Merger may be responsible for snuffing out star formation in this massive galaxy

An international team of astronomers observing a newly-dormant galaxy have discovered that the galaxy had stopped forming stars because most of its star-forming fuel had been tossed out of the system as it merged with another galaxy.

Using the Atacama Large Millimeter/submillimeter Array (ALMA) and the Hubble Space Telescope (HST), the team was observing SDSS J1448+1010 - a massive galaxy, which was born when the Universe was about half its current age. When two galaxies meet, they tend to rip each other apart, leaving a trail, called a tidal tail, of gas and dust in their wake.

In this case, the team discovered tidal tails containing roughly half of the entire system's cold, star-forming gas. The discovery of the forcefully discarded material, equal to 10 billion times the mass of Earth's Sun, suggested that the merger may be responsible for snuffing out star formation, which was something unexpected.

"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," said Justin Spilker, an astronomer at Texas A&M University and the lead author of the paper published in The Astrophysical Journal Letters.

The discovery challenges long-held theories about exactly how star formation stops and galaxies die and also challenges scientists to find more such examples.

"Astronomers used to think that the only way to make galaxies stop forming stars was through really violent, fast processes, like a bunch of supernovae exploding in the galaxy to blow most of the gas out of the galaxy and heat up the rest. Our new observations show that it doesn't take a 'flashy' process to cut off star formation. The much slower merging process can also put an end to star formation and galaxies," Spiker added.