Tag Archives: galaxy evolution

Astronomers witness the ‘death’ of a galaxy

The process that causes the end of star formation in galaxies, their transition to an inactive phase and thus their figurative ‘death’ has been a puzzle for astronomers and astrophysicist for some time. Many researchers believe that ‘galactic death’ begins with the ejection of a massive quantity of gas, but thus far, researchers have failed to capture evidence of the escape of this star-forming fuel in such volumes. Thus the confirmation of how this transition to galactic quintessence occurs has also proved elusive.

Now an international team of astronomers have used the  Atacama Large Millimeter/submillimeter Array (ALMA) located in the desert region of Chile to spot a distant galaxy in which such a massive ejection of gas is progressing.

“Using ALMA we have discovered a distant galaxy, ID2299, which is ejecting about half of its cold gas reservoir out of the galaxy,” Annagrazia Puglisi, Centre for Extragalactic Astronomy, Durham University, lead researcher on the study, tells ZME Science. “This is the first time we have observed a typical massive star-forming galaxy in the distant Universe about to ‘die’ because of a massive cold gas ejection.”

This artist’s impression of ID2299 shows the galaxy, the product of a galactic collision, and some of its gas being ejected by a “tidal tail” as a result of the merger. New observations made with ALMA, in which ESO is a partner, have captured the earliest stages of this ejection, before the gas reached the very large scales depicted in this artist’s impression. (ESO/M. Kornmesser)
This artist’s impression of ID2299 shows the galaxy, the product of a galactic collision, and some of its gas being ejected by a “tidal tail” as a result of the merger. New observations made with ALMA, in which ESO is a partner, have captured the earliest stages of this ejection before the gas reached the very large scales depicted in this artist’s impression. (ESO/M. Kornmesser)

ID2299 is so distant that the light it emits takes 9 billion years to reach Earth, which means the team were able to observe it at a time when the universe was just 4.5 billion years old.

The rate of gas ejection that ID2299–a galaxy with a similar mass to the Milky way– is experiencing is equivalent to 10,000 Suns per year, removing an extraordinary 48% of its total cold gas content. In addition to this, the galaxy is still forming stars at a rapid rate, hundreds of times faster than the star formation rate of our own galaxy.

Puglisi explains that the gas ejection, together with a large amount of star formation in the nuclear regions of the galaxy, will eventually deprive the galaxy of the fuel need to make new stars.

“This would stop star formation in the object, effectively halting the galaxy’s development.”

Annagrazia Puglisi, Centre for Extragalactic Astronomy, Durham University

The team’s research, published in the latest edition of the journal Nature Astronomy, is significant because it represents three ‘firsts’ for astronomy. “This is the first time we observe a typical massive star-forming galaxy in the distant Universe about to ‘die’ because of a massive cold gas ejection,” explains Puglisi. “Also, for the first time, we were able to tell that massive gas ejection might be frequent enough to cause the cessation of star formation in a large number of massive distant galaxies. Finally, we were able to study the physical properties of the ejected gas in a distant galaxy.”

The researcher goes on to explain that these factors are important in the understanding of the triggering mechanism of the ejection– the galaxy’s distinct tidal tail.

Galactic Collisions and Tidal Tails

The research team that discovered ID2299 believe that it was created during a collision between two galaxies and their eventual merger. Ironically this process seems to have triggered the rapid gas loss that will eventually cause it to become inactive.

Another stunning example of a tidal tail is the ‘Tadpole’s Tail’ emerging from the galaxy Arp 188. This tail stretches a stunning 280 thousand light years and was caused by a gravitational interaction with another galaxy. (Hubble Legacy Archive/ NASA/ ESA)

“ID2299 is a galaxy with a large mass in stars and is forming new stars at a rate 300 times faster than our Galaxy– a result of the collision between two galaxies,” co-author Chiara Circosta, Department of Physics & Astronomy, University College London, tells ZME.

The main clue that points towards ID2299’s creation by collision is the fact its ejected gas has taken the form of a tidal tail. These elongated streams of stars and gas that reach into interstellar space are often too faint to see and are theorised to be the result of galactic mergers.

“Collisions between galaxies are very powerful and spectacular phenomena. During the interaction, tidal forces develop and can trigger ejection of gas through tidal tails,” says Circosta. “Our study suggests that these ejections could be frequent enough to stop the formation of new stars in a large number of massive galaxies in the distant Universe.

“Our research shows that these interactions can have an important role in the life-cycles of galaxies.

Chiara Circosta, Department of Physics & Astronomy, University College London


What makes the team’s findings even more impressive is the fact that it’s a discovery that occurred predominantly through good fortune.

Serendipity and a Series of Firsts

Because tidal tails of gas such as the one that the team observed being ejected from ID2299 are extremely faint and thus, difficult for astronomers to observe. In fact, the team weren’t looking for a galaxy like ID2299 at all.

“The discovery of this object was serendipitous. I was inspecting the spectra of 100 star-forming galaxies from the ALMA telescope,” says Puglisi, who goes on to explain that the spectrum of galaxy ID2299 immediately caught her attention as it displayed an excess of emission near the very prominent emission line from the galaxy. “I was very surprised when I measured the flux of this excess emission because it indicated that the galaxy was expelling a large amount of gas.

 “I was thrilled to discover such an exceptional galaxy! I was eager to learn more about this weird object because I was convinced that there was some important lesson to be learned about how distant galaxies evolve.

Annagrazia Puglisi, Centre for Extragalactic Astronomy, Durham University

The discovery of ID2299 sparked a discussion within the team about the mechanism that is causing the gas ejection of gas at such a rapid rate. They concluded that alternative mechanisms simply couldn’t account for ejection in such large amounts.

“We discussed a lot to understand what could have been the possible cause of this phenomenon. Broad components are fairly common in the spectra of distant galaxies and are typically associated with galactic winds,” says Puglisi. “Nor the active black hole nor the strong star formation hosted in ID2299 were powerful enough to produce this ejection.

“The numbers didn’t just add up.”

The ALMA antennas at the Llano Chajnantor–above them, the bright Milky Way is visible–played a vital role in the discovery of ID2299 and will now assist in the further investigation of gas movements in the galaxy (ESO/Y. Beletsky)

The next steps for the team are to use ALMA to make high-resolution observations of ID2299 and the motion of gas within it in order to better understand the gas ejection occurring there. Looking beyond this galaxy, Puglisi says she will also look for similar occurrences in other galaxies.

“I personally find quite fascinating the study of galaxy interactions and mergers. These phenomena are visually spectacular,” the researcher adds. “I find quite poetic that galaxies can get close to each other and influence their life and evolution so dramatically.”

The research the team presents could either overturn current theories that suggest star-forming material is actually ejected by the activity of supermassive black holes at the centre of galaxies or could provide another mechanism by which this can occur. Either way, the discovery represents a significant step forward in our understanding of how galaxies develop.

“I see galaxy evolution as a complex puzzle that researchers are trying to complete through their studies,” Circosta concludes. “A crucial part of the puzzle is about the mechanisms that halt the formation of new stars and ‘kill’ galaxies.

“Witnessing such a massive disruption event allowed us to shed new light on one of the possible culprits responsible for the death of distant galaxies. This adds an important piece to the puzzle of galaxy evolution!”

Chiara Circosta, Department of Physics & Astronomy, University College London

Original research:

Puglisi. A., Daddi. E., Brusa. M., et al, ‘A titanic interstellar medium ejection from a massive starburst galaxy at z=1.4,’ Nature Astronomy, [2021], [DOI: 10.1038/s41550-020-01268-x].

A UA-led team of scientists generated millions of different universes on a supercomputer, each of which obeyed different physical theories for how galaxies should form. (Image: NASA, ESA, and J. Lotz and the HFF Team/STScI)

Researchers simulate millions of virtual universes to study star formation

Researchers have turned to a massive supercomputer — dubbed the ‘UniverseMachine’ — to model the formation of stars and galaxies. In the process, they created a staggering 8 million ‘virtual universes’ with almost 10¹⁴ galaxies.

A UA-led team of scientists generated millions of different universes on a supercomputer, each of which obeyed different physical theories for how galaxies should form. (Image: NASA, ESA, and J. Lotz and the HFF Team/STScI)
A UA-led team of scientists generated millions of different universes on a supercomputer, each of which obeyed different physical theories for how galaxies should form. (Image: NASA, ESA, and J. Lotz and the HFF Team/STScI)

To say that the origins and evolution of galaxies and the stars they host have been an enigma that scientists have sought to explore for decades is the ultimate understatement.

In fact, desire to understand how the stars form and why they cluster the way they do, predates science, religion and possibly civilisation itself. As long as humans could think and reason — way before we knew what either a ‘star’ or a ‘galaxy’ was— we looked to the heavens with a desire to have knowledge of its nature.

We now know more than we ever have, but the heavens and their creation still hold mysteries for us. Observing real galaxies can only provide researchers with a ‘snapshot’ of how they appear at one moment. Time is simply too vast and we exist for far too brief a spell to observe galaxies as they evolve.

Now a team of researchers led by the University of Arizona have turned to supercomputer simulations to bring us closer to an answer for these most ancient of questions.

Astronomers have used such computer simulations for many years to develop and test models of galactic creation and evolution — but it only works for one galaxy at a time — thus failing to provide a more ‘universal’ picture.

To overcome this hurdle, Peter Behroozi, an assistant professor at the UA Steward Observatory, and his team generated millions of different universes on a supercomputer. Each universe was programmed to develop with a separate set of physical theories and parameters.

As such the team developed their own supercomputer — the UniverseMachine, as the researchers call it —to create a virtual ‘multiverse’ of over 8-million universes and at least 9.6 x 10¹³ galaxies.

The results could solve a longstanding quirk of galaxy-formation — why galaxies cease forming new stars when the raw material — hydrogen — is not yet exhausted.

The study seems to show that supermassive black holes, dark matter and supernovas are far less efficient at stemming star-formation than currently theorised.

The team’s findings — published in the journal Monthly Notices of the Royal Astronomical Society — challenges many of the current ideas science holds about galaxy formation. In particular, the results urge a rethink of how galaxies form, how they birth stars and the role of dark matter — the mysterious substance that makes up 80% of the universe’s matter content.

Behroozi, the study’s lead author. says: “On the computer, we can create many different universes and compare them to the actual one, and that lets us infer which rules lead to the one we see.”

What makes the study notable is it is the first time each universe simulated has contained 12 million galaxies, spanning a time period of 400 million years after the ‘big bang’ to the present day. As such, the researchers have succeeded in the creation of self-consistent universes which closely resemble our own.

Putting the multiverse to the test — how the universe is supposed to work

To compare each universe to the actual universe, each was put through a series of tests that evaluated the appearance of the simulated galaxies they host in comparison to those in the real universe.

Common theories of how galaxies form stars involve a complex interplay between cold gas collapsing under the effect of gravity into dense pockets giving rise to stars. As this occurs, other processes are acting to counteract star formation.

The Hubble Space Telescope took this image of Abell 370, a galaxy cluster 4 billion light-years from Earth. Several hundred galaxies are tied together by gravity. The arcs of blue light are distorted images of galaxies far behind the cluster, too faint for Hubble to see directly. (Image: NASA, ESA, and J. Lotz and the HFF Team/STScI)

For example, we believe that most galaxies harbour supermassive black holes in their centres. Matter forming accretion discs around these black holes and eventually being ‘fed’ into them, radiate tremendous energies. As such, these systems act almost as a ‘cosmic blowtorch’ heating gas and preventing it from cooling down enough to collapse into stellar nurseries.

Supernova explosions — the massive eruption of dying stars — also contribute to this process. In addition to this, dark matter provides most of the gravitational force acting on the visible matter in a galaxy — thus, pulling in cold gas from the galaxy’s surroundings and heating it up in the process.

Behroozi elaborates: “As we go back earlier and earlier in the universe, we would expect the dark matter to be denser, and therefore the gas to be getting hotter and hotter.

“This is bad for star formation, so we had thought that many galaxies in the early universe should have stopped forming stars a long time ago.”

But what the team found was the opposite.

Behroozi says: “Galaxies of a given size were more likely to form stars at a higher rate, contrary to the expectation.”

Bending the rules with bizarro universes

In order to match observations of actual galaxies, the team had to create virtual universes in which the opposite was the case — universes in which galaxies continued to birth stars for much longer.

Had the researchers created universes based on current theories of galaxy formation — universes in which the galaxies stopped forming stars early on — those galaxies appeared much redder than the galaxies we see in the sky.

Ancient galaxies such as z8_GND_5296 appear red for two reasons; the lack of young blue stars and the stretching in the wavelength of emitted light due to cosmic redshift. (V. Tilvi, Texas A&M University/S.L. Finkelstein, University of Texas at Austin/C. Papovich, Texas A&M University/CANDELS Team and Hubble Space Telescope/NASA)
Ancient galaxies such as z8_GND_5296 appear red for two reasons; the lack of young blue stars and the stretching in the wavelength of emitted light due to cosmic redshift. (V. Tilvi, Texas A&M University/S.L. Finkelstein, University of Texas at Austin/C. Papovich, Texas A&M University/CANDELS Team and Hubble Space Telescope/NASA)

Galaxies appear red for major two reasons. If the galaxy formed earlier in the history of the universe cosmic expansion — the Hubble flow — means that it will be moving away from us more rapidly, causing significant elongation in the wavelength of the light it emits shifting it to the red end of the electromagnetic spectrum. A process referred to as redshift.

In addition to this, another reason an older galaxy may appear red is intrinsic to that galaxy and not an outside effect like redshift. If a galaxy has stopped forming stars, it will contain fewer blue stars, which typically die out sooner, and therefore be left with older — redder — stars.

Behroozi point out that isn’t what the team saw in their simulations, however. He says: “If galaxies behaved as we thought and stopped forming stars earlier, our actual universe would be coloured all wrong.

“In other words, we are forced to conclude that galaxies formed stars more efficiently in the early times than we thought. And what this tells us is that the energy created by supermassive black holes and exploding stars is less efficient at stifling star formation than our theories predicted.”

Computing the multiverse is as difficult as it sounds

Creating mock universes of unprecedented complexity required an entirely new approach that was not limited by computing power and memory, and provided enough resolution to span the scales from the “small” — individual objects such as supernovae — to a sizeable chunk of the observable universe.

Behroozi explains the computing challenge the team had to overcome: “Simulating a single galaxy requires 10 to the 48th computing operations. All computers on Earth combined could not do this in a hundred years. So to just simulate a single galaxy, let alone 12 million, we had to do this differently.”

In addition to utilizing computing resources at NASA Ames Research Center and the Leibniz-Rechenzentrum in Garching, Germany, the team used the Ocelote supercomputer at the UA High-Performance Computing cluster.

Two-thousand processors crunched the data simultaneously over three weeks. Over the course of the research project, Behroozi and his colleagues generated more than 8 million universes.

He explains: “We took the past 20 years of astronomical observations and compared them to the millions of mock universes we generated.

“We pieced together thousands of pieces of information to see which ones matched. Did the universe we created look right? If not, we’d go back and make modifications, and check again.”

Behroozi and his colleagues now plan to expand the Universe Machine to include the morphology of individual galaxies and how their shapes evolve over time.

As such they stand to deepen our understanding of how the galaxies, stars and eventually, life came to be.


Original research:https://academic.oup.com/mnras/article/488/3/3143/5484868