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
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.