Feedback from Galaxy Formation: Production and Photodissociation of Primordial H₂

We use one-dimensional radiative transfer simulations to study the evolution of H₂ gas-phase (H^- catalyzed) formation and photodissociation regions in the primordial universe. We find a new positive feedback mechanism capable of producing shells of H₂ in the intergalactic medium (IGM), 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 photodissociation spheres around each luminous object. The maximum background opacity of the IGM in the H₂ Lyman-Werner bands is τ ≈ 1-2 for a relic molecular fraction x = 2 × 10^-6, about 6 times greater than that found by Haiman, Abel, & Rees. Therefore, the relic molecular hydrogen can decrease the photodissociation rate by about an order of magnitude. The problem is relevant to the formation of small primordial galaxies with masses M_DM ≲ 10⁸ 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 T_vir ≳ 10⁴ K formed.