Astronomers confirmed the longest-known tail of superheated gas behind a galaxy group

Galaxies go on a deep dive and leave fiery tail behind.


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Cosmological simulations of structure formation predict that galaxy clusters continue to grow and evolve through ongoing mergers with group-scale systems. During these merging events, the ram pressure applied by the intracluster medium strips the gas from the infalling groups, forming large tails of stripped gas.

Recently, scientists presented a detailed analysis of the galaxy group NGC 4839 using deep Chandra observations. The observations confirmed the longest known tail of superheated gas behind a galaxy group- plunging into the Coma galaxy cluster.

This tail is, in fact, 1.5 million light-years long, or hundreds of thousands of times the distance between the Sun and the nearest star, making it the most extended tail ever seen trailing behind a group of galaxies.

The observations help astronomers better understand how galaxy clusters grow to their enormous sizes.

Located near the edge of the Coma galaxy cluster, NGC 4839 is one of the largest known clusters in the universe, about 340 million light-years away. The hot gas in NGC 4839’s galaxy group is taken away by its collision with gas in the Coma cluster as it advances toward its center. As a result, a tail develops in the back of the galaxy group.

The image on the left combines optical data from the Sloan Digital Sky Survey (yellow) with an X-ray view of the Coma galaxy cluster captured by the European Space Agency’s XMM-Newton (blue). In the lower right corner of that image is the galaxy group NGC 4839. The Chandra image (purple) of the area enclosed by the square can be found in the inset to the right. The brightest galaxy in the cluster and the densest gas are found in NGC 4839’s head, which is on the left side of the Chandra image. The tail extends rightward.

The hot gas in the outer parts of the Coma cluster, through which NGC 4839 is moving, emits X-rays that are too faint to be observed in the XMM image displayed here but are visible in an additional XMM-only image. This mosaic of photos reveals dark holes where point X-ray sources were eliminated and gaps between individual images when data could not be collected.

Scientists also uncovered a shock wave while analyzing the gas in front of the galaxy group; the researchers found a shock wave –– similar to a sonic boom from a supersonic jet – showing that NGC 4839 is traveling at about 3 million miles per hour through the galaxy cluster.

They also investigated how much gas in the tail was turbulent. Turbulence is the term used to describe the erratic air movements in our atmosphere that can cause choppy airplane journeys. They discovered only a little turbulence, which suggests that NGC 4839 has poor heat conduction.

On one side of the tail, the scientists might have detected Kelvin-Helmholtz’s instabilities and unique formations. These structures are discovered by scientists in various environments in space and on Earth, including cloud formations. They result from variations in the speed of nearby flowing gas or fluid layers. The gas in the tail of NGC 4839 has a weak magnetic field or a low degree of viscosity because Kelvin-Helmholtz instabilities are present there. (Water)

Journal Reference:

  1. M. S. Mirakho3, S. A Walker nd J. Runge. A deep dive: Chandra observations of the NGC 4839 group falling into the Coma cluster. Astrophysics of Galaxies. DOi: 10.48550/arXiv.2304.05419