The circumgalactic medium (CGM) is the gas that lies outside the main stellar distribution of a galaxy, but inside its virial radius. The first part of our own galaxy’s CGM to be discovered was a population of high-velocity clouds, discovered through the 21 cm emission of their neutral hydrogen. The high-velocity clouds, however, are embedded within hotter components of the CGM, with temperatures ranging from 104 K to 106 K. These hotter components can be detected through absorption and emission lines of ionized metals such as oxygen. The intracluster medium (ICM) is the gas that lies inside the virial radius of a cluster of galaxies, but which is not associated with any individual galaxy. The ICM can be detected and studied through its free--free emission, which indicates temperatures as high as 108 K.
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