THE METAGALACTIC IONIZING BACKGROUND: A CRISIS IN UV PHOTON PRODUCTION OR INCORRECT GALAXY ESCAPE FRACTIONS?

Recent suggestions of a photon underproduction crisis have generated concern over the intensity and spectrum of ionizing photons in the metagalactic ultraviolet background (UVB). The balance of hydrogen photoionization and recombination determines the opacity of the low-redshift intergalactic medium (IGM). We calibrate the hydrogen photoionization rate (Gamma(H)) by comparing Hubble Space Telescope spectroscopic surveys of the low-redshift column density distribution of H I absorbers and the observed (z < 0.4) mean Ly alpha flux decrement, D-A = (0.014) (1 + z)(2.2), to new cosmological simulations. The distribution, f(NH (I, z) ) d(2) N/d (logN(H I) )dz, is consistent with an increased UVB that includes contributions from both quasars and galaxies. Our recommended fit, Gamma H(z) = (4.6 x 10(-14) s(-1))(1 + z)(4.4) for 0 < z < 0.47, corresponds to unidirectional LyC photon flux Phi(0) approximate to 5700 cm(-2) s(-1) at z = 0. This flux agrees with observed IGM metal ionization ratios (C III/C IV and Si III/ Si IV) and suggests a 25%-30% contribution of Lya absorbers to the cosmic baryon inventory. The primary uncertainties in the low-redshift UVB are the contribution from massive stars in galaxies and the LyC escape fraction (f(esc)), a highly directional quantity that is difficult to constrain statistically. We suggest that both quasars and low-mass starburst galaxies are important contributors to the ionizing UVB at z < 2. Their additional ionizing flux would resolve any crisis in photon underproduction.