Intergalactic H 2 Photodissociation and the Soft Ultraviolet Background Produced by Population III Objects

Abstract

We study the effects of the ionizing and dissociating photons produced by Population III objects on the surrounding intergalactic medium. We find that the typical size of an H2 photodissociated region, Rd ≈ 1-5 kpc, is smaller than the mean distance between sources at z ≈ 20-30 but larger than the ionized region by a factor depending on the detailed properties of the emission spectrum. This implies that clearing of intergalactic H2 occurs before reionization of the universe is complete. In the same redshift range, the soft-UV background in the Lyman-Werner bands, when the intergalactic H and H2 opacity is included, is found to be JLW ≈ 10-30 to 10-27 ergs cm-2 s-1 Hz-1. This value is well below the threshold required for the negative feedback of Population III objects on the subsequent galaxy formation to be effective in that redshift range.