Searching for the earliest galaxies in the 21 cm forest

We use a model developed by Xu, Ferrara, and Chen ( Mon Not Roy Astron Soc, submitted) to compute the 21 cm line absorption signatures imprinted by star-forming dwarf galaxies (DGs) and starless minihalos (MHs). The method, based on a statistical comparison of the equivalent width ( W ν ) distribution and flux correlation function, allows us to derive a simple selection criteria for candidate DGs at very high ( z ⩽ 8) redshift. We find that ≈18% of the total number of DGs along a line of sight to a target radio source (GRB or quasar) can be identified by the condition W ν < 0; these objects correspond to the high-mass tail of the DG distribution at high redshift, and are embedded in large HII regions. The criterion W ν > 0.37 kHz instead selects ≈11% of MHs. Selected candidate DGs could later be re-observed in the near-IR by the JWST with high efficiency, thus providing a direct probe of the most likely reionization sources.