant professor of astronomy at the University of Massachusetts, Amherst. Whitaker is also associate faculty at the Cosmic Dawn Center in Copenhagen, Denmark. "So what happened to all the cold gas in these galaxies so early on?"

This study is a classic example of the harmony between Hubble and ALMA observations. Hubble pinpointed where in the galaxies the stars exist, showing where they formed in the past. By detecting the cold dust that serves as a proxy for the cold hydrogen gas, ALMA showed astronomers where stars could form in the future if enough fuel were present.

Using Nature's Own Telescopes

The study of these early, distant, dead galaxies was part of the appropriately named REQUIEM program, which stands for Resolving QUIEscent Magnified Galaxies At High Redshift. (Redshift happens when light is stretched by the expansion of space and appears shifted toward the red part of the spectrum. The farther away a galaxy is with respect to the observer, the redder it appears.)

The REQUIEM team uses extremely massive foreground galaxy clusters as natural telescopes. The immense gravity of a galaxy cluster warps space, bending and magnifying light from background objects. When an early, massive, and very distant galaxy is positioned behind such a cluster, it appears greatly stretched and magnified, allowing astronomers to study details that would otherwise be impossible to see. This is called "strong gravitational lensing."

Only by combining the exquisite resolution of Hubble and ALMA with this strong lensing was the REQUIEM team able to able to understand the formation of these six galaxies, which appear as they did only a few billion years after the big bang.

"By using strong gravitational lensing as a natural telescope, we can find the distant, most massive, and first galaxies to shut down their star formation," said Whitaker. "I like to think about it like doing science of the 2030s or 40s -- with powerful next-generation space telescopes -- but today instead by combining the capabilities of Hubble and ALMA, which are boosted by strong lensing."

"REQUIEM pulled together the largest sample to date of these rare, strong-lensed, dead galaxies in the early universe, and strong lensing is the key here," said Mohammad Akhshik, principal investigator of the Hubble observing program. "It amplifies the light across all wavelengths so that it's easier to detect, and you also get higher spatial resolution when you have these galaxies stretched across the sky. You can essentially see inside of them at much finer physical scales to figure out what's happening."

Live Fast, Die Young

These sorts of dead galaxies don't appear to rejuvenate, even through later minor mergers and accretions of nearby, small galaxies and gas. Gobbling up things around them mostly just "puffs up" the galaxies. If star formation does turn back on, Whitaker described it as "a kind of a frosting." About 11 billion years later in the present-day universe, these formerly compact galaxies are thought to have evolved to be larger but are still dead in terms of any new star formation.

These six galaxies lived fast and furious lives, creating their stars in a remarkably short time. Why they shut down star formation so early is still a puzzle.

Whitaker proposes several possible explanations: "Did a supermassive black hole in the galaxy's center turn on and heat up all the gas? If so, the gas could still be there, but now it's hot. Or it could have been expelled and now it's being prevented from accreting back onto the galaxy. Or did the galaxy just use it all up, and the supply is cut off? These are some of the open questions that we'll continue to explore with new observations down the road."

Read more at Science Daily