Monster Galaxies' Appetites Diminish With Age: Study

Our universe consists of a large number of galaxies that are held together by gravity to form bigger families called clusters. Within these clusters are the monster galaxies that increase in size by eating the small galaxies surrounding them, and this process is referred to as galactic cannibalism.

The formation of galaxies is extremely chaotic and it takes billions of years for the scientists to watch a galaxy grow. Our own Milky Way also practices galactic cannibalism by merging with the smaller galaxies that are close to it..

But a latest study contradicts these previous findings by stating that monster galaxies lose their appetite with age. The new finding provides researchers with information on how galaxy clusters form and evolve.

The new study based on data from NASA's Spitzer Space Telescope and Wide-Field Infrared Survey Explorer (WISE) opposes the previous findings by stating that these gargantuan galaxies appear to slow their growth over time, feeding less and less off neighboring galaxies, according to a press release.

According to Yen-Ting Lin of the Academia Sinica in Taipei, Taiwan, and the study lead, these large monster galaxies were on a diet from the last 5 billion years and that's the reason why they haven't gained a lot of mass in the present times.

Peter Eisenhardt, co-author from NASA's Jet Propulsion Laboratory in Pasadena, Calif., said, "WISE and Spitzer are letting us see that there is a lot we do understand -- but also a lot we don't understand -- about the mass of the most massive galaxies."

As mentioned above, a galaxy cluster consists of thousands of galaxies surrounding the biggest member that is referred to as the brightest cluster galaxy (BCG) by astronomers. BCG's can grow to a dozen of times the mass of our Milky Way. They get enormous in size by snacking on the other galaxies. They even take in stars that are directed into the centre of a growing cluster.

In order to understand this process, nearly 300 galaxy clusters were surveyed by astronomers across 9 billion years of cosmic time. The farthest cluster was formed when the universe was 4.3 billion years old and the closet one was formed when the universe was 13 billion years old.

Since the scientists can't watch a galaxy grow, they worked on a population census. With the new approach the researchers were able to link the average properties of the clusters observed in the recent past with the ones that were formed earlier in the universe.

They noticed that the growth of the BCD continued along the rates that were predicted by theories until 5 billion years ago i.e. the time when the universe was 8 billion years old. But after this period most of the galaxies stopped munching on the surrounding galaxies.  The scientists have no clue regarding BCD's gradual loss of appetite.

"BCGs are a bit like blue whales -- both are gigantic and very rare in number. 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," said Lin. 

The researchers also state that the survey could be missing the large stars that are present in more fully grown clusters. If  these observations failed to detect those stars, it is possible that the enormous galaxies are still bulking up. 

The study was published in the Astrophysical Journal.