![]() | Annu. Rev. Astron. Astrophys. 1993. 31:
689-716 Copyright © 1993 by Annual Reviews. All rights reserved |
3.4. High Redshift Phenomena
All currently viable theories have been selected from among their
unsuccessful rivals by their ability to match local, low redshift
phenomena;
and to a significant extent, their adjustable parameters have been chosen to
match these phenomena. Thus, moderate and high redshift observations
provide a crucial test. We have already seen that the large-scale CBR
fluctuations, measured by COBE and thought to originate at
z 1000,
are problematic for CDM. While they provide support for the general
approach of the theory, quantitatively they engender conflicts. The
1.0°-3.5°
Gaier et al (1992)
upper bounds are less secure. According to the interpretation of
Gorski (1992),
they either are in conflict with all n = 1
models or indicate reionization which could smear out fluctuations on this
angular scale. To achieve the latter, ionization must be maintained until
z
100, which
would indicate significant nonlinearity at this early epoch
- an impossibility in the standard CDM picture.
At lower redshifts we note the existence of quasars up to z
5, quite
complete ionization of the intercloud component of the IGM to the same
redshift, cloud-cloud correlations up to z
2.5, and quasar-quasar
correlations in the vicinity of redshift z = 1. How do these
observations fit into the standard CDM picture?
Efstathiou & Rees (1988)
argued that even in a biased
(8 = 0.4) CDM
model quasars up to a redshift of 4.43 (the record at that time) would not
cause difficulties, and, with the COBE normalization, redshifts
somewhat in excess of z
5 would be
acceptable. The reason is that the number
density of high redshift quasars is so low that a many sigma, very rare
event can, in principle, suffice (given our ignorance concerning the
physical origin of quasars). Ordinary seeming elliptical galaxies
(Lilly 1990)
observed to a redshift of z > 2 are more problematic since large
masses
and high metallicities appear to be involved. But, the observations are too
fragmentary at present for definitive statements to be made.
The high density of ellipticals and spiral bulges presents an indirect piece
of evidence cited by many for the origin of these objects at moderately
high redshift, but here it is our poor theoretical understanding that
handicaps
us. It would seem that efficient dissipation during collapse could lead
to the formation of objects with (effective, half mass) density not greatly
in excess of 100
at the
time of collapse. Combining this with the observed
density of an L* elliptical, indicates a formation redshift, z
30, far too
high for standard CDM.
The clustering observations (cf Shaver et al 1989 for a review), while tantalizing, are similarly difficult to interpret, given our ignorance of the detailed provenance of quasars and the quasar metal line absorption systems.
The Gunn-Peterson
(1965)
test is cleaner, since it measures neutral
hydrogen density along the average line of sight to high redshift. According
to Jenkins & Ostriker
(1991),
optical depth
at a given redshift
z can be translated to density as follows:
![]() | (4) |
where I
is the intergalactic gas density (with assumed primordial
composition) in units of the critical density, and y is the
neutral fraction. In
the second line, photo-ionization equilibrium is assumed with
J-21(z) being
the ionizing radiation density in familiar units and f being the
clumping
factor of the gas. We see the very steep dependence on redshift even if,
magically, J-21(z) is maintained constant in
time. If it decreases with
increasing redshift at early epochs, as it must before galaxy and quasar
formation, the difficulty of maintaining a transparent
(
GP << 1)
medium is even greater. Current limits on
GP are consistent
with the CDM model
only if 10-3 of the baryons have collapsed into, stars by
z = 5. These
stars must have at least the normal fraction of high mass UV emitting
components and at least half of the ionizing radiation must escape the
environs of the galaxy. This is marginally possible in the standard CDM
picture, but the discovery of new, higher redshift quasars could severely
test the theory.
Finally, gravitational lensing of distant quasars measures mass
concentrations along the line of sight, and it is particularly sensitive
to the redshift interval z = 0.5-1.5. A preliminary study by the
author and others,
based on extensive numerical modeling, indicates that too many large
splitting events (
= 10"-30") would
occur in a standard CDM universe
as compared to current observational evidence. An earlier paper by
Narayan & White (1988)
is consistent with this. Using the Press-Schechter
formalism, they found good agreement with observations for
8 = 0.48,
but examination of their Figure 2 shows far too much large splitting
predicted for
8
= 1.05, the COBE normalization.
Thus, in sum, the evidence for redshifts 100 >l z > 0.5 is
fragmentary
but appears to indicate that theories for which galaxy formation was
already well developed at z
10 would be
preferable, and that the density in very nonlinear lumps at z
0.5-1.0 should be
small compared to the
critical density. Both lines of evidence argue weakly against standard
(
= 1) CDM and for
an open variant (
0.2) or for a
baryonic universe.