Galactopause Formation and Gas Precipitation During Strong Galactic Outflows

Using X-ray constrained beta-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 (dM/dt)_w >> 1 M_sun/yr. We develop analytic models of halo clouds as they cool radiatively over condensation time scales t_c = (390 Myr)(T_6 /n_{-4}) (Z/Z_sun)^-1 for hydrogen number densities n_H = (10^{-4} cm^{-3}) n_{-4}, gas temperatures T = (10^6 K)T_6, and metallicities (Z/Z_sun). Halo gas can form kpc-scale clouds out to galactocentric distances r = 30-65 kpc, where efficient radiative cooling from 10^6 K down to 10^4 K occurs at Z > 0.3 Z_sun on timescales less than 1 Gyr. After condensing to column densities N_H > 4x10^{16} cm^{-2}, these clouds lose hydrostatic pressure support and fall inward on dynamical time scales 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.