Exploring the astrophysics of dark atoms
  A component of the dark matter could consist of two darkly charged particles
with a large mass ratio and a massless force carrier. This `atomic' dark sector
could behave much like the baryonic sector, cooling and fragmenting down to
stellar-mass or smaller scales. Past studies have shown that cosmic microwave
background and large-scale structure constraints rule out $\gtrsim 5\%$ of the
dark matter to behave in this manner. However, we show that, even with percent
level mass fractions, a dark atomic sector could affect some extragalactic and
galactic observables. We track the cooling and merger history of an atomic dark
component for much of the interesting parameter space. Unlike the baryons,
where stellar feedback (driven by nuclear physics) delays the formation and
growth of galaxies, cooling dark atomic gas typically results in disks forming
earlier, leaving more time for their destruction via mergers. Rather than disks
in Milky Way sized halos, we find the end product is typically spheroidal
structures on galactic scales or dark atom fragments distributed on halo
scales. This result contrasts with previous studies, which had assumed that the
dark atoms would result in dark disks. Furthermore the dark atoms condense into
dense clumps, analogous to how the baryons fragment on solar-mass scales. We
estimate the size of these dark clumps, and use these estimates to show that
viable atomic dark matter parameter space is ruled out by stellar microlensing,
by the half-light radii of ultra-faint dwarf galaxies, and by Milky Way
mass-to-light inferences.
