Galactopause Formation and Gas Precipitation during Strong Galactic Outflows

Abstract

Using X-ray constrained β -models for the radial distribution of gas in the outskirts of galaxies, we analyze the termination of galactic winds and the formation and evolution of halo clouds by thermal instability. At low mass-loss rates ( ) galactic winds are trapped within the halo, but they burst into the intergalactic medium during intermittent strong outflows with . We develop analytic models of halo clouds as they cool radiatively over condensation timescales for hydrogen number densities , gas temperatures T  ≈ (10 6 K) T 6 , and metallicities ( Z / Z ⊙ ). Halo gas can form kiloparsec-scale clouds out to galactocentric distances r  ≈ 30–65 kpc, where efficient radiative cooling from 10 6  K down to T  ≈ 10 4–5 K occurs at Z  ≥ 0.3 Z ⊙ on timescales less than 1 Gyr. After condensing to column densities N H  ≥ 4 × 10 16 cm −2 , these clouds lose hydrostatic pressure support and fall inward on dynamical timescales of ∼200 Myr. Our baseline analysis will be followed by numerical calculations to understand the governing principles of halo cloud formation and transport of gas to the galactic disk.