Annihilation Signatures of Hidden Sector Dark Matter Within
  Early-Forming Microhalos
  If the dark matter is part of a hidden sector with only very feeble couplings
to the Standard Model, the lightest particle in the hidden sector will
generically be long-lived and could come to dominate the energy density of the
universe prior to the onset of nucleosynthesis. During this early
matter-dominated era, density perturbations will grow more quickly than
otherwise predicted, leading to a large abundance of sub-earth-mass dark matter
microhalos. Since the dark matter does not couple directly to the Standard
Model, the minimum halo mass is much smaller than expected for weakly
interacting dark matter, and the smallest halos could form during the
radiation-dominated era. In this paper, we calculate the evolution of density
perturbations within the context of such hidden sector models and use a series
of $N$-body simulations to determine the outcome of nonlinear collapse during
radiation domination. The resulting microhalos are extremely dense, which leads
to very high rates of dark matter annihilation and to large indirect detection
signals that resemble those ordinarily predicted for decaying dark matter. We
find that the Fermi Collaboration's measurement of the high-latitude gamma-ray
background rules out a wide range of parameter space within this class of
models. The scenarios that are most difficult to constrain are those that
feature a very long early matter-dominated era; if microhalos form prior to the
decay of the unstable hidden sector matter, the destruction of these microhalos
effectively heats the dark matter, suppressing the later formation of
microhalos.
