Photoevaporation of Minihalos during Cosmic Reionization: Primordial and Metal-Enriched Halos

TL;DR

This study uses radiation hydrodynamics simulations to analyze how minihalos in the early universe are photoevaporated by UV radiation, considering various metallicities, masses, and redshifts, revealing conditions for gas survival and star formation.

Contribution

It provides a comprehensive parametric analysis of minihalo photoevaporation, including effects of metallicity, mass, and redshift, and highlights the potential for early star formation in metal-enriched halos.

Findings

Small halos evaporate in tens of millions of years.

Larger halos survive ten times longer.

Metal-enriched halos develop dense, self-shielded cores.

Abstract

The density distribution of the inter-galactic 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 photoevaporated by the emerging ultra-violet background (UVB) radiation. We perform a suite of radiation hydrodynamics simulations to study the photoevaporation of minihalos. Our simulations follow hydrodynamics, non-equilibrium chemistry, and the associated cooling processes in a self-consistent manner. We conduct a parametric study by considering a wide range of gas metallicity ($0\,Z_\odot \leq Z \leq 10^{-3}\,Z_\odot$), halo mass ($10^3 M_\odot \leq M \leq 10^8 M_\odot$), UVB intensity ($0.01 \leq J_{21} \leq 1$), and turn-on redshift of ionizing sources ($10\leq z_{\rm IN} \leq 20$). We show that small halos are evaporated in a few tens million years, whereas larger mass halos survive for ten times longer. We show that 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.