Annu. Rev. Astron. Astrophys. 1992. 30: 499-542 Copyright © 1992 by Annual Reviews. All rights reserved

4.5 Quasar Absorption Line Statistics

Gas clouds, believed to be associated with the halos of galaxies, are distributed through intergalactic space and cause narrow absorption features in quasar spectra. These are the most numerous objects that can be counted to high redshift. Their statistical distribution with redshift, dN / dz, offers another possible test somewhat akin to the dV / dz count tests. Potentially, the evolution of ionized gas clouds might be more easily understandable than that of physically much more complex whole galaxies; at least in the case of the clouds, one probably knows what the relevant fundamental equations and physical effects are. This gives some hope that evolution and cosmology might someday be clearly disentangled for quasar absorption line statistics.

In an early application of this idea, Tytler (1981) used the absence of strong features in the distribution of quasar absorption line redshifts to argue against loitering cosmologies. Turner & Ikeuchi (1992) have studied quasar absorption line statistics in cosmological models corresponding to our cases A and C, using an identical simple physical model for the clouds in each case to account for their evolution. They find that no clear choice between these two extreme possibilities can be made from the available data; however, the extrapolation to low redshift of high-redshift fits of various absorption lines predicts quite different frequencies for the two models. Typically 2-3 times more frequent absorptions are predicted for model C than for model A. When space far-UV spectroscopy of quasars provides a substantial body of data on low-redshift quasar absorption lines (not now available), this effect may well provide an interesting test. Also, flat -dominated models predict an inflection in dN / dz versus z for all types of lines at a redshift of about ( / _M)^1/3 due to the universes transition into roughly exponential expansion. Consistent detection of such a feature in a wide variety of classes of absorbers might give some confidence that it was due to a cosmological, rather than an evolutionary, effect. This is clearly an area which deserves further work, especially as data on low-redshift absorption systems becomes available.

The first Hubble Space Telescope (HST) data on low-redshift Lyman-alpha clouds (Morris et al 1991, Bahcall et al 1991) have been tentatively interpreted on this basis as evidence against -dominated models by Fukugita & Lahav (1991), with, however, a variety of caveats (Turner & Ikeuchi 1992, Ikeuchi & Turner 1991).