Radiative Transfer in a Clumpy Universe. III. The Nature of Cosmological Ionizing Sources

The history of the transition from a neutral intergalactic medium (IGM) toone that is almost fully ionized can reveal the character of cosmologicalionizing sources. We study the evolution of the volume filling factor of HIIand HeIII regions in a clumpy IGM, and discuss the implications for rivalreionization scenarios of the rapid decline in the space density of radio-loudquasars and of the large population of star-forming galaxies recently observedat z>3. The hydrogen component in a highly inhomogeneous universe is completelyreionized when the number of photons emitted above 1 ryd in one recombinationtime equals the mean number of hydrogen atoms. If stellar sources areresponsible for keeping the IGM ionized at z=5, the rate of star formation atthis epoch must be comparable or greater than the one inferred from opticalobservations of galaxies at z=3, and the mean metallicity per baryon in theuniverse of order 1/500 solar. An early generation of stars in dark matterhalos with circular velocities v_circ=50 km/s, possibly one of the main sourceof UV photons at high-z, could be detectable with the Next Generation SpaceTelescope. Models in which the quasar emissivity declines rapidly at z>3predict a late HeII reionization epoch, a feature that could explain the recentdetection of patchy HeII Lyman-alpha at z=2.9 by Reimers et al. (1997) and theabrupt change observed by Songaila (1998) at about the same epoch of theSiIV/CIV ratio, but appear unable to provide the required number ofhydrogen-ionizing photons at z=5.Comment: LaTeX, 29 pages, 5 figures, submitted to the Ap