THE REIONIZATION OF THE INTERGALACTIC MEDIUM AND ITS OBSERVATIONAL CONSEQUENCES

Citation
Ml. Giroux et Pr. Shapiro, THE REIONIZATION OF THE INTERGALACTIC MEDIUM AND ITS OBSERVATIONAL CONSEQUENCES, The Astrophysical journal. Supplement series, 102(2), 1996, pp. 191-238
Citations number
131
Categorie Soggetti
Astronomy & Astrophysics
ISSN journal
00670049
Volume
102
Issue
2
Year of publication
1996
Pages
191 - 238
Database
ISI
SICI code
0067-0049(1996)102:2<191:TROTIM>2.0.ZU;2-Y
Abstract
We have calculated the thermal and ionization evolution of a uniform d ensity intergalactic medium (IGM) composed of H and He in a postrecomb ination Friedmann universe as sources inject ionizing radiation and en ergy into it. We have solved nonequilibrium rate equations for ionizat ion and recombination, together with the equations of energy conservat ion, including the effects of cosmological expansion, radiative and Co mpton cooling, and the diffuse flux emitted by the gas, and radiative transfer, in this coarse-grained-average description of the IGM. For t he radiative transfer, we also include the mean effect of gas clumps ( the quasar absorption-line clouds [QALCs]) embedded in the smoothly di stributed ambient gas. We focus on the presumed transition within the IGM from cold neutral gas to a highly ionized state. We have considere d the effect of a metagalactic ionizing radiation background, such as would be contributed by quasars and primeval galaxies, as well as the possibility that hydrodynamical processes deposit thermal energy in th e IGM. We describe our numerical method and apply this method here for the purpose of elucidating the minimum requirements for reionizing th e IGM by z approximate to 5 to the extent required by the hydrogen Gun n-Peterson (GP) constraint, by the energy release associated with cosm ological structure formation. These minimum requirements are a signifi cant constraint on theories of the origin of structure in the universe . For a photoionized universe, we determine the minimum required ioniz ing photon emissivity of the universe at z > 4. We find that the requi red emissivity exceeds that due to the observed quasar population unle ss the IGM density Omega(IGM)h(2) < 4-6 X 10(-3). We consider the cons equences for primordial galaxy luminosity density and metal production if early-type stars are the photoionization source instead. Such reio nization by stars implies that a substantial metallicity per galactic baryon was generated at high redshift (i.e., z > 3). We also consider scenarios in which the IGM is primarily collisionally ionized as a res ult of heating to temperatures T> 10(5) K by bulk hydrodynamical heati ng processes. In this case, the GP constraint may be satisfied with no violation of current COBE Limits on the Compton y-parameter. If the s ource of this heating is the supernova explosions of early-type stars, then there is a substantial metallicity production at high redshift i mplied that is similar to that of the photoionized case. If such bulk heating occurs in conjunction with photoionization, however, then the average ionizing photon emissivity from sources at high z required to photoionize the universe enough to satisfy the GP constraint is substa ntially reduced. Our calculations yield a set of observational diagnos tics of the nature and source of the ionization of the IGM and of quas ar absorption-line clouds. This includes predictions of the H, He I, a nd He II Gunn-Peterson Ly alpha absorption troughs from the smoothly d istributed IGM, dependent on the nature of the ionizing source and the physical state of the Ly alpha cloud absorbers. We compare our result s with the latest observational constraints.