Formation of mini-galaxies in defunct cosmological HII regions

Abstract

Using a large set of high resolution numerical simulations incorporating non-equilibrium molecular hydrogen chemistry and a constant source of external radiation, we study gas collapse in previously photo-ionized mini-galaxies with virial temperatures less than 10^4 K in the early universe (redshifts z=10-20). We confirm that the mechanism of positive feedback of ionizing radiation on star formation in mini-galaxies proposed by Ricotti, Gnedin, & Shull (2002) can be efficient despite a significant flux of metagalactic photo-dissociating radiation. We derive critical fluxes for the Lyman-Werner background radiation sufficient to prevent the collapse of gas in mini-galaxies as a function of the virial mass of the halo and redshift. In our model, the formation of mini-galaxies in defunct HII regions is most efficient at large redshifts (z>15) and/or for large local gas overdensity delta>10. We show that non-equilibrium chemistry plays an important dynamical role not only during the initial evolutionary phase leading to the gas becoming gravitationally unstable inside the mini-halo, but also at the advanced stages of the core collapse, resulting in efficient gas accretion in the core region. We speculate on a possible connection between our objects and metal-poor globular clusters and dwarf spheroidal galaxies.