Galactic Outflows and Photoionization Heating in the Reionization Epoch

TL;DR

This study uses cosmological radiation hydrodynamic simulations to investigate how galactic outflows and photoionization heating affected star formation and reionization during the early universe, highlighting the role of faint galaxies and the impact of inhomogeneous UV backgrounds.

Contribution

It presents new simulations that quantify the effects of galactic outflows and photoionization heating on reionization, revealing a steeper star formation dependence on halo mass and emphasizing the importance of faint galaxies.

Findings

EUVB suppresses galaxy luminosity function by less than 50% at z=6

Galactic outflows restore agreement with observed galaxy abundance

Star formation rate scales as M_h^1.3-1.4 in halos with M_h=10^{8.2-10.2} Msun

Abstract

We carry out a new suite of cosmological radiation hydrodynamic simulations and explore the relative impacts on reionization-epoch star formation of galactic outflows and photoionization heating. By itself, an extragalactic ultraviolet background (EUVB) suppresses the luminosity function by less than 50% at z=6, overproducing the observed galaxy abundance by a factor of 3-5. Galactic outflows restore agreement with observations without preventing Population II star formation from reionizing the Universe by z=6. The resulting EUVB suppresses star formation in halos with virial temperatures below 10^5K but has a weaker impact in more massive halos. Nonetheless, the low-mass halos contribute up to 50% of all ionizing photons owing to the EUVB's inhomogeneity. Overall, star formation rate scales as halo mass M_h to the 1.3-1.4 in halos with $M_h=10^{8.2--10.2}\msun$. 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, indicating that reionization was driven by faint galaxies (M_1600 >= -15) that have not yet been observed. Our models cannot simultaneously explain observations of galaxies, the cosmic microwave background, and the intergalactic medium. Increased dynamic range will alleviate the existing discrepancies, but observations may still require additional physics such as a variable ionizing escape fraction (abridged).