SUPERBUBBLES in the MULTIPHASE ISM and the LOADING of GALACTIC WINDS

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^-3and intervals between SNe Δt_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 ∼ 10⁶-10⁷ K, with an expansion velocity v_h ∼ 10²-10³ km s^-1 (both highest for low Δt_SN). At late times, the warm shell gas velocities are several tens to ∼100 km s^-1. 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 tens of Km s^-1. We measure the mass in hot gas per SN, M_h, and the total radial momentum of the bubble per SN, p_b. After t_sf,m, M_h ∼ 10-100 M_⊙ (highest for low n_avg ), while p_b ∼0.7-3 × 10⁵ M_⊙ km s^-1(highest for high Δt_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 usual in galactic disks, as may occur for nuclear starbursts, can SBs breakout while hot and expel up to 10 times the mass locked up in stars. The momentum injection values, p_b, are consistent with requirements to control star formation rates within galaxies at observed levels.