The monstrous galaxies that lie at the center of galaxy clusters are not the voracious predators scientists once stereotyped them as, according to new research from NASA's Spitzer Space Telescope.
Fed through merging with neighboring galaxies, a process called galactic cannibalism, astronomers have used the help of Spitzer's Wide-field Infrared Survey Explorer (WISE) to discover that these massive space bodies actually grow relatively slowly over time, feeding less and less off neighboring galaxies.
"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.
According to the team of researchers, the new findings will help scientists understand how galaxy clusters -- among the most massive structures in our universe -- form and evolve.
Galaxy clusters are made up of thousands of galaxies that gather around the group's largest, called the brightest cluster galaxy, or BCG. BCGs can be up to dozens of times the mass of galaxies like our own Milky Way, growing not only as they devour their peers, but through assimilating stars that are funneled into the middle of a growing cluster as well.
In order to better understand this process, the team led by Lin surveyed nearly 300 galaxy clusters spanning 9 billion years of cosmic time.
"You can't watch a galaxy grow, so we took a population census," Lin said. "Our new approach allows us to connect the average properties of clusters we observe in the relatively recent past with ones we observe further back in the history of the universe."
The findings revealed that BCG growth proceeded along rates predicted by theories until 5 billion years ago when the universe was about 8 billion years old. At this point, the data suggest that most galaxies engaged in some kind of truce, ceasing to munch on others around them.
"BCGs are a bit like blue whales -- both are gigantic and very rare in number," Lin said. "Our census of the population of BCGs is in a way similar to measuring how the whales gain their weight as they age. In our case, the whales aren't gaining as much weight as we thought. Our theories aren't matching what we observed, leading us to new questions."
Though, Lin admits the possibility remains that the surveys are missing large numbers of stars in the more mature clusters. Clusters can be violent environments. It's not unknown, for example, for stars to be stripped from colliding galaxies and flung into space. In the case that the recent observations are not detecting those stars, it's possible that the enormous galaxies are, in fact, continuing to bulk up.
Going forward, the researchers hope to reveal more about the feeding habits of one of nature's largest galactic species.