Why do galaxies stop making stars? A huge collision in space provides new clues

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A torrent of gas was thrown hundreds of millions of light years out by the collision of two galaxies six billion years ago. This uncommon trait, reported this week by a team that included Pitt researchers, offers a fresh hypothesis as to why galaxies stop creating stars.

The giant galaxies are dead is "one of the major puzzles in astronomy," according to David Setton, a sixth-year Ph.D. student in physics and astronomy at the Kenneth P. Dietrich School of Arts and Sciences. We discovered that colliding two galaxies may really cause the galaxy's gas to be ripped out.

The majority of giant galaxies in the region of space we live in have long since stopped producing new stars. With the skills to discover recently deceased galaxies and understand how they became that way, astronomers have recently begun looking farther away—and hence farther back in time.

Black holes or supernovae are two ways the cold gas coalescing to produce stars might leave galaxies. Another, even more straightforward explanation is that galaxies stop expanding after they run out of resources to make new stars.

The study team utilized the Sloan Digital Sky Survey, which has cataloged millions of galaxies with a telescope at Apache Point Observatory in New Mexico, to look for examples of galaxies that recently stopped star creation. The researchers discovered a "post-starburst" galaxy seven billion light years away that nevertheless had signals of abundant star-forming fuel using images from the ground-based radio astronomy network ALMA. Then, Setton said, "We needed an explanation." "Why isn't it creating stars if it contains gas?"

The unique "tail" of gas emanating from the galaxy was later discovered by the Hubble Space Telescope after doing a second flyby. The researchers reconstructed the galaxies' collision and the powerful gravitational force that tore apart stars and threw a stream of gas a distance greater than two Milky Ways placed end-to-end from that spot, much like forensic investigators working through a telescope.

Setton declared, "That was the smoking gun." We were all very moved by it. This much gas is simply uncommon at this distance from the galaxy."

The team, which included Margaret Verrico (A&S '21), an alumna of Pitt Physics and Astronomy, Texas A&M University, and numerous other universities, published its findings in the Astrophysical Journal Letters on August 30.

According to Setton, a meeting of galaxies of this extreme magnitude is probably uncommon. Still, it happens more frequently than you might think since gravity attracts massive objects into tight clusters. Although there are several significant gaps in space, he said that all giant galaxies are found there. For an enormous system, "you expect to see these big collisions once every 10 billion years or so."

Aside from the tail, Setton's task in the study was establishing the galaxy's size and form. He found that the post-merger galaxy appeared pretty typical. It may resemble any other dead galaxy after the tail fades in a few hundred million years, further indicating that the process may be more frequent than it seems. The team is currently pursuing this idea with another scan.

According to Setton, such collisions depict one potential outcome for our own galaxy's future and offer hints as to how the universe came to be as it is.

In five billion years, the Andromeda Galaxy, seen if you go somewhere dark and stare up towards the night sky, may do exactly this to our Milky Way, according to Setton. The fundamental question of what will happen to the Milky Way in the future is being addressed by this research.


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