The 21 cm Forest: Radio Absorption Spectra as Probes of Minihalos Before   Reionization
  We study the absorption along lines of sight toward high-z radio sources caused by the 21 cm transition of neutral hydrogen in the intergalactic medium (IGM) before reionization. Using semi-analytic methods, we compute the number density of observable features caused by both ``minihalos'' (bound objects that are unable to cool efficiently because of their small virial temperatures) and protogalactic disks. We show that both sets of features should be observable by the next generation of low-frequency radio telescopes, including the Low Frequency Array and the Square Kilometer Array, provided that sufficiently bright background sources exist. The statistics of minihalo absorption features seen along lines of sight to radio-loud quasars offer a way to measure the evolution of the radiation background and the IGM temperature with cosmic time. Intersections with disks are much less common, but also cause much deeper absorption features that would be visible in the spectra of both radio-loud quasars and gamma-ray bursts (GRBs). The absorption feature caused by HI in the host galaxy of a GRB should be observable, offering a route to determine spectroscopically the burst redshift. 
Steven Furlanetto, Abraham Loeb (Harvard University)