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

We explore the possibility of detecting HI 21cm 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 to 200 MHz). The epoch considered is between the time when the first ionizing sources form and when the bulk of the neutral IGM becomes ionized. Expected IGM absorption signal includes $\sim 1%$ absorption by the mean neutral IGM (the radio 'Gunn-Peterson' effect'), plus deeper, narrow lines ($\ge 5%$, a few km s$^{-1}$) arising in mild density inhomogeneities with typical values of cosmic overdensity $\delta\sim 10$, precisely the structures that at later times give rise to the Ly$\alpha$ forest (the `21cm forest'). Absorption can also arise in gas associated with collapsed structures ($\delta\ge 100$), including 'minihalos' ($\le 10^7$ M$_\odot$) and protodisks ($\ge 10^8$ M$_\odot$). 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.