coma cluster of galaxies
September 20, 2013

Long Arms Of Hot Gas Discovered In The Coma Cluster

April Flowers for - Your Universe Online

In the Coma cluster of galaxies, a team of astronomers has discovered enormous arms of hot gas using NASA's Chandra X-ray Observatory and ESA's XMM-Newton. These arms span at least a half a million light years. The scientists say they will provide insight into how the Coma cluster has grown through mergers of smaller groups and clusters of galaxies to become one of the largest structures in the Universe held together by gravity.

A new composite image featuring these spectacular arms has been released with the Chandra data in pink and optical data from the Sloan Digital Sky Survey appearing in blue and white. The Chandra data has been processed in such a way that extra detail can be seen. The data gathered represents six days of observational time on Chandra, and the results were recently published in the journal Science.

Multimillion-degree gas causes the X-ray emission shown, and the optical data shows galaxies in the Coma Cluster that contain only approximately one-sixth the mass in hot gas. In the image, only the brightest X-ray emissions are shown to emphasize the arms, however, the hot gas is present over the entire field of view.

The arms were most likely formed, the research team says, when smaller galaxy clusters had their gas stripped away by the head wind created by the motion of the cluster through the hot gas. This is similar to the way that the headwind created by a roller coaster blows the hats off riders.

Coma contains not one, but two giant elliptical galaxies near its center, making it very unusual. The two giants are probably the vestigial remains from each of the two largest clusters that merged with Coma in the past. The data shows evidence of other past collisions and mergers as well.

The length, and the speed of sound in the hot gas - approximately 2.5 million miles per hour - allow the scientists to estimate the age of the arms to be around 300 million years old. They appear to have a smooth shape, which gives researchers some clues about the conditions of the hot gas in Coma.

Most models predict that mergers between clusters like those in Coma will produce strong turbulence, like ocean water that has been churned by passing ships, however the smooth nature of these arms points to a rather calm setting for the hot gas in the Coma cluster, even after many mergers.

The small amount of turbulence visible in Coma is probably the result of large-scale magnetic fields. Astrophysicists have found estimating the amount of turbulence in a galaxy cluster challenging, with a range of conflicting answers being found, so observations of other clusters are needed.

Two of the arms in Coma appear to be attached to a group of galaxies located about two million light years from the center of Coma, with one, or both, of these arms connecting to a larger structure seen in the XMM-Newton data. The arms span a distance of at least 1.5 million light years. As evidence of gas being stripped from a single galaxy, a very thin tail appears behind one of the galaxies in Coma.