Abstract
The hot gas of clusters is a very useful plasma : it contains a significant proportion of baryons, and its thermal structure is the fossil record of the cluster's formation. An entropy floor exists, due to the non-gravitational energy released by supernovae, the active nuclei, during the life of the cluster. The X-raysalso provide a spectrum of lines that can be used to determine the abundance of heavy elements (iron). The hydrodynamics of the gas gives the distribution of dark matter, based on the equilibrium achieved. More than half of all clusters are relaxed, cold-core systems. The gas does not cool at the center of the cluster, as might have been expected from the higher density. But feedback from the active core heats it up at the center, and radio jets from the AGN carve out cavities in the very hot gas. These bubbles rise due to buoyancy, and the gas finally cools at the edges of the cavities, ~ 20 kpcfrom the center.
The problem of cooling streams, the cold gas that was expected to form stars, and which has not yet been observed, is about to be solved. The gas is indeed cooling, but 10 times less than expected, due to feedback phenomena from the central AGN.
On the other hand, clusters are constantly undergoing collisions and interactions with other groups/clusters. Their outer shapes are often those of spiral arms, or spiral lobes. In addition to the internal energy generated by AGNfeedback and the bubbling of radio cocoons, the hot gas is constantly being stirred, and metallicity can be high at the edges. The gas cools as a result of thermal instability, as soon as the cooling time equals 10 times the free-fall time. Cold molecular gas can thus contain metals and become carbon monoxide (CO). In the Perseus cluster, substantial quantities of molecular gas (1010 Mo) have been detected, as well as heated dust (far-infrared radiation detected by Herschel).
