Hydrodynamical simulations of galaxy formation: effects of supernova feedback

In this paper we numerically simulate some of the most critical physical processes in galaxy formation: the supernova feedback loop, in conjunction with gas dynamic processes and gravitational condensations, plays a crucial role in determining how the observable properties of galaxies arise within the context of a model for large-scale structure. Our treatment incorporates a multiphase model of the interstellar medium and includes the effects of cooling, heating and metal enrichment by supernovae, and evaporation of cold clouds. Star formation occurs inside cold gas clouds produced via thermal instability. In the present work, we simulate galaxy formation within the framework of a standard biased cold dark matter (CDM) model for a variety of parameters and for several resolutions. In our picture, supernova feedback regulates the evolution of the gas components and star formation. The efficiency of cold cloud evaporation by supernovae strongly influences star formation rates. This feedback results in a steady rate of star formation in `large' galaxies [mass larger than (2-3)x10^11 M_solar within 100 kpc radius] at a level of 1-10 M_solar per year for z<3 (H_0=50 km s^-1 Mpc^-1). Supernova feedback has an even stronger effect on the evolution of `dwarf' galaxies. Most of the dwarf galaxies in our models have a small fraction of stars and extremely low luminosities: M_R<~-15 for parent dark halo masses M_tot<(2-3)x10^10 M_solar within a 50-kpc radius. The observational properties (colours, luminosities) of galaxies identified in the simulations are computed using a stellar population synthesis model. In the case of both large and small galaxies, the distribution of luminous matter (stars) is strongly biased with respect to the dark matter. For a range of parameter values and resolutions we find an approximate biasing measure of the form rho_lum=(rho_dm/133)^1.7, for overdensities exceeding about 1000. Deviations from this relation depend strongly on the environment. For halo masses exceeding 2x10^10 M_solar, the dependence of the absolute visual magnitude M_V on the total mass can be approximated as M_V=-18.5-4 log (M_tot/10^11 M_solar), with a scatter of less than 1/2 magnitude.