Dying Galaxies Shoot Out Gases That Boomerang Back, Slow Their Aging: Study

Scientists have observed a galaxy that appears to be recycling ejected gases in order to slow its dying process.

Until now, researchers had seen the phenomenon in simulations, but after observing the spiral galaxy NGC 4921 they may now have proof that it happens.

The scientists made the observations using the Atacama Large Millimeter/submillimeter Array (ALMA) observatory in Chile, and their results are due to be published in The Astrophysical Journal. The lead author is William Cramer, an astronomer at Arizona State University.

When a galaxy moves through space, it experiences an effect known as ram pressure stripping, in which gas gets stripped away from the galaxy by a sort of cosmic wind that exists in between it and other galaxies.

Galaxies need this gas in order to form new stars, so by losing enough of it galaxies begin to starve and run out of the fuel needed to remain active.

However, in order for this gas to escape the galaxy entirely it has to reach a certain speed, otherwise it will simply fall back into the galaxy it came from in a process known as re-accretion.

Scientists observing NGC 4921 found several clouds of molecular gas located away from the main gas ring of the galaxy that appeared to be falling back towards it.

A pre-print version of the study states: "Simulations have long predicted that some gas removed from the galaxy disk will fall back during ram pressure stripping.

"This may be the first clear observational evidence of gas re-accretion in a ram pressure stripped galaxy."

Understanding ram pressure stripping is important for scientists who want to predict how long it might take for a galaxy to stop forming new stars and die, Jeff Kenney, an astronomer at Yale University and co-author of the study, said in a statement.

"If you don't know that gas can fall back onto the galaxy and continue to recycle and form new stars, you're going to overpredict the quenching of the stars," he said.

NGC 4921 is the largest spiral galaxy in the Coma Cluster of more than 1,000 individual galaxies. It is roughly 320 million light-years away from Earth.

NASA states that NGC 4921 has been informally referred to as "anemic"—a medical term referring to a low number of red blood cells—because of its low rate of star formation.

According to Kenney, while some parts of the galaxy have been "almost completely cleaned out" by ram pressure, the process of re-accretion could mean gas is falling back into the galaxy in a number of locations.

More research is needed to determine just how much gas is being recycled and how many new stars are born as a result of this recycling process.

NGC 4921
NGC 4921 is seen in this photo released by NASA/ESA Hubble Space Telescope on February 5, 2009. Researchers have observed the galaxy to examine the effect of ram pressure stripping. AFP / Getty / NASA / ESA Hubble