Superbubbles in the Multiphase ISM and the Loading of Galactic Winds

Abstract

We use numerical simulations to analyze the evolution and properties of superbubbles (SBs), driven by multiple supernovae (SNe), that propagate into the two-phase (warm/cold), cloudy interstellar medium (ISM). We consider a range of mean background densities n_avg=0.1-10 cm^{-3} and intervals between SNe dt_sn=0.01-1 Myr, and follow each SB until the radius reaches (1-2)H, where H is the characteristic ISM disk thickness. Except for embedded dense clouds, each SB is hot until a time t_sf,m when the shocked warm gas at the outer front cools and forms an overdense shell. Subsequently, diffuse gas in the SB interior remains at T_h 10^6-10^7K with expansion velocity v_h~10^2-10^3km/s (both highest for low dt_sn). At late times, the warm shell gas velocities are several 10's to ~100km/s. While shell velocities are too low to escape from a massive galaxy, they are high enough to remove substantial mass from dwarfs. Dense clouds are also accelerated, reaching a few to 10's of km/s. We measure the mass in hot gas per SN, M_h/N_SN, and the total radial momentum of the bubble per SN, p_b/N_SN. After t_sf,m, M_h/N_SN 10-100M_sun (highest for low n_avg), while p_b/N_SN 0.7-3x10^5M_sun km/s (highest for high dt_sn). If galactic winds in massive galaxies are loaded by the hot gas in SBs, we conclude that the mass-loss rates would generally be lower than star formation rates. Only if the SN cadence is much higher than typical in galactic disks, as may occur for nuclear starbursts, SBs can break out while hot and expel up to 10 times the mass locked up in stars. The momentum injection values, p_b/N_SN, are consistent with requirements to control star formation rates in galaxies at observed levels.