Physical regimes for feedback in galaxy formation

Abstract

We present a new (semi-)analytic model for feedback in galaxy formation. The ISM is modeled as a two-phase medium in pressure equilibrium. The remnants of exploding type II SNe percolate into super-bubbles (SBs) that sweep the ISM, heating the hot phase (if the SB is adiabatic) or cooling it (in the snowplow stage, when the interior gas of the SB has cooled). The resulting feedback regimes occur in well-defined regions of the space defined by vertical scale-length and surface density of the structure. When SBs blow out in the adiabatic regime, the efficiency of SNe in heating the ISM is ~5 per cent, with \~80 per cent of the energy budget injected into the external halo, and the outcoming ISM is self-regulated to a state similar to that found in the Milky Way. Feedback is most efficient when SBs are pressure-confined in the adiabatic regime. In some significant regions of the parameter space confinement takes place in the snowplow stage; then the hot phase has a lower temperature and star formation is quicker. In some critical cases, the hot phase is strongly depleted and the cold phase percolates the whole volume, giving rise to a sudden burst of star formation. Strong galactic winds are predicted to happen only in critical cases. This model provides a starting point for constructing a realistic grid of feedback solutions to be used in galaxy formation codes. The predictive power of this model extends to many properties of the ISM, so that most parameters can be constrained by reproducing the main properties of the Milky Way. (Abridged)