COSMOLOGY
(2) The Reionization of the Intergalactic Medium ("IGM")
The IGM and clumpy gas within it were highly ionized by the time quasars seen
at redshifts close to 5 appeared. This is inferred from the absence of the
appreciable H I absorption trough in quasar spectra on the blue side of each
quasar's H I Lyman alpha emission line expected if even a small fraction of the
mean cosmic density of baryons were present in a smoothly distributed, diffuse,
neutral IGM - the Gunn-Peterson effect - and from the observations of a
"forest" of H I Lyman alpha quasar absorption lines caused by intervening gas
clumps, most of which are individually optically thin to ionizing radiation.
At some time between z=5 and the well-known recombination epoch at z=1000,
therefore, the universe experienced a "reionization epoch." Shapiro and his
collaborators have made a number of fundamental contributions to the study of
the nature and source of this reionization and its implications for cosmology.
An extensive summary of this work and of the subject of IGM reionization, in
general, was made by
Shapiro (1995) in an invited review chapter for
the book The Physics of the Interstellar Medium
and the Intergalactic Medium,
including a number of new calculations and results which have not appeared
elsewhere. These latter new results included: the calculation of how early the
reionization by starlight in the CDM model could have been; new calculations
of the He II Gunn-Peterson opacity of the IGM as a discriminant of a
stellar- versus a quasar-dominated ionizing radiation background for
comparison with
recent detections of He II absorption in quasar UV spectra at z=3; and two
new quantitative arguments which demonstrated that the IGM must contain most of
the baryons in the universe at close to the maximum mean density allowed by
Big Bang Nucleosynthesis constraints (one argument was based upon the large
sizes for Lyman alpha forest clouds indicated by recent observations of close
quasar pairs and the other was based upon the observed ratio of X-ray-measured
baryon mass to optically-measured stellar luminosity inside galaxy clusters,
together with the observed universal luminosity density of galaxies both
within and outside of clusters). Among the things summarized in this review
paper which were published by us in more detail elsewhere in this time period
were the many results of the detailed studies of IGM reionization by Shapiro
and Giroux which culminated in two ApJ articles (
1994, "Paper I";
1996, "Paper II"), summarized below.
The universe was reionized by redshift z ~ 6 by a small fraction of
the baryons in the universe, which released energy following
their condensation out of a cold, dark, and neutral IGM into
the earliest galaxies. The theory of this reionization is a
critical missing link in the theory of galaxy formation.
Its numerous observable consequences include effects on the
spectrum, anisotropy and polarization of the cosmic microwave
background and signatures of high-redshift star and quasar formation.
This energy release also created feedback on galaxy formation
which left its imprint on the mass spectrum and internal
characteristics of galaxies and on the gas between galaxies long
after reionization was complete. Recent work suggests that the
photoevaporation of dwarf galaxy minihalos may have consumed most of
the photons required to reionize the currently-favored Lambda-CDM universe.
Recent developments in our understanding of this process were reviewed
by
Shapiro (2001).