The 21 Centimeter Forest: Radio Absorption Spectra as Probes of Minihalos before Reionization

We study the absorption along lines of sight toward high-z radio sourcescaused by the 21 cm transition of neutral hydrogen in the intergalactic medium(IGM) before reionization. Using semi-analytic methods, we compute the numberdensity of observable features caused by both ``minihalos'' (bound objects thatare unable to cool efficiently because of their small virial temperatures) andprotogalactic disks. We show that both sets of features should be observable bythe next generation of low-frequency radio telescopes, including the LowFrequency Array and the Square Kilometer Array, provided that sufficientlybright background sources exist. The statistics of minihalo absorption featuresseen along lines of sight to radio-loud quasars offer a way to measure theevolution of the radiation background and the IGM temperature with cosmic time.Intersections with disks are much less common, but also cause much deeperabsorption features that would be visible in the spectra of both radio-loudquasars and gamma-ray bursts (GRBs). The absorption feature caused by HI in thehost galaxy of a GRB should be observable, offering a route to determinespectroscopically the burst redshift.Comment: 24 pages, 7 figures, version to appear in ApJ (minor modifications from submitted version, conclusions unchanged