Intergalactic H2 Photodissociation and the Soft Uv Background Produced by Population Iii Objects

We study the effects of the ionizing and dissociating photons produced by Pop III objects on the surrounding intergalactic medium. We find that the typical size of a 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 ≈ 10−10 erg cm s Hz. This value is well below the threshold required for the negative feedback of Pop III objects on the subsequent galaxy formation to be effective in that redshift range. Subject headings: galaxies: formation cosmology: theory