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(-3) and intervals between SNe Delta t(SN) = 0.01-1 Myr, and follow each SB until the radius reaches similar to(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 similar to 10(6)-10(7) K , with an expansion velocity v(h) similar to 10(2)-10(3) km s(-1)(both highest for low Delta t(SN)). At late times, the warm shell gas velocities are several tens to similar 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 similar to 10-100M(circle dot) (highest for low n(avg)), while p(b)similar to 0.7-3 x 10(5)M(circle dot) km s(-1)(highest for high Delta t(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.