Formation of Dwarf Galaxies during the Cosmic Reionization

Abstract

We reanalyze the photoevaporation problem of subgalactic objects irradiated by UVB radiation in a reionized universe. For the purpose, we perform three-dimensional radiation smoothed-particle-hydrodynamics calculations, where the radiative transfer is solved by a direct method and also the non-equilibrium chemistry of primordial gas including H$_2$ molecules is incorporated. Attention is concentrated on radiative transfer effects against UVB for the formation of subgalactic objects with $T_{\rm vir}\la 10^4$K. We consider the reionization model with $z_{\rm reion} \approx 7$ and also the earlier reionization model ($z_{\rm reion} \approx 17$) inferred by the WMAP. We find that the star formation is suppressed appreciably by UVB, but baryons at high-density peaks are self-shielded even during the reionization, forming some amount of stars eventually. In that sense, the photoevaporation for subgalactic systems is not so perfect as argued by one-dimensional spherical calculations. The final stellar fraction depends on the collapse epoch and the mass of system, but almost regardless of the reionization epoch. For instance, a few tenths of formed stars are born after the cosmic reionization in $z_{\rm reion} \approx 7$ model, while more than 90% stars are born after the reionization in the WMAP reionization model. Thus, effects of UVB feedback on the substructure problem with CDM scenario should be evaluated with careful treatment of the radiative transfer. The star clusters formed at high-density peaks coalesce with each other in a dissipationless fashion in a dark matter potential, resultantly forming a spheroidal system. As a result, these low-mass galaxies have large mass-to-light ratios such as observed in dwarf spheroidals in the Local Group.