Why Do Quenched Galaxies Seem To Keep Growing?
August 2, 2013

Hubble COSMOS Survey Helps Solve Mystery Of Quenched Galaxy Growth

redOrbit Staff & Wire Reports - Your Universe Online

Thanks to new data obtained from the Hubble Cosmological Evolution Survey (COSMOS), researchers have solved the mystery as to why some galaxies appear to grow larger even after they no longer form new stars.

Once galaxies reach a point in their lives when they cease star formation, they are known as "quenched" galaxies, report the authors of the new study. Quenched galaxies in the distant past appear to be much smaller than those in the universe today - a phenomenon which has long puzzled the scientific community.

The answer is "surprisingly simple," the astronomers said in a statement. Scientists had been under the impression these quenched galaxies grew into the larger ones we witness nearby. Now, thanks to observations collected from the COSMOS survey (as well as the Canada-France-Hawaii and Subaru Telescopes in Hawaii), the investigators have learned this is not the case.

Using those instruments to take a peek back to when the Universe was less than half of its current age, the astronomers mapped an area of sky nearly nine times that of the full moon. They found the quenched galaxies at that time appeared to be small and compact, and they appeared to remain that way rather than growing by merging with other quenched galaxies over time.

Since the small galaxies mostly appear to keep the size they had when their star formation stopped, why does it seem like they are growing larger over time? According to co-author and ETH Zurich researcher Simon Lilly, "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."

"It's like saying that the increase in the average apartment size in a city is not due to the addition of new rooms to old buildings, but rather to the construction of new, larger apartments," added co-author Alvio Renzini of Italy's INAF Padua Observatory.

Lilly, Renzini and their colleagues report their findings, which have been presented for publication in The Astrophysical Journal, provide new information about how galaxies have evolved over the last eight billion years of the universe's history.

It had already been established those galaxies that were actively forming stars were smaller in the early universe, they noted, explaining why they were smaller when their star formation first ceased.

"No single collection of images has been large enough to enable us to study very large numbers of galaxies in exactly the same way - until Hubble's COSMOS," said co-author Nick Scoville of the California Institute of Technology (Caltech).

Likewise, his Caltech colleague and fellow co-author, Dr. Peter Capak, said COSMOS, "provided us with simply the best set of observations for this sort of work - it lets us study very large numbers of galaxies in exactly the same way, which hasn't been possible before."

"The apparent puffing up of quenched galaxies has been one of the biggest puzzles about galaxy evolution for many years," added lead author Marcella Carollo of ETH Zurich. "Our study offers a surprisingly simple and obvious explanation to this puzzle. Whenever we see simplicity in nature amidst apparent complexity, it's very satisfying."