Is Reionization Regulated by H₂ in the Early Universe?

Summary

Molecular hydrogen is a key species for the formation of the first luminous objects in the early universe. It is therefore crucial to understand the various physical processes leading to its formation and destruction and the feedbacks regulating this chemical network. Here we review both the radiative and SN-induced feedbacks and we assess the role of the objects relying on H₂ for their collapse in the evolution of the reionization of the universe.

Introduction

At z ≈ 1100 the intergalactic medium (IGM) is expected to recombine and remain neutral until the first sources of ionizing radiation form and reionize it. Until recently, QSOs were thought to be the main source of ionizing photons, but observational constraints suggest the existence of an early population of pregalactic objects (Pop III hereafter) which could have contributed to the reheating, reionization and metal enrichment of the IGM at high redshift. In order to virialize in the potential well of dark matter halos, the gas must have a mass greater than the Jeans mass (M_b > M_J), which, at z ∼ 20 – 30 corresponds to very low virial temperatures (T_vir < 10⁴ K). To have a further collapse and fragmentation of the gas, and to ignite star formation, additional cooling is required. It is well known that in these conditions the only efficient coolant for a plasma of primordial composition, is molecular hydrogen (Abel et al. 1997; Tegmark et al. 1997; Ciardi, Ferrara & Abel 2000 [CFA]).