Observational constraints on cosmic reionization

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 similar to 6, while the large scale polarization of the cosmic microwave background (CMB) implies a significant ionization fraction extending to higher redshifts, z similar to 11 +/- 3. These results, as well as observations of galaxy populations, suggest that reionization is a process that begins as early as z similar to 14, and ends with the "percolation" phase at z similar to 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 anisotropy will provide details of the dynamics of the reionized IGM.