Observational constraints on cosmic reionization

Xiaohui Fan, C. L. Carilli, B. Keating

Research output: Chapter in Book/Report/Conference proceedingChapter

661 Scopus citations


Observations have set the first constraints on the epoch of reionization (EoR), corresponding to the formation epoch of the first luminous objects. Studies of Gunn-Peterson (GP) absorption indicate a rapid increase in the neutral fraction of the intergalactic medium (IGM) from x HI < 10 -4 at z ≤ 5.5, to x HI > 10 -3, perhaps up to 0.1, at z ∼ 6, while the large scale polarization of the cosmic microwave background (CMB) implies a significant ionization fraction extending to higher redshifts, z ∼ 11 ± 3. These results, as well as observations of galaxy populations, suggest that reionization is a process that begins as early as z ∼ 14, and ends with the "percolation" phase at z ∼ 6 to 8. Low luminosity star-forming galaxies are likely the dominant sources of reionizing photons. Future low-frequency radio telescopes will make direct measurements of HI 21-cm emission from the neutral IGM during the EoR, and measurements of secondary CMB temperature anisorropy will provide details of the dynamics of the reionized IGM.

Original languageEnglish (US)
Title of host publicationAnnual Review of Astronomy and Astrophysics
EditorsRoger Blandford, John Kormendy, Ewine Dishoeck
Number of pages48
StatePublished - 2006

Publication series

NameAnnual Review of Astronomy and Astrophysics
ISSN (Print)0066-4146


  • Cosmic reionization
  • Galaxy formation
  • Observational cosmology

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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