Quasar absorption lines with a nonzero cosmological constant

Edwin L. Turner, Satoru Ikeuchi

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Flat but nonzero cosmological constant model universes contain substantially larger comoving volumes and path lengths within a given redshift than more conventional zero Λ ones. They also change the nature of their expansion at a characteristic critical redshift zc = (λ00)1/3, whereas zero Λ flat models show a smooth power-law expansion at all epochs. Both of these physical differences can affect intergalactic absorption and scattering processes. The former simply implies more absorption for a given fixed population of absorbers or, conversely, requires fewer absorbers to explain a given observed amount of absorption. The latter manifests itself as a break in the number of absorbers per unit redshift at about zc and gives rise to a quite different redshift dependence of the number of absorbers at z < zc. Thus, there is an interesting interaction between Λ effects and cosmic absorption phenomena which are explored in this paper. For example, the absolute frequency of metal and high column density H I absorption at high redshift is significantly closer to that expected from the properties of low-redshift galaxies in Λ-dominated cosmologies than in conventional ones. The equations describing IGM absorption statistics are developed for nonzero Λ cosmologies, both for unevolving absorber populations and including the effects of the expected absorber evolution in some of the more popular physical models for the IGM and intergalactic clouds. These results are compared to observations for the cases of Lyman-α forest lines, damped Lyman-α lines, Lyman limit systems, C IV 1549 Å lines, Mg II 2800 Å lines, diffuse H I absorption (Gunn-Peterson effect), and the Compton y-parameter. Cosmological models with (Ω0, λ0) of (1, 0) and (0.1, 0.9) are contrasted. In general, the available data are consistent with both models, but the (0.1, 0.9) model extrapolates to much less absorption at low redshift and requires less evolution with redshift than the (1, 0) model. Hubble Space Telescope and ASTRO space UV spectroscopy of low-redshift absorption lines should allow a fairly direct test of Λ. The Mg II absorption line data currently extend to fairly low redshifts and allow the best Λ discrimination available with present data; the (0.1, 0.9) model gives a beter fit to the Mg II observations, but the size of the statistical errors is such that the (1, 0) model is not excluded with great significance. It is also found that the available Gunn-Peterson effect and Compton y-parameter limits are somewhat more difficult to satisfy in the (0.1, 0.9) model than in the (1, 0) one.

Original languageEnglish (US)
Pages (from-to)478-490
Number of pages13
JournalAstrophysical Journal
Issue number2
StatePublished - Apr 20 1992

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • Cosmology: theory
  • Dust, extinction
  • Intergalactic medium
  • Quasars: absorption lines


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