Evolution of the Intergalactic Medium and UV Background Radiation

Abstract

The absence of an HI Gunn-Peterson trough extending to the blue side of the Lyman α emission in the spectra of high-redshift quasars has placed a severe limit on the density of neutral hydrogen in the intergalactic medium. In addition, a probable detection of a HeII Gunn-Peterson trough has been reported, though it is controversial. On the other hand, numerous and narrow absorption lines in quasar spectra have been interpreted as the so-called Lyman α forest. They would play an important role in relation to the evolution of the UV background radiation at high redshifts. We calculate the evolution of the UV background radiation, and explore the ionization states of intergalactic gas and clouds while trying to solve any mutual relations among them consistently. We examine two such models for the Lyman α clouds as pressure-confined and for the clouds confined by cold dark matter, the minihalos. For the former, the evolution is not consistent with other observational constraints, because these clouds with low HI column densities evolve too rapidly to be missed in detection at low redshifts, z ≲ 2, contrary to the HST observations that more numerous Lyman α absorption lines exist than predicted by extrapolation from high redshift. Otherwise, the minihalos cannot reproduce the evolution of the Lyα forest at high redshifts if assumed that the evolution of the UV background is proportional to the number of QSOs. Consequently, we propose a two-component model: pressure-confined clouds at high redshifts and minihalos at low redshifts, while satisfying all of the observational constraints marginally within resonable parameters.