The heating of gas in a galaxy cluster by X-ray cavities and large-scale shock fronts

Galactic clusters on heat Astronomers using the latest generation of orbiting X-ray observatories are making some surprising discoveries, one of which is the relatively slow rate of cooling of gas in the cores of galaxy clusters. The Chandra X-ray observatory is now routinely observing giant cavities in the centre of galaxy clusters, and these may hold the key to this phenomenon. The gaseous halo of the galaxy cluster MS0735.6+7421 has been found to contain two such cavities, caused by interaction between a radio source and the hot gas surrounding it. This is producing the most powerful radio outburst known, and sufficient heat to counteract the expected cooling. The most likely power source is a supermassive black hole. Most of the baryons in galaxy clusters reside between the galaxies in a hot, tenuous gas 1 . The densest gas in their centres should cool and accrete onto giant central galaxies at rates of 10–1,000 solar masses per year 1 . No viable repository for this gas, such as clouds or new stars, has been found 1 . New X-ray observations, however, have revealed far less cooling below X-ray temperatures than expected 2 , altering the previously accepted picture of cooling flows. As a result, most of the gas must be heated to and maintained at temperatures above ∼2 keV (ref. 3 ). The most promising heating mechanism is powerful radio jets emanating from supermassive black holes in the central galaxies of clusters 4 . Here we report the discovery of giant cavities and shock fronts in a distant ( z = 0.22) cluster caused by an interaction between a radio source and the hot gas surrounding it. The energy involved is ∼6 × 10 61 erg, the most powerful radio outburst known. This is enough energy to quench a cooling flow for several Gyr, and to provide ∼1/3 keV per particle of heat to the surrounding cluster.