Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halo

Abstract

Recent observations on the cosmic microwave background by Wilkinson Microwave Anisotropy Probe(WMAP) strongly suggest that the reionization of the universe took place quite early (z \sim 17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from the substructure problem that subgalactic halos are overproduced than the observed dwarf galaxies in the Local Group. In this paper, as a potential mechanism to solve this problem, we consider the feedback effects of the early reionization on the formation of small-scale structures. For this purpose, we perform 3D radiation hydrodynamic simulations with incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is so devastating for low-mass systems with M_{vir}\la 10^8 M_\odot or v_{circ}\la 20km/s, and almost all gas is photo-evaporated in more than 95% of low-mass systems. Such a strong negative feedback on the formation of low-mass galaxies may solve the substructure problem and support the picture that Local Group dwarf galaxies are descendants of the more massive halos that experienced and survived tidal stripping.