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

Abstract

The history of the transition from a neutral intergalactic medium (IGM) to one that is almost fully ionized can reveal the character of cosmological ionizing sources. We study the evolution of the volume filling factors of H II and He III regions in a clumpy IGM, and discuss the implications for rival reionization scenarios of the rapid decline in the space density of radio-loud quasars and of the large population of starforming galaxies recently observed at z ∼> 3. The hydrogen component in a highly inhomogeneous universe is completely reionized when the number of photons emitted above 1 ryd in one recombination time equals the mean number of hydrogen atoms. If stellar sources are responsible for keeping the IGM ionized at z = 5, the rate of star formation at this epoch must be comparable or greater than the one inferred from optical observations of galaxies at z ≈ 3, and the mean metallicity per baryon in the universe ∼ 1/500 solar. An early generation of stars in dark matter halos with circular velocities vcirc ≈ 50 km s −1, possibly one of the main source of UV photons at highz, could be detectable with the Next Generation Space Telescope. Models in which the quasar emissivity declines rapidly at z ∼> 3 predict a late He II reionization epoch, a feature that could explain the recent detection of patchy He II Lyα at z = 2.9 by Reimers et al. (1997) and the abrupt change observed by Songaila (1998) at about the same epoch of the Si IV /C IV ratio, but appear unable to provide the required number of hydrogen-ionizing photons at z ≈ 5. Subject headings: cosmology: theory – diffuse radiation – intergalactic medium – galaxies: evolution – quasars: general – 2 – 1.