Redshifted 21 cm Emission from Minihalos before Reionization

Before reionization, the intergalactic medium (IGM) may have beensufficiently cold for low-mass "minihalos" to condense out of the gas andsubsequently affect reionization. Previous work has shown that minihalosgenerate reasonably large 21 cm fluctuations. Here we consider this signal inits proper cosmological context and show that isolating minihalos from the restof the IGM is extremely difficult. Using the well-known halo model, we computethe power spectrum of 21 cm fluctuations from minihalos and show that thesignal decreases rapidly as feedback increases the Jeans mass. We then showthat even a small Lyman-alpha background increases the 21 cm fluctuations ofthe diffuse IGM well beyond those of the minihalos; because the mass fractionin the IGM is much larger, minihalos will lie buried within the IGM signal. Thedistinctive signatures of non-linear bias and minihalo structure emerge only atmuch smaller scales, well beyond the resolution of any upcoming instruments.Using simple, but representative, reionization histories, we then show that therequired Lyman-alpha background level is most likely achieved at z>15, whileminihalos are still rare, so that they are almost always degenerate with thediffuse IGM.Comment: 8 pages, 6 figures, submitted to Ap