CONSTRAINING THE MINIMUM MASS OF HIGH-REDSHIFT GALAXIES AND THEIR CONTRIBUTION TO THE IONIZATION STATE OF THE INTERGALACTIC MEDIUM

We model the latest HST WFPC3/IR observations of greater than or similar to 100 galaxies at redshifts z = 7-8 in terms of a hierarchical galaxy formation model with starburst activity. Our model provides a distribution of UV luminosities per dark matter halo of a given mass and a natural explanation for the fraction of halos hosting galaxies. The observed luminosity function is best fit with a minimum halo mass per galaxy of 10(-0.9)(9.4)(+0.3) M-circle dot, corresponding to a virial temperature of 10(-0.7)(4.9)(+0.2) K. Extrapolating to faint, undetected galaxies, the total production rate of ionizing radiation depends critically on this minimum mass. Future measurements with JWST should determine whether the entire galaxy population can comfortably account for the UV background required to keep the intergalactic medium ionized.