Feedback from Galaxy Formation: Production and Photodissociation of Primordial H2

We use 1D radiative transfer simulations to study the evolution of H2 gas-phase (H − catalyzed) formation and photo-dissociation regions in the primordial universe. We find a new positive feedback mechanism capable of producing shells of H2 in the intergalactic medium, which are optically thick in some Lyman-Werner bands. While these shells exist, this feedback effect is important in reducing the H2 dissociating background flux and the size of photo-dissociation spheres around each luminous object. The maximum background opacity of the IGM in the H2 Lyman-Werner bands is τH2 ≈ 1−2 for a relic molecular fraction xH2 = 2× 10−6, about 6 times greater than found by Haiman et al. (2000). Therefore, the relic molecular hydrogen can decrease the photo-dissociation rate by about an order of magnitude. The problem is relevant to the formation of small primordial galaxies with masses MDM . 10 8 M⊙, that rely on molecular hydrogen cooling to collapse. Alternatively, the universe may have remained dark for several hundred million years after the birth of the first stars, until galaxies with virial temperature Tvir & 10 4 K formed. Subject headings: Galaxies: formation – Cosmology: theory