Anne’s Image of the Day: The Coma Cluster of Galaxies

November 15, 2013

The Coma Cluster of Galaxies in Coma Berenices

Image Credit & Copyright: Adam Block/Mount Lemmon SkyCenter/University of Arizona (http://skycenter.arizona.edu/gallery/Galaxies/ComaCluster)

The Coma Cluster (also known as Abell 1656) is a large cluster of galaxies that contains over 1,000 identified galaxies in a region of about 2 million light-years across, located around 321 million light-years away from Earth in the constellation of Coma Berenices (Berenice’s Hair), while it is speeding away from us at roughly 7,000 kilometers (4,300 miles) per second.

The cluster is within a few degrees of the north galactic pole on the sky. Along with the Leo Cluster (Abell 1367) which contains about 100 galaxies, the Coma Cluster is one of the two galaxy clusters comprising the Coma Supercluster or Local Supercluster, the nearest massive cluster of galaxies to our own Virgo Supercluster.

As is usual for clusters of this richness, the galaxies in the Coma Cluster are overwhelmingly elliptical and lenticular galaxies (an intermediate between an elliptical galaxy and a spiral galaxy), with only a few spiral galaxies of younger age, and many of them near the outskirts of the cluster. Most of the galaxies that inhabit the central portion of the Coma Cluster are ellipticals. Both dwarf and giant ellipticals are found in abundance in the Coma Cluster.

The central region is dominated by two giant elliptical galaxies: NGC 4889 (over 250,000 light-years across) is the brightest galaxy within the Coma Cluster and is known for containing the largest known supermassive black hole, and NGC 4874 (about ten times larger than the Milky Way). In this image, NGC 4889 is the bright galaxy to the left, with NGC 4874 at the right. The bright star on the upper right is HD 112887, a foreground star which lies at less than a millionth of the distance to the Coma Cluster.

Recently, long X-ray arms of hot, multimillion-degree gas have been discovered in the Coma Cluster. These arms, that are estimated to be about 300 million years old, span at least a half a million light-years. They were likely formed by hot gas being stripped and left behind by smaller clusters of galaxies as they merged with the Coma Cluster. NGC 4889 and NGC 4874 are probably the remnants from each of the two largest clusters that merged with the Coma Cluster in the past.

The Coma Cluster is one of the first places where observed gravitational anomalies were considered to be indicative of unobserved mass. In 1933 Fritz Zwicky showed that the galaxies of the Coma Cluster were moving too fast for the cluster to be bound together by the visible matter of its galaxies, though the idea of dark matter would not be accepted for another fifty years. About 90% of the mass of the Coma cluster is currently believed to be dark matter.

A strong X-ray source was observed close to the center of the Coma cluster and this source was suggested be designated Coma X-1. As the source is extended, a single galaxy is not likely to be responsible for the emission. Until now, it is unknown what is.

This image was taken in February 2012 with a SBIG STX16803 CCD Camera on the 32-inch Schulman Telescope (RCOS) at the Mount Lemmon SkyCenter.