Gas emitted by merging galaxies is stopping stars from forming

Most stars in the universe today are found in massive galaxies called ellipticals, named for their stretched-out-circle shape. Unlike our own galaxy, which is a spiral with arms extending out from the center, the edges of elliptical galaxies are smooth.

At first glance, these galaxies might seem like simple systems. However, they are among the most mysterious objects in the cosmos. Elliptical galaxies host extremely old stars and are not forming new stars.

Exactly how these dead galaxies form is a question both observational and theoretical astronomers have tried to answer for a long time. Our new study, published in Nature Astronomy, could help solve this puzzle.

Stars form when huge clouds of gas within galaxies collapse under gravity. Eventually, enough mass clumps together, and a star is formed. But galaxies need gas for this to happen. Our team detected a galaxy, called ID2299, that is ejecting nearly half of its star-forming gas. The galaxy is throwing out the equivalent of 10,000 Suns per year in gas, removing 46% of the total cold gas the galaxy contains.

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Because the galaxy is also forming stars very rapidly, hundreds of times faster than our Milky Way, the gas that remains will be quickly consumed. At the current rate, ID2299 will shut down in just a few tens of million years. This is much faster than the typical duration of star formation episodes in galaxies, which is a few billion years.

Tidal tails

This exceptional massive ejection is being caused by a tidal tail, produced by the galaxy’s merger with another galaxy. Tidal tails are elongated streams of stars and gas extending into the interstellar space, as a result of tidal forces caused by the interaction – like the moon’s tidal pull on Earth.

Tidal tails are commonly seen in nearby merging galaxies, but it is difficult to identify them in the distant universe because of their low luminosity. Luckily, even though ID2299 was observed at a time when the universe was only 4.5 billion years old (our universe is now about 14 billion years old), we were able to see this ejection when it first started happening, when these tails are usually at their brightest.