August 2, 2013
Old Age Makes Monster Galaxies Picky Eaters
Brett Smith for redOrbit.com - Your Universe Online
A new analysis from scientists at NASA's Jet Propulsion Laboratory (JPL) revealed that supermassive galaxies have stopped cannibalizing their neighbors over the past few billion years. The finding calls current models for galactic growth into question.
"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 study authors said their findings will help to refine models for the formation and evolution of galaxy clusters. These clusters are comprised of thousands of smaller galaxies that all swarm around the biggest member -- the brightest cluster galaxy, or BCG. BCGs can grow tens of times the size of the Milky Way by assimilating other galaxies and random stars that are funneled into the center of a growing cluster.
The JPL scientists said they used two very different infrared telescopes in the analysis, NASA's Spitzer Space Telescope and Wide-field Infrared Survey Explorer (WISE). Spitzer is able to create more detailed images, making it better at capturing the farthest clusters. On the other hand, WISE, an infrared all-sky survey, has a larger field of view and is better at seeing nearby galaxies. While Spitzer still has an ongoing mission, WISE was decommissioned in 2011 after scanning the sky twice.
"WISE and Spitzer are letting us see that there is a lot we do understand -- but also a lot we don't understand,â€ said study co-author Peter Eisenhardt, a JPL scientist.
Because the light from the most distant galaxies takes billions of years to reach Earth, the scientists examined almost 300 galaxy clusters that included 9 billion years of time. The farthest cluster in the analysis dated back to when theÂ universe was just 4.3 billion years old and the closest to when the universe was 13 billion years old; the universe is believed to be 13.8 billion years old.
"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 research team found that BCG growth followed established models until around 5 billion years ago, when it appears they stopped feasting on the galaxies around them. They said the reason for this 'diet' remains a mystery.
"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."
The study authors theorized that violent forces within these clusters may have forced out large numbers of stars. If this is happening, it may account for why these supermassive star clusters appear to have stopped growing.