Indirect Dark Matter Signatures in the Cosmic Dark Ages II. Ionization,
  Heating and Photon Production from Arbitrary Energy Injections
  Any injection of electromagnetically interacting particles during the cosmic
dark ages will lead to increased ionization, heating, production of Lyman-alpha
photons and distortions to the energy spectrum of the cosmic microwave
background, with potentially observable consequences. In this note we describe
numerical results for the low-energy electrons and photons produced by the
cooling of particles injected at energies from keV to multi-TeV scales, at
arbitrary injection redshifts (but focusing on the post-recombination epoch).
We use these data, combined with existing calculations modeling the cooling of
these low-energy particles, to estimate the resulting contributions to
ionization, excitation and heating of the gas, and production of low-energy
photons below the threshold for excitation and ionization. We compute corrected
deposition-efficiency curves for annihilating dark matter, and demonstrate how
to compute equivalent curves for arbitrary energy-injection histories. These
calculations provide the necessary inputs for the limits on dark matter
annihilation presented in the accompanying Paper I, but also have potential
applications in the context of dark matter decay or de-excitation, decay of
other metastable species, or similar energy injections from new physics. We
make our full results publicly available at
http://nebel.rc.fas.harvard.edu/epsilon, to facilitate further independent
studies. In particular, we provide the full low-energy electron and photon
spectra, to allow matching onto more detailed codes that describe the cooling
of such particles at low energies.
