TITLE: How the Galaxy got its Mass and Other Just So Stories

Abstract:
The standard treatment of cooling in Cold Dark Matter halos assumes
that all of the gas within a ``cooling radius'' cools and contracts
monolithically to fuel galaxy formation. Taking into account the
expectation that the hot gas in galactic halos is thermally unstable 
and prone to fragmentation during cooling. I show that the
implications are more far-reaching than previously expected: allowing
multi-phase cooling fundamentally alters expectations about gas infall
in galactic halos and naturally gives rise to a characteristic upper-
limit on the masses of galaxies, as observed. I argue that cooling 
should proceed via the formation of  high-density, clouds, pressure-
confined within a hot gas background.  The background medium that 
emerges has a low density, and can survive as a hydrostatically stable
corona with a long cooling time.  In this scenario,
galaxy formation is fueled by the infall of pressure-supported clouds.
For Milky-Way-size systems,  clouds that formed or merged within the 
last several Gyrs should still exist as a residual population in the 
halo.  The baryonic mass of the Milky Way galaxy is explained naturally 
in this model, and is a factor of two smaller than would result in the
standard treatment without feedback.  The predicted properties of Milky
Way clouds match well the observed radial velocity distribution, 
angular sizes, column densities, and velocity widths of High Velocity 
Clouds around our Galaxy.  The clouds we predict are also of the type 
needed to explain high-ion absorption systems at z<1, and the 
predicted covering factor around external galaxies is consistent with 
observations.