(2a) The Reionization of the Universe and its Observational
Consequences
Giroux and Shapiro (1996), described their
detailed numerical
method and results for calculations of the thermal and ionization history of
the IGM and of the metagalactic ionizing radiation background contributed by
quasars and stars, from the postrecombination era during which the IGM became
cold, dark, neutral, and highly opaque to ionizing radiation through the
epoch
of reionization to the present. They solved nonequilibrium rate equations for
ionization balance and hydrogen molecule formation in a gas of H and He,
together with the equations of energy conservation and radiative transfer,
including cosmological expansion, radiative and Compton cooling, the diffuse
flux emitted by the gas, the emissivity in ionizing radiation by quasars and
stars, a possible bulk-heating rate such as that from SNe, and the bound-free
opacity of the observed quasar absorption-line gas, in a coarse-grained average
description of the IGM. These calculations elucidated the minimum requirements
for ionizing the IGM to the extent required by the observations and presented
a host of observable consequences. The conclusion was reached, for example,
that additional radiation sources beyond the observed quasars are required
to explain the ionization level of the IGM at high redshift (z greater than 3),
such as massive stars, a view
which overturned the conventional wisdom when it was first advocated by
Shapiro and Giroux in the 1980's and early 1990's
[e.g.
Shapiro and Giroux 1987] but which is
now widely held. The massive-star formation rate
implied by the required emissivity in starlight was shown to imply the
enrichment of the universe with a substantial metallicity by the same stars thatemitted the ionizing radiation background. The spectrum of the evolving
radiation background was shown to possess a significant downward jump on the
blue side of the He II Lyman edge, introduced by He II in the H I Lyman alpha
forest gas. The ratio of He II to H I in this gas was found to be consistent
with that required to explain the recently detected He II opacity of the IGM
at z=3 in terms of blended, weak He II Lyman alpha forest lines, depending
upon the relative importance of quasars and stars in supplying the ionizing
background, but a contribution from He II in the IGM outside of the known H I
Lyman alpha forest is also likely.