Intergalactic H₂ Photodissociation and the Soft UV Background Produced by Population III Objects

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We study the effects of the ionizing and dissociating photons produced by PopIII 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 1d-30 - 1d-27 erg cm⁻² s⁻¹ Hz⁻¹. This value is well below the threshold required for the negative feedback of PopIII objects on the subsequent galaxy formation to be effective in that redshift range.