Cold Cloud Infall and Galaxy Formation

We present a pair of high‐resolution SPH (smoothed particle hydrodynamics) simulations that explore the nature of cool gas infall into galaxies, and the physical conditions necessary to support the type of gaseous halos that seem to be required by observations. Observations of local X‐ray absorbers, high‐velocity clouds, and distant quasar absorption line systems suggest that a significant fraction of baryons may reside in multi‐phase, low‐density, extended, ∼100 kpc, gaseous halos around normal galaxies. The two simulations are identical other than their initial gas density distributions: one is initialized with a standard hot gas halo that traces the cuspy profile of the dark matter, and the other is initialized with a cored hot halo with a high central entropy, as might be expected in models with early pre‐heating feedback. Galaxy formation proceeds in dramatically different fashions in these two cases. While the standard cuspy halo cools rapidly, primarily from the central region, the cored halo is qu...