Galactic Outflows and Photoionization Heating in the Reionization Epoch

We carry out a new suite of cosmological radiation hydrodynamic simulations that explores the relative impacts on reionization-epoch star formation of galactic outflows and photoionization heating from a self-consistently grown extragalactic ultraviolet ionizing background (EUVB). We compare the predictions with observational constraints from the cosmic microwave background, the ultraviolet continuum luminosity function of galaxies, and the Lyα forest. By itself, an EUVB suppresses the luminosity function by less than 50% at z = 6 even if it is orders of magnitude stronger than observed. This overproduces the observed galaxy abundance by a factor of 3-5, indicating the need for an additional feedback process. We confirm that outflows readily suppress both the EUVB and the luminosity function into improved agreement with observations. Population I-II star formation can reionize the universe by z = 6 even in the presence of strong feedback from photoheating and outflows. The resulting EUVB suppresses star formation in halos with virial temperatures below 10⁵ K but has a weaker impact on more massive halos. Nonetheless, halos with virial temperatures below 10⁵ K contribute up to ~50% of all ionizing photons owing to the EUVB's inhomogeneity. Overall, star formation rate scales with halo mass M_h as M ^1.3-1.4 _h for halos with M_h = 10^8.2-10^10.2 M _⊙. This is a steeper dependence than is often assumed in reionization models, boosting the expected power spectrum of 21 centimeter fluctuations on large scales. The luminosity function rises steeply to at least M ₁₆₀₀ = -13 even in models that treat both outflows and an EUVB, indicating that reionization was driven by faint galaxies (M ₁₆₀₀ >= -15) that have not yet been observed. Outflows and an EUVB interfere with each other's feedback effects in two ways. Outflows weaken the EUVB, limiting Jeans suppression of low-mass halos; this leads to overall de-amplification of suppression at early times (z > 8). Meanwhile, they amplify each other's impact on more massive halos, leading to overall amplification of suppression at later times. Our models cannot simultaneously explain observations of galaxies, the cosmic microwave background, and the intergalactic medium. Correcting for dynamic range limitations and adjusting our physical treatments will alleviate discrepancies, but observations may still require additional physical scalings such as a mass-dependent ionizing escape fraction.