Observational constraints on Cosmic Reionization

The epoch of reionization (EoR) is the last unexplored phase of cosmic evolution, corresponding to a crucial era in cosmic structure formation -- the formation of the first luminous objects. We review recent data that have set the first observational constraints on the EoR. These observations paint an interesting picture: spectroscopic studies of Gunn-Peterson (GP) absorption, and related phenomena, suggest a qualitative change in the state of the intergalactic medium (IGM) at z \sim 6, indicating a rapid increase in the neutral fraction of the IGM, from x_{HI} 10^{-3}, and perhaps as high as 0.1, at z \ge 6. The IGM characteristics at this epoch are consistent with the end of the `percolation' stage of reionization. On the other hand, transmission spikes in the GP trough, and observations of the evolution of the \lya galaxy luminosity function indicate x_{HI} < 50% at z\sim 6.5. Similarly, the measurement of the large scale polarization of the CMB implies a significant ionization fraction extending to higher redshifts, z \sim 15 to 20. Overall, the current data suggest that reionization is less an event than a process, extended in both time and space, with the process beginning as early as z \sim 20, and with the last vestiges of the neutral IGM extending down to z \sim 6. Current data are consistent with star forming galaxies, in particular low luminosity galaxies, as being the dominant sources of reionizing photons. We also show that low frequency radio telescopes currently under construction should be able to make the first direct measurements of HI 21cm emission from the neutral IGM during the EoR, and that upcoming measurements of secondary CMB temperature anisotropy will provide fine details of the dynamics of the reionized IGM