Violent mergers can help understand how galaxies stop producing stars

A study recently published in Letters from the Astrophysical Journal reports a discovery that could solve the mystery surrounding the “death” (when they stop producing stars) of galaxies.

This is thanks to observations of a violent galactic merger that took place billions of years ago. The unusual features of the result of this ancient collision may indicate that these catastrophic events eject the cold gas responsible for star formation in a process that eventually wipes out the galaxy.

Galactic collision: New research suggests that mergers like this could eject star-forming gas, saturating star production. Image: NASA Hyperwall

Scientists have suggested possible causes for this phenomenon, including supermassive black holes or supernovae “blowing” heat into the gas so it can no longer clump together, or galaxies simply decompressing, made of star-forming materials.

“One of the biggest questions in astronomy is why the biggest galaxies died,” said David Setton, a doctoral candidate in astronomy at the University of Pittsburgh in the US and co-author of the new research. , in a press release. Setton was part of a multi-agency team studying the galaxy, and his role was to study its size and shape.

“What we’ve seen is that if you take two galaxies and smash them together, it can actually extract gas from the resulting galaxy,” Setton said.

The Milky Way and other nearby galaxies began to slow down their star production long ago. This indicates that to find out what causes them to “die”, astronomers need to look further back in time (and further into space) to discover galaxies that have only recently stopped forming stars .

To search for an example of a galaxy with recently saturated star formation, the team used data from Sloan Digital Sky Surveyan attempt to map millions of galaxies based on observations from a telescope at the Apache Point Observatory in New Mexico.

The researchers combined this data with observations from the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 radio telescopes spread across the Atacama Desert in northern Chile.

Simulating the merger between galaxies

This allowed the team to discover a “post-merger” galaxy between 6 and 7 billion light-years away that still showed signs of having cold star-forming gas. “So we needed an explanation,” Setton said. “If it contains gas, why doesn’t it form stars?”

The team looked again with the Hubble Space Telescope and discovered a tail of gas created by the merger. Using this unique structure, they reconstructed the violent event and intense gravitational forces that would have ripped gas and stars from galaxies and flung them hundreds of millions of light-years away, twice the width of the Milky Way.

“That was the smoking gun. We were all very impressed with that,” Setton said. “You just don’t see that much gas this far out in the galaxy. Such events can be quite common when gravity pulls large objects into close groups.

“There are all these big voids in space, but all the biggest galaxies live in the spaces where all the big ones live,” Setton said. “One would expect to see these kinds of large collisions once every 10 billion years or so for such a massive system.”

Setton added that when the galaxy’s tail is separated, it looks like any other dead galaxy. This suggests that other stars that have recently stopped forming may look like this on closer inspection – a question the researchers intend to investigate further.

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