Multiphase galaxy formation: high-velocity clouds and the missing baryon problem

The standard treatment of cooling in cold dark matter haloes assumes that all of the gas within a 'cooling radius' cools and contracts monolithically to fuel galaxy formation. Here we take into account the expectation that the hot gas in galactic haloes is thermally unstable and prone to fragmentation during cooling and we show that the implications are more far-reaching than previously expected: allowing multiphase cooling fundamentally alters expectations about gas infall in galactic haloes and naturally gives rise to a characteristic upper limit on the masses of galaxies, as observed. Specifically, we argue that cooling should proceed via the formation of high-density, similar to 10(4) K 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. The fraction of halo baryons contained in the residual hot core component grows with halo mass because the cooling density increases with gas temperature, and this leads to an upper-mass limit in quiescent, non-merged galaxies of similar to10(11) Mcircle dot. In this scenario, galaxy formation is fuelled by the infall of pressure-supported clouds. For Milky-Way-size systems, clouds of mass similar to5 x 10(6) Mcircle dot that formed or merged within the last several Gyr should still exist as a residual population in the halo, with a total mass in clouds of similar to2 x 10(10) Mcircle dot. The baryonic mass of the Milky Way galaxy is explained naturally in this model, and is a factor of 2 smaller than would result in the standard treatment without feedback. We expect clouds in galactic haloes to be similar to1 kpc in size and to extend similar to150 kpc from galactic centres. 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.