ALMA witnesses a deadly showdown between merging galaxies

By observing a recently dormant galaxy using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope (HST), researchers have discovered that it has stopped forming stars not because it had used up all its gas, but because most of its star-forming fuel had been ejected from the system when it merged with another galaxy. The result is a first for ALMA researchers. Additionally, if it turns out to be common, the findings could change the way scientists think about galaxy mergers and deaths. Research results are published in The Astrophysical Journal Letters.

As galaxies move through the universe, they sometimes encounter other galaxies. As they interact, each galaxy pulls on the other’s gravity. The ensuing standoff knocks gas and stars away from the galaxies, leaving streams of matter known as tidal tails.

And that’s exactly what scientists think happened to SDSS J1448+1010, but with a plot twist. The massive galaxy, which formed when the universe was about half its current age, has nearly completed its merger with another galaxy. During observations with HST and ALMA – an international collaboration in which the National Radio Astronomy Observatory (NRAO) of the United States National Science Foundation is a partner – the researchers discovered tidal tails that contained about half of the gas-cold star formation in the entire system. The discovery of the ejected material – equivalent to 10 billion times the mass of Earth’s Sun – was an indication that fusion could be responsible for stifling star formation, something the scientists did not expect.

“What originally made this massive galaxy interesting was that for some reason it suddenly stopped forming stars about 70 million years ago, immediately after a burst of star-forming activity. stars. Most galaxies are happy to keep forming stars,” said Justin Spilker, an astronomer at Texas A&M University and lead author of the paper. “Our observations with ALMA and Hubble have proven that the real reason the galaxy stopped forming stars, is that the fusion process ejected about half of the gaseous fuel needed for star formation into intergalactic space. Without fuel, the galaxy could not continue to form stars. »

This discovery sheds light on the processes by which galaxies live or die and helps scientists better understand their evolution.

“When we look at the universe, we see galaxies that are actively forming new stars, like our own Milky Way, and others that are not. But these ‘dead’ galaxies contain many old stars, so they must have formed all these stars on at some point and then stopped making new ones,” says University of California cosmology researcher Wren Suess. in Santa Cruz and co-author of the article. . “We still do not understand all the processes that prevent galaxies from forming stars, but this discovery shows how powerful these large galaxy mergers are and how much they can affect the growth and evolution of a galaxy over time. »

Since the new result comes from a single observation, it is currently unclear how common this fight and resulting silence might be. However, the discovery challenges long-held theories about exactly how star formation stops and galaxies die, and has given scientists an exciting new challenge: finding more examples.

“While it’s pretty clear from this system that cold gas can indeed end up well outside a galaxy-closing merger system, the sample size of a galaxy tells us very little about the frequency of this process,” said David Setton, a graduate student. in the Department of Physics and Astronomy at the University of Pittsburgh and co-author of the paper. “But there are many galaxies like J1448+1010 that we can catch right in the middle of these collisions and study exactly what happens to them as they pass through this phase. The ejection of cold gas is an exciting new piece of the quiescence puzzle, and we are excited to try and find more examples of that.”

Spilker added: “Astronomers used to think that the only way to stop galaxies from forming stars was through really violent and fast processes, like a bunch of supernovae exploding in the galaxy to kick most of the gas out of the galaxy and heat up . rest. . Our new observations show that it does not require a “rapid” process to cut off star formation. The much slower merger process can also halt star and galaxy formation. »

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Materials provided by National Radio Astronomy Observatory. Note: Content can be edited for style and length.

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