A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources:
  implications for annihilating dark matter
  We make a first attempt to find dwarf galaxies in eight \Fermi-LAT extended,
unassociated, source fields using \Gaia\ DR2. We probe previously unexplored
heliocentric distances of $d<20$~kpc with an extreme-deconvolution (XD)
technique. We find no signature of a dwarf galaxy in any of these fields
despite \Gaia's excellent astrometric accuracy. We estimate our detection
limits by applying the XD method to mock data, obtaining a conservative limit
on the stellar mass of $M_* < 10^4$~M$_{\sun}$ for $d < 20$\, kpc. Such a low
stellar mass implies either a low-mass subhalo, or a massive stripped-down
subhalo. We use an analytic model for stripped subhalos to argue that, given
the sizes and fluxes of the \Fermi-LAT sources, we can reject the hypothesis
that they owe to dark matter annihilation.
