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Galaxy Clusters & Cannibalism

 When looking at the universe at a macroscale, astronomers have noted numerous large groupings of galaxies, (each containing billions of stars) that all have a similar center of mass. Our own galaxy, the Milky Way, is a part of one such grouping of galaxies (containing an upward of 70 galaxies), commonly referred to as the local group. (our local group is a member of the Virgo Supercluster, its larger counterpart) Among the hundreds of known galaxy clusters, our local group is small peas compared to some of the others. Most are known to contain one colossally huge galaxy at the center. Said galaxies typically start out small, but become much larger by merging with their celestial neighbors (something we commonly refer to as galactic cannibalism). Eventually, they can acquire more mass than a dozen galaxies with a similar mass threshold as the Milky Way.

In the past, our theories said that the galaxy anchoring smaller members of the cluster should inevitably experience a drastic decrease in growth over time, but new research undertaken by NASA’s Spitzer Space Telescope (an infrared observatory) has reveled evidence to the contrary about the growth and evolution of these monster galaxies (known as “brightest cluster galaxy” [orBCG’s]).

In conjunction with Wide-field Infrared Survey Explorer (WISE), the Spitzer team recently surveyed nearly 300 galaxy clusters stretching out over 9 billion light-years. Among the participating clusters, the team uncovered a cluster that originated during a time in which the universe was hardly a billion years in age. (making it some 12 billion light-years away) Another cluster noted in the survey, the youngest, formed less than a billion years ago.

This image shows two of the galaxy clusters observed by NASA’s Wide-field Infrared Survey Explorer (WISE) and Spitzer Space Telescope missions. Galaxy clusters are among the most massive structures in the universe. The central and largest galaxy in each grouping, called the brightest cluster galaxy or BCG, is seen at the center of each image.

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Image credit: NASA/JPL-Caltech/SDSS/NOAO
Looking at two galaxies with vastly different ages gave the team an opportunity to take part in a population census, allowing them to study how (and if) the properties of the clusters change with age. In doing so, they discovered that the predicted growth of BGC’s began to deviate from our models after the universe was about 8 billion years old. After that period of time, the galaxies pretty much stopped absorbing smaller galaxies — an indication that our previous models need to be adjusted.

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 The cause for this deviation is uncertain, but perhaps there’s a simple explanation. We might be overlooking a huge percentage of the older, more evolved stars in the clusters involved in the survey. If so, this could account for the discrepancy in mass, which would mean that the galaxies are, in fact, still actively consuming their neighbors.

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