Constraints on Dark Matter Microphysics from the Milky Way Satellite
  Population
  Alternatives to the cold, collisionless dark matter (DM) paradigm in which DM
behaves as a collisional fluid generically suppress small-scale structure.
Herein we use the observed population of Milky Way (MW) satellite galaxies to
constrain the collisional nature of DM, focusing on DM-baryon scattering. We
first derive analytic upper limits on the velocity-independent DM-baryon
scattering cross section by translating the upper bound on the lowest mass of
halos inferred to host satellites into a characteristic cutoff scale in the
linear matter power spectrum. We then confirm and improve these results through
a detailed probabilistic inference of the MW satellite population that
marginalizes over relevant astrophysical uncertainties. This yields $95\%$
confidence upper limits on the DM-baryon scattering cross section of
$2\times10^{-29}\ \rm{cm}^2$ ($6\times 10^{-27}\ \rm{cm}^2$) for DM particle
masses $m_\chi$ of~$10\ \rm{keV}$ ($10\ \rm{GeV}$); these limits scale as
$m_\chi^{1/4}$ for $m_\chi \ll 1\ \rm{GeV}$ and $m_\chi$ for~$m_\chi \gg 1\
\rm{GeV}$. This analysis improves upon cosmological bounds derived from
cosmic-microwave-background anisotropy measurements by multiple orders of
magnitude over a wide range of DM masses, excluding regions of parameter space
previously unexplored by other methods, including direct-detection experiments.
Our work reveals a mapping between DM-baryon scattering and other alternative
DM models, and we discuss the implications of our results for warm and fuzzy DM
scenarios.
