Photoevaporation of Minihalos During Cosmic Reionization: Primordial and Metal-enriched Halos

The density distribution of the intergalactic medium is an uncertain but highly important issue in the study of cosmic reionization. It is expected that there are abundant gas clouds hosted by low-mass "minihalos" in the early universe, which act as photon sinks until being photoevaporated by the emerging ultraviolet background (UVB) radiation. We perform a suite of radiation-hydrodynamic simulations to study the photoevaporation of minihalos. Our simulations follow hydrodynamics, nonequilibrium chemistry, and the associated cooling processes in a self-consistent manner. We conduct a parametric study by considering a wide range of gas metallicities (0 Z_⊙ ≤ Z ≤ 10^-3 Z_⊙), halo mass (10³ M_⊙ ≤ M ≤ 10⁸ M_⊙), UVB intensity (0.01 ≤ J₂₁ ≤ 1), and turn-on redshift of ionizing sources (10 ≤ z_IN ≤ 20). We show that small halos are evaporated in a few tens of millions of years, whereas larger mass halos survive 10 times longer. The gas mass evolution of a minihalo can be characterized by a scaling parameter that is given by a combination of the halo mass, background radiation intensity, and redshift. Efficient radiative cooling in metal-enriched halos induces fast condensation of the gas to form a dense, self-shielded core. The cold, dense core can become gravitationally unstable in halos with high metallicities. Early metal enrichment may allow star formation in minihalos during cosmic reionization.