Diagnosing the reionization of the universe: The absorption spectrum of the intergalactic medium and Lyman α clouds

Abstract

We have calculated in detail the thermal and ionization evolution of a uniform intergalactic medium (IGM) composed of H and He undergoing ‘‘reionization,’’ including the mean effect of gas clumps embedded in a smoothly distributed ambient gas. We have solved the rate equations for ionization and recombination together with the equations of energy conservation including the effects of cosmological expansion, radiative and Compton cooling, and the diffuse flux emitted by the gas, and radiative transfer. We have included the contribution to the continuum opacity of the universe due to the observed quasar absorption line clouds (QALC’s). We have considered a variety of sources of photoionization, including quasars and primeval galaxies, as well as the possibility that hydrodynamical processes deposit thermal energy in the IGM.−1–3 We limit our discussion in what follows, to two questions.(1) Can metal line ratios for Lyman limit quasar absorption line systems diagnose the metagalactic ionizing radiation background at z∼3? The interesting suggestion has been made4 that metal absorption line strengths for Lyman limit (LL) QALC’s (NHi≳1017 cm−2) at z∼3 can be compared with photoionized cloud predictions in order to diagnose the nature and origin of the metagalactic ultraviolet and/or soft x‐ray background. Previous results have strongly favored an AGN‐type, nonthermal source spectrum for this background over that emitted by a normal star‐forming galaxy. The input spectra for these theoretical comparison models, however, were those of the sources in their own rest frame, unaffected by the intervening IGM and unevolved by cosmological expansion and redshift effects. We have reexamined this question by using our detailed thermal/ionization and radiative transfer calculations of the IGM to predict the proper cosmologically evolving metagalactic ionizing radiation field for each of several types of underlying source spectra. We find that wider range of source type is acceptable than previously thought.(2) Can the He ii Gunn‐Peterson effect be used to diagnose the reionization source and/or the nature of the Ly α forest clouds? Our detailed IGM calculations predict the strength of the Gunn‐Peterson (GP) He ii Lyman α absorption troughs from the smoothly‐distributed IGM, detectable by the HST, for a variety of assumptions about the nature of the Lyα absorbers and of the sources of ionization of the IGM and Ly α clouds, including a hydrodynamical source (e.g. shock‐heating) for the former.