Molecular cooling is essential for studying the formation of sub-structure of dissipative dark-matter halos that may host compact objects such as black holes. Here, we analyze the reaction rates relevant for the formation, dissociation, and transition of hydrogenic molecules while allowing for different values of the physical parameters: the coupling constant, the proton mass, and the electron mass. For all cases, we re-scale the reaction rates for the standard molecular hydrogen, so our results are valid as long as the dark matter is weakly coupled and one of the fermions is much heavier than the other. These results will allow a robust numerical treatment of cosmic structure, in particular for mini-halos for which molecular cooling is important, in a dissipative dark matter scenario.

Molecular cooling is essential for studying the formation of substructure of dissipative dark-matter halos that may host compact objects such as black holes. Here, we analyze the reaction rates relevant for the formation, dissociation, and transition of hydrogenic molecules while allowing for different values of the physical parameters: the coupling constant, the proton mass, and the electron mass. For all cases, we rescale the reaction rates for the standard molecular hydrogen, so our results are valid as long as the dark matter is weakly coupled and one of the fermions is much heavier than the other. These results will allow a robust numerical treatment of cosmic structure, in particular for minihalos for which molecular cooling is important, in a dissipative dark-matter scenario.

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 rescaling the rates for normal hydrogen, and evolved the abundances for ionized, atomic, and molecular states using a modified version of Recfast++ (which we have released publicly at a a https://github.com/jamesgurian/RecfastJulia ). We also provide an analytical approximation for the final abundances. We then calculate the effects of atomic dark matter on the linear power spectrum, which enter through a dark photon diffusion and dark acoustic oscillations. At formation time, the atomic dark matter model suppresses halo abundances on scales smaller than the diffusion scale, just as 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.

We study the fragmentation scale of dark gas formed in dissipative dark-matter halos and show that the simple atomic-dark-matter model consistent with all current observations can create low-mass fragments that can evolve into compact objects forbidden by stellar astrophysics. We model the collapse of the dark halo’s dense core by tracing the thermochemical evolution of a uniform-density volume element under two extreme assumptions for density evolution: hydrostatic equilibrium and pressure-free collapse. We then compute the opacity-limited minimum fragment mass from the minimum temperature achieved in these calculations. The results indicate that much of the parameter space is highly unstable to small-scale fragmentation.

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