Observational constraints of cosmic reionization

The epoch of reionization is the last unexplored phase of cosmic evolution; corresponding to a, crucail 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 similar to 6, indicating a. rapid increase in the neutral fraction of the IGM, from x(HI) < 10(-4) at z <= 5.5, to x(HI) > 10(-3) and perhaps as high as 0.3, at z >= 6. On the other band; transmission spikes in the GP trough, and observations of the evolution of the Ly alpha galaxy luminosity function indicate x(HI) < 50% at z similar to 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.