|Annu. Rev. Astron. Astrophys. 1993. 31:
Copyright © 1993 by . 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:
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.