Novel constraints on the particle nature of dark matter from stellar
  streams
  Tidal streams are highly sensitive to perturbations from passing dark matter
(DM) subhalos and thus provide a means of measuring their abundance. In a
recent paper, we analyzed the distribution of stars along the GD-1 stream with
a combination of data from the Gaia satellite and the Pan-STARRS survey, and we
demonstrated that the population of DM subhalos predicted by the cold dark
matter (CDM) paradigm are necessary and sufficient to explain the perturbations
observed in the linear density of stars. In this paper, we use the measurements
of the subhalo mass function (SHMF) from the GD-1 data combined with a similar
analysis of the Pal 5 stream to provide novel constraints on alternative DM
scenarios that predict a suppression of the SHMF on scales smaller than the
mass of dwarf galaxies, marginalizing over uncertainties in the slope and
normalization of the unsuppressed SHMF and the susceptibility of DM subhalos in
the inner Milky Way to tidal disruption. In particular, we derive a 95% lower
limit on the mass of warm dark matter (WDM) thermal relics $m_{\rm
WDM}>3.6\,\mathrm{keV}$ from streams alone that strengthens to $m_{\rm
WDM}>6.2\,\mathrm{keV}$ when adding dwarf satellite counts. Similarly, we
constrain the axion mass in ultra-light ("fuzzy") dark matter (FDM) models to
be $m_{\rm FDM}>1.4\times10^{-21}\,\mathrm{eV}$ from streams alone or $m_{\rm
FDM}>2.2\times10^{-21}\,\mathrm{eV}$ when adding dwarf satellite counts.
Because we make use of simple approximate forms of the streams' SHMF
measurement, our analysis is easy to replicate with other alternative DM models
that lead to a suppression of the SHMF.
