Massive Galaxy Strips Star-Forming Gas from Smaller Galaxy during Encounter

Massive Galaxy Strips Star-Forming Gas from Smaller Galaxy during Encounter

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) and the NASA/ESA Hubble Space Telescope have observed SDSS J144845.91+101010.5, a newly-dormant massive galaxy that was born when the Universe was at roughly half of its current age, and is nearing the completion of a merger with another galaxy; during the course of this encounter, the force of gravity flung what amounts to nearly half of the system’s star-forming gas away from the galaxy, leaving it unable to form new stars.

This artist’s conception shows the stream of gas and stars that were flung away from a massive galaxy during its merger with a smaller galaxy. Image credit: ALMA / ESO / NAOJ / NRAO / S. Dagnello, NRAO, AUI & NSF.

This artist’s conception shows the stream of gas and stars that were flung away from a massive galaxy during its merger with a smaller galaxy. Image credit: ALMA / ESO / NAOJ / NRAO / S. Dagnello, NRAO, AUI & NSF.

As galaxies move through the Universe, they sometimes encounter other galaxies. As they interact, each galaxy’s gravity pulls on the other.

The ensuing tug-of-war flings gas and stars away from the galaxies, leaving behind streams of material known as tidal tails.

And that’s just what Texas A&M University astronomer Justin Spilker and colleagues believe happened to SDSS J144845.91+101010.5, but with a plot twist.

The massive galaxy, which was born when the Universe was about half its current age, has nearly completed merging with another galaxy.

Using ALMA and Hubble data, the astronomers discovered tidal tails containing roughly half of the entire system’s cold, star-forming gas — equal to 10 billion times the mass of Earth’s Sun.

“What initially made this massive galaxy interesting was that, for some reason, it suddenly stopped forming stars about 70 million years ago immediately following a burst of star-forming activity,” Dr. Spilker said.

“Most galaxies are happy to just keep forming stars.”

“Our observations with ALMA and Hubble proved that the real reason the galaxy stopped forming stars is that 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.”

This composite image combines blue/white data from Hubble and red/orange data from ALMA to show the post-merger distribution of gas and stars from SDSS J144845.91+101010.5 into streams of material known as tidal tails. Image credit: ALMA / ESO / NAOJ / NRAO / Spilker et al. / S. Dagnello, NRAO, AUI & NSF.

This composite image combines blue/white data from Hubble and red/orange data from ALMA to show the post-merger distribution of gas and stars from SDSS J144845.91+101010.5 into streams of material known as tidal tails. Image credit: ALMA / ESO / NAOJ / NRAO / Spilker et al. / S. Dagnello, NRAO, AUI & NSF.

“When we look out at the Universe, we see some galaxies that are actively forming new stars, like our own Milky Way, and some that aren’t,” said Dr. Wren Suess, an astronomer at the University of California Santa Cruz.

“But those ‘dead’ galaxies have many old stars in them, so they must have formed all of those stars at some point and then stopped making new ones.”

“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.”

Because the new result is from a single observation, it is currently unclear just how common this tug-of-war and its resultant quiescence may be.

“While it’s pretty clear from this system that cold gas really can end up way outside of a merger system that shuts off a galaxy, the sample size of one galaxy tells us very little about how common this process is,” said David Setton, a graduate student at the University of Pittsburgh.

“But, there are many galaxies out there like SDSS J144845.91+101010.5 that we’re able to catch right in the middle of those crashes and study exactly what happens to them when they go through that stage.”

“The ejection of cold gas is an exciting new piece of the quiescence puzzle, and we’re excited to try to find more examples of this.”

The study is published in the Astrophysical Journal Letters.

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Justin S. Spilker et al. 2022. Star Formation Suppression by Tidal Removal of Cold Molecular Gas from an Intermediate-redshift Massive Post-starburst Galaxy. ApJL 936, L11; doi: 10.3847/2041-8213/ac75ea

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