Formation of Minigalaxies in Defunct Cosmological H II Regions

Using a large set of high-resolution numerical simulations incorporating nonequilibrium molecular hydrogen chemistry and a constant source of external radiation, we study gas collapse in previously photoionized minigalaxies with virial temperatures less than 104 K in the early universe (redshifts z = 10-20). We confirm that the mechanism of positive feedback of ionizing radiation on star formation in minigalaxies proposed by Ricotti and coworkers can be efficient despite a significant flux of metagalactic photodissociating radiation. We derive critical fluxes for the Lyman-Werner background radiation sufficient to prevent the collapse of gas in minigalaxies as a function of the virial mass of the halo and redshift. In our model, the formation of minigalaxies in defunct H II regions is most efficient at large redshifts (z ≳ 15) and/or for large local gas overdensity δ ≳ 10. We show that nonequilibrium chemistry plays an important dynamical role not only during the initial evolutionary phase, leading to the gas becoming gravitationally unstable inside the minihalo, 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.