Star Formation and Feedback in Dwarf Galaxies

We examine the star formation history and stellar feedback effects of dwarfgalaxies under the influence of extragalactic ultraviolet radiation. Weconsider the dynamical evolution of gas in dwarf galaxies using aone-dimensional, spherically symmetric, Lagrangian numerical scheme to computethe effects of radiative transfer and photoionization. We include aphysically-motivated star formation recipe and consider the effects offeedback. Our results indicate that star formation in the severe environment ofdwarf galaxies is a difficult and inefficient process. For intermediate masssystems, such as the dSphs around the Galaxy, star formation can proceed within early cosmic epochs despite the intense background UV flux. Triggeringprocesses such as merger events, collisions, and tidal disturbance can lead todensity enhancements, reducing the recombination timescale, allowing gas tocool and star formation to proceed. However, the star formation and gasretention efficiency may vary widely in galaxies with similar dark matterpotentials, because they depend on many factors, such as the baryonic fraction,external perturbation, IMF, and background UV intensity. We suggest that thepresence of very old stars in these dwarf galaxies indicates that their initialbaryonic to dark matter content was comparable to the cosmic value. Thisconstraint suggests that the initial density fluctuation of baryonic matter maybe correlated with that of the dark matter. For the more massive dwarfelliptical galaxies, the star formation efficiency and gas retention rate ismuch higher. Their mass to light ratio is regulated by star formation feedback,and is expected to be nearly independent of their absolute luminosity. Theresults of our theoretical models reproduce the observed $M/L-M_v$ correlation.Comment: 35 pages, 13 figure