A Gaia DR2 search for dwarf galaxies towards Fermi-LAT sources: implications for annihilating dark matter.

We make the first attempt to find dwarf galaxies in eight Fermi -LAT extended, unassociated, source fields using Gaia DR2. After probing previously unexplored heliocentric distances of d < 20 kpc with an extreme-deconvolution (XD) technique, we find no sign of a dwarf galaxy in any of these fields despite Gaia 's excellent astrometric accuracy. Our detection limits are estimated by applying the XD method to mock data, obtaining a conservative limit on the stellar mass of M _* < 10⁴ M_⊙ 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 subhaloes 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. [ABSTRACT FROM AUTHOR]