Researchers Discover the Secret Behind a Long-held Mystery of Galactic Evolution

Some galaxies eventually hit a point in their lives when they're no longer able to produce new stars. Called "quenched" galaxies, astronomers have long puzzled over why those of the distant past appear much smaller than today's seeing as star formation is no longer present.

The answer, according to a new study based on observations from the Hubble COSMOS survey offers, is surprisingly simple, as well as contrary to the prevailing belief that it is the result of galactic collisions and mergers.

"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," co-author Simon Lilly of ETH Zurich said in a statement.

According to co-author Alvio Renzini of INAF Padua Observatory in Italy, the process is similar to one much closer to home.

"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," the scientist said.

Key to this surprisingly basic answer, according to the researchers, was the Hubble COSMOS survey. As the largest survey ever undertaken by the famous telescope, the project is designed to reveal new insights into the formation and evolution of galaxies. In this case, the survey enabled scientists to examine these postmenopausal galaxies going back 8 billion years in cosmic history.

"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," co-author Nick Scoville of Caltech said.

As a result, scientists now have a much clearer understanding of how galaxies have evolved over the last eight billion years of the universe's history.

"Our study offers a surprisingly simple and obvious explanation to this puzzle," Marcella Carollo also of of ETH Zurich concluded. "Whenever we see simplicity in nature amidst apparent complexity, it's very satisfying."