THE PHOTON UNDERPRODUCTION CRISIS

We examine the statistics of the low-redshift Ly alpha forest from smoothed particle hydrodynamic simulations in light of recent improvements in the estimated evolution of the cosmic ultraviolet background (UVB) and recent observations from the Cosmic Origins Spectrograph (COS). We find that the value of the metagalactic photoionization rate (Gamma(HI)) required by our simulations to match the observed properties of the low-redshift Ly alpha forest is a factor of five larger than the value predicted by state-of-the art models for the evolution of this quantity. This mismatch in Gamma(HI) results in the mean flux decrement of the Ly alpha forest being overpredicted by at least a factor of two (a 10 sigma discrepancy with observations) and a column density distribution of Ly alpha forest absorbers systematically and significantly elevated compared to observations over nearly two decades in column density. We examine potential resolutions to this mismatch and find that either conventional sources of ionizing photons (galaxies and quasars) must contribute considerably more than current observational estimates or our theoretical understanding of the low-redshift universe is in need of substantial revision.