Abstract
We study the atomic physics and the astrophysical implications of a model in which the dark matter is the analog of hydrogen in a secluded sector. The self interactions between dark matter particles include both elastic scatterings as well as inelastic processes due to a hyperfine transition. The self-interaction cross sections are computed by numerically solving the coupled Schr\"{o}dinger equations for this system. We show that these self interactions exhibit the right velocity dependence to explain the low dark matter density cores seen in small galaxies while being consistent with all constraints from observations of clusters of galaxies. For a viable solution, the dark hydrogen mass has to be in 10--100 GeV range and the dark fine-structure constant has to be larger than 0.02. Precisely for this range of parameters, we show that significant cooling losses may occur due to inelastic excitations to the hyperfine state and subsequent decays, with implications for the evolution of low-mass halos and the early growth of supermassive black holes. Cooling from excitations to higher $n$ levels of dark hydrogen and subsequent decays is possible at the cluster scale, with a strong dependence on halo mass. Finally, we show that the minimum halo mass is in the range of $10^{3.5}$ to $10^7 M_\odot$ for the viable regions of parameter space, significantly larger than the typical predictions for weakly-interacting dark matter models. This pattern of observables in cosmological structure formation is unique to this model, making it possible to rule in or rule out hidden sector hydrogen as a viable dark matter model.
Highlights
While the standard ΛCDM cosmological model with collisionless, cold dark matter (CDM) is successful in explaining the observed large-scale structure in the Universe, there are many puzzles on galactic scales yet to be explained convincingly by a CDM-based scenario
We have investigated a model of self-interacting dark matter that mimics the properties of atomic hydrogen
Dark matter in the late Universe takes the form of dark hydrogen, which is neutral under a new U(1) gauge force
Summary
While the standard ΛCDM cosmological model with collisionless, cold dark matter (CDM) is successful in explaining the observed large-scale structure in the Universe, there are many puzzles on galactic scales yet to be explained convincingly by a CDM-based scenario. Explore the cosmological consequences of, both collisional scattering (which transfers energy between between dark atoms) as well as inelastic hyperfine upscattering (which results in energy loss through excitations and subsequent decays) These cross sections are velocity-dependent, allowing the self-interactions to modify the halo profile to varying degrees in different astrophysical systems. Additional atomic interactions, such as collisional excitations to the n = 2 state and ionization, may begin to affect the structure of cluster-sized halos For this interesting range of parameter space, where we see competing effects from collisional heating and cooling processes on the evolution of halos, we find additional features in the small-scale halo mass function that allow us to distinguish atomic dark matter from CDM cosmologically.
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