Gas Dynamics in Clusters of Galaxies

Abstract

One of the more surprising results from X-ray astronomy is that the great volumes of space between galaxies in clusters of galaxies are not empty, as they appear in optical images. Instead, they are filled with a diffuse, hot plasma, with typical temperatures of T ∼ 107−108 K. At this temperature, the sound speed in the gas is comparable to the orbit velocities of the galaxies in the cluster, which is consistent with the gas being in hydrostatic equilibrium with the same gravitational potential as binds the galaxies. This intracluster medium (ICM) is highly rarefied, with electron number densities of ne ∼ 10−4−10−2 cm−3. At least on large scales, the gas is stably stratified, with the density decreasing with increasing radius r. The gas extends out to distances of r ∼> Mpc from the cluster center. The total mass of hot gas is typically Mgas ∼ 10M ; this mass exceeds the total mass of all the galaxies in a typical rich clusters, although even more of the mass is in the form of unseen “dark matter.” At temperatures of 106−108 K, the dominant radiation mechanism of a plasma is X-ray emission. As a result, clusters of galaxies are generally very luminous X-ray emitters, with luminosities of LX ∼ 1043− 10 ergs s−1. Clusters are second only to quasars as the most luminous X-ray sources in the Universe, and are the most luminous extended sources. While X-ray emission is the primary observational diagnostic for the intracluster medium, the ICM has a number of other important physical effects. It confines and distorts radio galaxies within the cluster. The cosmic ray and magnetic field components of the intracluster medium can also produce diffuse radio emission (see Feretti & Giovannini this volume). The ICM can strip interstellar gas from galaxies as they move through the cluster. Intracluster gas cools at the centers of many clusters, producing lower temperature gas. If the ICM contains dust, the dust will be strongly heated by the plasma, and may emit strongly in the infrared. The ICM also has a number of opacity effects; for example, it scatters and