Galaxy Halos'
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Introduction
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The stellar velocity dispersion in the halos of galaxies similar to the Milky Way exceeds 100 km s$^{-1}$. The gravitational potential that binds the stars to their host galaxies is dominated by collisionless dark matter. According to the widely adopted cold dark matter (CDM) scenario, these normal galaxies are formed through the mergers of much smaller entities, dwarf galaxies. After violent relaxation, the dark matter is well mixed in phase space and attains an extended 3-D spatial distribution. In spiral galaxies, the formation and concentration of stars in extended, flattened, rotating disks requires the detachment of ordinary matter from the dark-matter halos of the original host dwarf galaxies. The dominance of the dark-matter halo to the galactic potential at large radii, and the separation of the ordinary matter imply that, during the epoch of galactic buildup, the ordinary matter was primarily in the form of gas which dissipated a substantial fraction of its initial potential energy.

In a previous paper (Lin & Murray 2000), we considered the dynamical evolution of infalling gas in the halos of normal galaxies. We showed that for typical values ($\sim 10^6$ K) of the virial temperature, the cooling timescale increases 