Astronomers Solve Mystery Behind 'Quenched' Galaxies

Galaxies are usually full of dust and gas, the raw materials that help them birth stars. Over time, though, these materials can be lost or used up, effectively snuffing out these galaxies and causing them to become "quenched." One aspect of these quenched galaxies has always puzzled astronomers: past quenched galaxies are far smaller than current ones. Now, researchers may have the answer to this riddle.

In the past, astronomers believed that these snuffed-out galaxies of the past grew into the larger quenched galaxies we see nearby. They thought that it was possible that they grew by colliding and merging with other smaller quenched galaxies that were five to ten times less massive. Yet these mergers would require many of these small galaxies floating around for the quenched galaxies to snack on--something that astronomers hadn't seen in their observations.

"The apparent puffing up of quenched galaxies has been one of the biggest puzzles about galaxy evolution for many years," said Marcella Carollo, one of the researchers, in a news release. Until now, no single collection of images had been large enough to allow astronomers to study very large numbers of galaxies in exactly the same way.

In order to investigate this phenomenon a bit further, the astronomers employed observations from the Hubble COSMOS survey. They identified and counted these quenched galaxies throughout the last eight billion years of cosmic history. They then used this data alongside additional observations from the Canada-France-Hawaii Telescope and the Subaru Telescope in order to peer back to when the universe was less than half its present age.

So what did they find? It turns out that small, compact quenched galaxies stayed that way; they didn't puff up and grow via mergers. How, then, do some of these galaxies seem to puff up?

"We found that a large number of the bigger galaxies instead switch off at later times, joining their smaller quenched siblings and giving the mistaken impression of individual galaxy growth over time," said Simon Lilly, co-author of the new study, in a news release.

The findings reveal a little bit more about how these galaxies evolved over the last eight billion years. In addition, it explains why actively star-forming galaxies were smaller when their star formation was switched off.

The findings are published in The Astrophysical Journal.