Tour: Galaxies Go on a Deep Dive and Leave Fiery Tail Behind

A group of galaxies is plunging into the Coma galaxy cluster and leaving behind an extraordinary tail of superheated gas. Astronomers have confirmed this is the longest known tail behind a galaxy group and used it to gain a deeper understanding of how galaxy clusters — some of the largest structures in the Universe — grow to their enormous sizes.

Astronomers trained NASA’s Chandra X-ray Observatory on the galaxy group NGC 4839. Galaxy groups are collections of about 50 galaxies or less that are bound together by gravity. Galaxy clusters are even larger and can contain hundreds or thousands of individual galaxies.

Both galaxy clusters and galaxy groups are enveloped by huge amounts of hot gas that are best studied using X-rays. These superheated pools of gas, though extremely thin and diffuse, represent a significant portion of the mass in galaxy groups or clusters and are crucial for understanding these systems.

NGC 4839 is located near the edge of the Coma galaxy cluster, one of the largest known clusters in the Universe about 340 million light-years away. As NGC 4839 moves toward the center of the Coma cluster, the hot gas in the galaxy group is stripped away by its collision with gas in the cluster. This results in a tail forming behind the galaxy group.

This comet-like tail is 1.5 million light-years long, or hundreds of thousands of times the distance between the Sun and the nearest star, making it the longest tail ever seen trailing behind a group of galaxies. This gas is a key ingredient in making future generations of stars and planets.

The current brightness of the tail gives astronomers a special chance to study the tail’s gas before it mixes in with the hot gas in the cluster and becomes too faint to study. The gas in the tail behind NGC 4839 will ultimately merge with the large amount of hot gas already present in the Coma Cluster.