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.