Dim baryons in the cosmic web

Research output: Contribution to journalArticlepeer-review


The distribution of baryons beyond galaxies is descibed. The majority of the baryons, which represent 4% of the cosmic mass and energy budget, lie far from individual galaxies in the diffuse intergalactic medium (IGM). Many of these baryons are in a warm phase that can be probed by quasar absorption in the Lyman- line of hydrogen. The mature field of quasar spectroscopy can diagnose the location, physical state, metallicity, and general geometry of this gas, which is called the cosmic web. The remainder of the gas is kept very hot by infall and shocks and is mostly in higher density regions such as filaments, groups and clusters. The hot gas is only detectable via X-rays and the absorption of highly ionized species of heavy elements. The baryons in low density regions of space are excellent tracers of underlying dark matter. The evolution of the cosmic web indicates where to look for the baryons in collapsed objects but the overall inefficiency of galaxy formation has conspired to keep most baryons dark.

Original languageEnglish (US)
Pages (from-to)157-166
Number of pages10
JournalProceedings of the International Astronomical Union
Issue numberS244
StatePublished - Jun 2007


  • Atomic processes
  • Cosmology: observations
  • Dark matter
  • Intergalactic medium
  • Large-scale structure of universe
  • Quasars: absorption lines
  • X-rays: diffuse background

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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