Space Science Reviews

, Volume 100, Issue 1–4, pp 89–99 | Cite as

Baryons in the diffuse intergalactic medium

  • D. Reimers


The baryon density of the universe ΩB is well measured indirectly from Big-Bang nucleo-synthesis, in particular by recent measurements of the D/H ratio in high-redshift QSO absorption systems. In addition, very recent measurements of the second maximum of the power-spectrum of the CMB fine scale anisotropy allow to constrain ΩB at z≈1000. Both results agree and yield ΩB=0.02h−2. Direct measurements of the diffuse baryonic component (intergalactic gas) at redshifts z=3 and 1.5 and in the local universe are reviewed and shown to be much more difficult. Available observations are consistent with the hypothesis that at z=3 and possibly still at z=1.5 nearly all baryons are located in the highly ionized Lα forest component, while at later epochs the contribution of a low-density, shock-heated component (105–107 K), the so called warm-hot intergalactic medium (WHIM), occupies with decreasing redshift an increasing fraction of all baryons. Methods to detect this component and the difficulty to make quantitative estimates are described. In the local universe \({\raise0.7ex\hbox{$1$} \!\mathord{\left/ {\vphantom {1 3}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$3$}}\) of all baryons may be hidden in the WHIM. Yet at z=1.5, this component contains at least a factor of 5 less material.


Anisotropy Direct Measurement Quantitative Estimate Fine Scale Absorption System 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.


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© Kluwer Academic Publishers 2002

Authors and Affiliations

  • D. Reimers
    • 1
  1. 1.Hamburger SternwarteUniversität HamburgGermany

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