  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 Lyman-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 non-linear bias and minihalo structure emerge only at
much smaller scales, well beyond the resolution of any upcoming instruments.
Using simple, but representative, reionization histories, we then show that the
required Lyman-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.
