With the aid of the Hubble Space Telescope (HST) and the Atacama Large Millimeter/submillimeter Array (ALMA), researchers studied a newly-dormant galaxy and found that it had stopped forming stars not because it had exhausted all of its gas, but rather because the majority of its star-forming fuel had been ejected from the system as it merged with another galaxy. The outcome is a first for ALMA researchers.
Furthermore, if the findings are confirmed to be widespread, they may alter how astronomers view galaxy mergers and extinctions. The Astrophysical Journal Letters publishes the study’s findings.
As galaxies move through the Universe, they sometimes encounter other galaxies. The gravity of each galaxy pushes on the other as they interact. As a result of the tug-of-war that results, stars and gas are flung away from the galaxies, leaving behind streams of material known as tidal tails.
And that’s just what scientists believe happened to SDSS J1448+1010, but with a plot twist. The massive galaxy, which was born when the Universe was about half its current age, has nearly completed merging with another galaxy.
During observations with the HST and ALMA an international collaboration in which the U.S. National Science Foundation’s National Radio Astronomy Observatory (NRAO) is a partner scientists discovered tidal tails containing roughly half of the entire system’s cold, star-forming gas.
Astronomers used to think that the only way to make galaxies stop forming stars was through really violent, fast processes, like a bunch of supernovae exploding in the galaxy to blow most of the gas out of the galaxy and heat up the rest. Our new observations show that it doesn’t take a ‘flashy’ process to cut off star formation. The much slower merging process can also put an end to star formation and galaxies.
Justin Spilker
Scientists were surprised to learn that the merger may have stopped star formation as evidenced by the discovery of material that had been forcibly ejected with a mass 10 billion times that of the sun.
“What initially made this massive galaxy interesting was that, for some reason, it suddenly stopped forming stars about 70 million years ago immediately following a burst of star-forming activity. Most galaxies are happy to just keep forming stars,” said Justin Spilker, an astronomer at Texas A&M University and the lead author of the paper.
“Our observations with ALMA and Hubble proved that the real reason the galaxy stopped forming stars is that the merger process ejected about half the gas fuel for star formation into intergalactic space. With no fuel, the galaxy couldn’t keep forming stars.”
Scientists can now better comprehend the mechanisms that govern galaxies’ emergence and demise thanks to the discovery.
“When we look out at the Universe, we see some galaxies that are actively forming new stars, like our own Milky Way, and some that aren’t. But those ‘dead’ galaxies have many old stars in them, so they must have formed all of those stars at some point and then stopped making new ones,” said Wren Suess, a cosmology fellow at the University of California Santa Cruz and a co-author of the paper.
“We still don’t yet understand all of the processes that make galaxies stop forming stars, but this discovery shows just how powerful these major galaxy mergers are, and how much they can affect how a galaxy grows and changes over time.”
It is now unknown how frequent this tug-of-war and the quiescence that results from it may be because the new result is from a single observation. The discovery, nevertheless, raises intriguing new questions for researchers about how star creation ends and galaxies die, challenging established theories in the process.
“While it’s pretty clear from this system that cold gas really can end up way outside of a merger system that shuts off a galaxy, the sample size of one galaxy tells us very little about how common this process is,” said David Setton, a graduate student in the department of physics and astronomy at the University of Pittsburgh and a co-author of the paper.
“But, there are many galaxies out there like J1448+1010 that we’re able to catch right in the middle of those crashes and study exactly what happens to them when they go through that stage. The ejection of cold gas is an exciting new piece of the quiescence puzzle, and we’re excited to try to find more examples of this.” Spilker added,
“Astronomers used to think that the only way to make galaxies stop forming stars was through really violent, fast processes, like a bunch of supernovae exploding in the galaxy to blow most of the gas out of the galaxy and heat up the rest. Our new observations show that it doesn’t take a ‘flashy’ process to cut off star formation. The much slower merging process can also put an end to star formation and galaxies.”
