Observations of HI 21-cm absorption by the neutral IGM during the epoch of re-ionization with the Square Kilometer Array

We explore the possibility of detecting HI 21-cm absorption by the neutral intergalactic medium (IGM) toward very high redshift radio sources, and by gas associated with the first collapsed structures, using the Square Kilometer Array at low frequency (100-200 MHz). The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral TGM becomes ionized. Expected IGM absorption signal includes similar tol% absorption by the mean neutral IGM (the radio 'Gunn-Peterson' effect), plus deeper, narrow lines (>5%, a few kin s(-1)) arising in mild density inhomogencities with typical values of cosmic overdensity delta similar to 10, precisely the structures that at later times give rise to the Lyalpha forest (the '21-cm forest'). Absorption can also arise in gas associated with collapsed structures (delta > 100), including 'minihalos' (less than or equal to 10(7)M(circle dot)) and protodisks (>10(8)M(circle dot)). We consider SKA sensitivity limits and the evolution of radio source populations, and conclude that it is reasonable to hypothesize the existence of an adequate number of high-z radio sources against which such absorption studies could be performed, provided that reionization occurs at z < 10. Lastly, we discuss the possibility of 'line confusion' due to radio recombination lines arising in the ionized IGM. Overall, SKA absorption studies should provide a fundamental probe of the thermal state of the neutral IGM during the epoch of reionization, as well as critical insight into the process and sources of reionization. (C) 2004 Elsevier B.V. All rights reserved.