Colliding Galaxies in the constellation Coma Berenices. Depositphotos.

“War of Galaxies” Reveals Surprising Details About Galactic Mergers

Two massive galaxies, about half as old as the universe are set to collide within the next few years. Currently, a tug-of-war is taking place between the two, as gravitational forces pull each galaxy towards the other.

Researchers found that a newly-dormant galaxy stopped forming stars not because it had exhausted all of its gas but because most of its star-forming fuel had been thrown out as the galaxy merged with another. This was discovered by observing the galaxy with the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope (HST). Moreover, if the results are confirmed to be common, they could change the way scientists view galaxy mergers and deaths. In the course of their motions through the universe, galaxies sometimes encounter other galaxies. Gravity pulls each galaxy towards the other as they interact. Tides tails are formed by streams of material that are flung away from galaxies during this tug-of-war.

In a nutshell, that is exactly what happened to SDSS J1448+1010. There is a growing likelihood that the massive galaxy, half as old as the universe, will merge with another galaxy within the next few hundred years. Scientists discovered tidal tails containing roughly half of the cold, star-forming gas in the system while working with the HST and ALMA, an international collaboration in which NRAO is a partner. Researchers discovered forcefully discarded material – equal to 10 billion times the mass of the sun – as evidence that the merger could have caused star formation to be halted.

This massive galaxy was initially fascinating because it suddenly stopped forming stars after a burst of star formation about 70 million years ago. Observers at Texas A&M University and the lead author of the paper Justin Spilker said most galaxies just want to keep forming stars. “Our observations with ALMA and Hubble proved that the galaxy stopped forming stars because 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.”

As a result of this discovery, scientists are gaining a better understanding of how galaxies live and die, as well as how they evolve. A look at the universe shows that some galaxies are actively forming new stars and others aren’t. According to Wren Suess, a cosmology fellow at the University of California Santa Cruz and co-author of the paper, ‘dead‘ galaxies have many old stars in them. They must have made all of those stars at some point and then stopped making new ones,” he said. “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.”

There is currently no idea just how common this tug-of-war and its resultant quiescence may be since the new result is based on a single observation. Scientists now have a new challenge to find more examples of how star formation stops and galaxies die as a result of this discovery, which challenges long-held theories. According to David Setton, a graduate student in physics and astronomy at the University of Pittsburgh and a co-author of the paper, “it’s pretty obvious from this system that cold gas can actually end up outside of a merger system that shuts off a galaxy,” but this sample size does not tell us a lot about how common this process is.

A number of galaxies are out there, such as J1448+1010, that scientists can catch right in the middle of these crashes and examine in detail to determine what happens to them. In addition to finding more examples of the ejection of cold gas, scientists are eager to investigate more of the quiescence puzzle.

“Astronomers used to think galaxies could only stop forming stars through violent, fast processes, such as supernovae exploding in the galaxy to blow most of the gas out and warm the rest of it up,” Spilker said. New observations show, however, that merging does not have to be a flashy thing to stop star formation. In fact, a slower merging process can effectively stop the formation of galaxies and stars as well.

The results of the research are published in The Astrophysical Journal Letters.


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Written by Ivan Petricevic

I've been writing passionately about ancient civilizations, history, alien life, and various other subjects for more than eight years. You may have seen me appear on Discovery Channel's What On Earth series, History Channel's Ancient Aliens, and Gaia's Ancient Civilizations among others.

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