Destruction of Cool Core Suspected at the Center of Massive Galaxy Cluster

May 5, 2015

The extent to which the cool, dense gas at the centers of massive galaxy clusters can be disrupted remains an outstanding question in astrophysics. Although physical processes such as mergers and central galaxy activity have been shown to suppress cooling and therefore star formation in the central gas, the cool core has almost always been observed to remain more or less intact. Recently, however, a team of KIPAC researchers has found the most extreme example yet of these processes disrupting the core.

The biggest reservoirs of dense hot gas in the universe are in of galaxy clusters, massive aggregations of galaxies, intergalactic gas, and dark matter. In clusters, the gas, which contains a mass several times larger than all of the stars, is heated to temperatures such that is glows in X-rays. At the clusters' centers, where the densities are high enough, the gas can give off its heat through the emission of photons giving rise to the phenomenon known as 'cooling core clusters', where the photon emission from the cooling centers is seen.

It has long been speculated that the reason that the cooling gas at the center of a cluster does not get cold enough to form stars is that it is periodically driven away or reheated by activity from the center of the cluster. The cool cores can also be mixed and displaced by the merging of two galaxy clusters, which often results in motion similar to sloshing in a wine glass, transporting the central gas outwards.

KIPAC researchers Steven Ehlert, Steve Allen, Anja von der Linden, Aurora Simionescu, Norbert Werner, Evan Million, Doug Applegate, Patrick Kelly, and Mark Allen,leading a team with colleagues from several other institutions, have analyzed data of the galaxy cluster MACS J1931.8-2634 in several different wavelengths. X-ray observations were carried out using the Chandra X-ray Observatory, optical observations were undertaken with the Subaru telescope in Hawaii, and radio observations were taken with the Very Large Array in New Mexico. These three instruments provided insight into the nature of the active central galaxy and the thermodynamic structure surrounding it.

What they found was clear evidence of both a past merger event stirring up the central gas and a very powerful outburst from the central galaxy. Most interestingly, however, was that the central regions of this cluster looked very different from what was expected from observations of similar clusters. The coolest gas was not observed at the location of the central galaxy as expected, but instead was displaced approximately 30 kpc away. Gas from the core also appears to be have been stripped away as it made its way from the central galaxy to its current location. Now that it is separated from the activity of the central galaxy, this gas is rapidly cooling and forming stars. The separation of the coolest gas and central galaxy seems be due to a combination of sloshing motion induced in the central core after the merger event and the activity of the central AGN itself. Both of these phenomena are seen in a large number of systems, but MACS J1931 is the first system that shows such profound disruption in the central regions.

This work is based on a paper to appear in Monthly Notices of the Royal Astronomical Society, which will soon be available from astro-ph (

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Steven Ehlert

Tidbit authors:  Steven Ehlert and Jack Singal