Monster galaxies lose their appetite with age

Monster galaxies appear to slow their growth over time, feeding less and less off neighbouring galaxies, astronomers have found.

Galaxy clusters are made up of thousands of galaxies, gathered around their biggest member, what astronomers call the brightest cluster galaxy, or BCG.

BCGs can be up to dozens of times the mass of galaxies like our own Milky Way. They plump up in size by cannibalising other galaxies, as well as assimilating stars that are funnelled into the middle of a growing cluster.

New research from NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE) found that contrary to previous theories, these gargantuan galaxies appear to slow their growth over time.

"We've found that these massive galaxies may have started a diet in the last 5 billion years, and therefore have not gained much weight lately," said Yen-Ting Lin of the Academia Sinica in Taipei, Taiwan, lead author of a study published in the Astrophysical Journal.

The new findings will help researchers understand how galaxy clusters - among the most massive structures in our universe - form and evolve.

Astronomers surveyed nearly 300 galaxy clusters spanning 9 billion years of cosmic time. The farthest cluster dates back to a time when the universe was 4.3 billion years old, and the closest, when the universe was much older, 13 billion years old (our universe is presently 13.8 billion years old).

The findings showed that BCG growth proceeded along rates predicted by theories until 5 billion years ago, or a time when the universe was about 8 billion years old. After that time, it appears the galaxies, for the most part, stopped munching on other galaxies around them.

The scientists are uncertain about the cause of BCGs' diminished appetites, but the results suggest current models need tinkering.

A possible explanation is that the surveys are missing large numbers of stars in the more mature clusters. Clusters can be violent environments, where stars are stripped from colliding galaxies and flung into space.

If the recent observations are not detecting those stars, it's possible that the enormous galaxies are, in fact, continuing to bulk up, researchers said.