Observations of a violent galactic merger between 6 and 7 billion years ago could help unravel the mystery of how galaxies “die” or stop forming stars.
The unusual features of the dead – or more accurately, “post-star-forming” – galaxy formed by this ancient collision may indicate that these cataclysms are expelling cold gas. Because this gas is a building block of star formation, its expulsion effectively “kills” what’s left of the galaxy, scientists say in a new study.
For some time now, astronomers have observed that some galaxies form stars at a high rate, while others, such as our own Milky Way, seem to have reduced star birth or almost completely stopped the process.
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However, why exactly this happens remains a mystery, although astronomers know that the cessation of star formation is associated with the loss of cold gas. Scientists have proposed possible reasons for this phenomenon, including that supermassive black holes or supernovae are “deflated” or heating gas so that it can no longer stick together, or that galaxies are simply running out of material for star formation.
“One of the biggest questions in astronomy is why the largest galaxies are dead,” David Setton, an astronomy doctoral student at the University of Pittsburgh and co-author of the new study, said in a statement. (will open in a new tab) Setton was part of a multidisciplinary team that studied this galaxy, and his role was to study the size and shape of the galaxy.
New observations of the merger and the tail of gas and dust it ripped out of the galaxy it created suggest that galactic collisions may play a role in stopping star formation.
“We saw that if you take two galaxies and push them together, it can actually rip gas out of the galaxy itself,” Setton said.
Reconstruction of the galactic “murder”
The Milky Way and other nearby galaxies have long since begun to slow down the production of stars. This means that to figure out what is causing these galaxies to “die,” astronomers need to look further into space – and therefore even further back in time – to detect galaxies that have only recently stopped forming stars.
To find an example of a galaxy with recently extinct star formation, the team used data from the Sloan Digital Sky Survey, an attempt to map millions of galaxies based on observations from the telescope at Apache Point Observatory in New Mexico.
The researchers combined this data with observations from the Atacama Large Millimeter/Submillimeter Array, which consists of 66 radio telescopes scattered across the Atacama Desert in northern Chile. This allowed the team to detect a “post-starburst” galaxy 6 to 7 billion light-years away that still showed signs of the presence of cold star-forming gas.
“So we needed an explanation,” Setton said. “If it has gas, why doesn’t it form stars?”
Taking another look at the galaxy with the Hubble Space Telescope, the team found a gaseous tail resulting from the merger. Using this unique structure, they reconstructed a violent event and intense gravitational forces that would rip gas and stars out of galaxies, hurling them hundreds of millions of light-years away—twice the width of the Milky Way.
“It was a smoking gun. We were all so blown away by it,” Setton said. “You just don’t see that much gas this far from the galaxy.” Such events can be quite frequent when gravity pulls large objects into tight clusters.
“There are all these big voids in space, but all the biggest galaxies live in places where all the other big galaxies live,” Setton said. “You would expect to see such large collisions once every 10 billion years or so for such a massive system.”
Once a galaxy’s massive tail breaks off, it becomes like any other dead galaxy, Setton added. This means that other galaxies that have recently ceased star formation may look like this upon closer inspection, a question the researchers intend to explore further.
The team’s research was published online in August. 30 in Astrophysical Journal Letters.
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