Feedback from First Radiation Sources: H - Photodissociation

Abstract

During the epoch of reionization, the formation of radiation sources is accompanied by the growth of a H- photodissociating flux. We estimate the impact of this flux on the formation of molecular hydrogen and cooling in the first galaxies, assuming different types of radiation sources (e.g., Population II and Population III stars, miniquasars). We find that H- photodissociation reduces the formation of H2 molecules by a factor of Fs ~ 1 + 103ksxfδ-1, where x is the mean ionized fraction in the intergalactic medium, fesc is the fraction of ionizing photons that escape from their progenitor halos, δ is the local gas overdensity, and ks is an order unity constant that depends on the type of radiation source. By the time a significant fraction of the universe becomes ionized, H- photodissociation may significantly reduce the H2 abundance and, with it, the primordial star formation rate, delaying the progress of reionization.