Hierarchical Formation of Galaxies with Dynamical Response to Supernova‐Induced Gas Removal

Abstract

We reanalyze the formation and evolution of galaxies in the hierarchical clustering scenario. Using a semianalytic model (SAM) of galaxy formation described in this paper, which we hereafter call the Mitaka model, we extensively investigate the observed scaling relations of galaxies among photometric, kinematic, structural, and chemical characteristics. In such a scenario, spheroidal galaxies are assumed to be formed by a major merger and subsequent starburst, in contrast to the traditional scenario of monolithic cloud collapse. As a new ingredient of SAMs, we introduce the effects of dynamical response to supernova-induced gas removal on size and velocity dispersion, which play an important role in dwarf galaxy formation. In previous theoretical studies of dwarf galaxies based on the monolithic cloud collapse given by Yoshii & Arimoto and Dekel & Silk, the dynamical response was treated in the extremes of a purely baryonic cloud and a baryonic cloud fully supported by surrounding dark matter. To improve this simple treatment, in our previous paper we formulated the dynamical response in more realistic, intermediate situations between the above extremes. While the effects of dynamical response depend on the mass fraction of removed gas from a galaxy, the amount of the gas that remains just after major merger depends on the star formation history. A variety of star formation histories are generated through the Monte Carlo realization of merging histories of dark halos, and it is found that our SAM naturally makes a wide variety of dwarf galaxies and their dispersed characteristics as observed. It is also found that our result strongly depends on the adopted redshift dependence of thestar formation timescale, because it determines the gas fraction in high-redshift galaxies for which major mergers frequently occur. We test four star formation models. The first model has a constant timescale of star formation independent of redshift. The last model has a timescale proportional to the dynamical timescale of the galactic disk. The other models have timescales intermediates of these two. The last model fails to reproduce observations, because it predicts only a small amount of the leftover gas at major mergers, therefore giving too weak a dynamical response on size and velocity dispersion of dwarf spheroidals. The models, having a constant timescale of star formation or a timescale very weakly dependent on redshift, associated with our SAM, succeed in reproducing most observations from giant to dwarf galaxies, except that the adopted strong supernova feedback in this paper does not fully explain the color-magnitude relation under the cluster environment and the Tully-Fisher relation. A direction for overcoming this remaining problem is also discussed.