Observational Constraints of the End of Cosmic Reionization

The epoch of reionization is the last unexplored phase of cosmic evolution, corresponding to a crucial era in cosmic structure formation—the formation of the first luminous objects. I review some recent constraints on the end of reionization using observations of the highest redshift quasars and galaxies. Spectroscopic studies of Gunn‐Peterson (GP) absorption, and related phenomena, suggest a qualitative change in the state of the intergalactic medium (IGM) at $z∼6,$ indicating a rapid increase in the neutral fraction of the IGM, from $xHI<10−4$ at $z⩽5.5,$ to $xHI>10−3,$ and perhaps as high as 0.3, at $z⩾6.$ On the other hand, transmission spikes in the GP trough, and observations of the evolution of the Lyα galaxy luminosity function indicate $xHI<50%$ at $z∼6.5.$ The IGM characteristics at this epoch are consistent with the end of the overlapping stage of reionization. Current data are consistent with star forming galaxies, in particular low luminosity galaxies, rather than quasars and AGNs, as being the dominant sources of reionizing photons, although large uncertainties still exist in estimating the total contribution from the galaxy population at high‐redshift.