Molecular Chemistry for Dark Matter II: Recombination, Molecule
  Formation, and Halo Mass Function in Atomic Dark Matter
  Dissipative dark matter predicts rich observable phenomena that can be tested
with future large-scale structure surveys. As a specific example, we study
atomic dark matter, consisting of a heavy particle and a light particle charged
under a dark electromagnetism. In particular, we calculate the cosmological
evolution of atomic dark matter focusing on dark recombination and
dark-molecule formation. We have obtained the relevant interaction-rate
coefficients by re-scaling the rates for normal hydrogen, and evolved the
abundances for ionized, atomic, and molecular states using a modified version
of RecFAST++. We also provide an analytical approximation for the final
abundances. We then calculate the effects of the atomic dark matter on the
linear power spectrum, which enter through a dark-photon diffusion and dark
acoustic oscillations. At the formation time, the atomic dark matter model
suppresses halo abundances on scales smaller than the diffusion scale, just
like the warm dark matter models suppress the abundance below the
free-streaming scale. The subsequent evolution with radiative cooling, however,
will alter the halo mass function further.
