Redshifted 21 cm emission from minihalos before reionization

Before reionization, the intergalactic medium (IGM) may have been sufficiently cold for low-mass minihalos to condense out of the gas and subsequently affect reionization. Previous work has shown that minihalos generate reasonably large 21 cm fluctuations. Here we consider this signal in its proper cosmological context and show that isolating minihalos from the rest of the IGM is extremely difficult. Using the well-known halo model, we compute the power spectrum of 21 cm fluctuations from minihalos and show that the signal decreases rapidly as feedback increases the Jeans mass. We then show that even a small Ly alpha background increases the 21 cm fluctuations of the diffuse IGM well beyond those of the minihalos; because the mass fraction in the IGM is much larger, minihalos will lie buried within the IGM signal. The distinctive signatures of nonlinear bias and minihalo structure emerge only at much smaller scales, well beyond the resolution of any upcoming instrument. Using simple but representative reionization histories, we then show that the required Ly alpha background level is most likely achieved at z >= 15, while minihalos are still rare, so that they are almost always degenerate with the diffuse IGM.