The Effects of Early Cosmic Reionization on the Substructure Problem in Galactic Halos
Susa, Hajime, & Umemura, Masayuki
Recent observations of the cosmic microwave background by the Wilkinson Microwave Anisotropy Probe strongly suggest that the reionization of the universe took place quite early (zåisebox-0.5ex~17). On the other hand, it has been pointed out that the cold dark matter cosmology suffers from a substructure problem in which more subgalactic halos are produced in the Local Group than dwarf galaxies. In this Letter, as a potential mechanism for solving this problem, we consider the feedback effects of early reionization on the formation of small-scale structures. For this purpose, we perform three- dimensional radiation hydrodynamic simulations, incorporating the radiative transfer for ionizing photons. As a result, it is found that the early reionization is devastating for low-mass systems with M$_vir$<\i̊sebox-0.5ex~10$^8$ M$_solar$ or v$_circ$<\rs̊ebox-0.5ex~20 km s$^-1$, and almost all gas is photoevaporated 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.