Star Formation and Feedback in Dwarf Galaxies

Abstract

We examine the star formation history and stellar feedback effects of dwarf galaxies under the influence of extragalactic ultraviolet radiation. We consider the dynamical evolution of gas in dwarf galaxies using a one-dimensional, spherically symmetric, Lagrangian numerical scheme to compute the effects of radiative transfer and photoionization. We include a physically-motivated star formation recipe and consider the effects of feedback. Our results indicate that star formation in the severe environment of dwarf galaxies is a difficult and inefficient process. For intermediate mass systems, such as the dSphs around the Galaxy, star formation can proceed with in early cosmic epochs despite the intense background UV flux. Triggering processes such as merger events, collisions, and tidal disturbance can lead to density enhancements, reducing the recombination timescale, allowing gas to cool and star formation to proceed. However, the star formation and gas retention efficiency may vary widely in galaxies with similar dark matter potentials, because they depend on many factors, such as the baryonic fraction, external perturbation, IMF, and background UV intensity. We suggest that the presence of very old stars in these dwarf galaxies indicates that their initial baryonic to dark matter content was comparable to the cosmic value. This constraint suggests that the initial density fluctuation of baryonic matter may be correlated with that of the dark matter. For the more massive dwarf elliptical galaxies, the star formation efficiency and gas retention rate is much higher. Their mass to light ratio is regulated by star formation feedback, and is expected to be nearly independent of their absolute luminosity. The results of our theoretical models reproduce the observed $M/L-M_v$ correlation.