  The Fermi-LAT collaboration has studied the gamma-ray emission from a stacked
population of dwarf spheroidal galaxies and used this information to set
constraints on the dark matter annihilation cross section. Interestingly, their
analysis uncovered an excess with a test statistic (TS) of 8.7. If interpreted
naively, this constitutes a 2.95 sigma local excess (p-value=0.003), relative
to the expectations of their background model. In order to further test this
interpretation, the Fermi-LAT team studied a large number of blank sky
locations and found TS>8.7 excesses to be more common than predicted by their
background model, decreasing the significance of their dwarf excess to 2.2
sigma (p-value=0.027). We argue that these TS>8.7 blank sky locations are
largely the result of unresolved blazars, radio galaxies, and starforming
galaxies, and show that multi-wavelength information can be used to reduce the
degree to which such sources contaminate the otherwise blank sky. In
particular, we show that masking regions of the sky that lie within 1 degree of
sources contained in the BZCAT or CRATES catalogs reduces the fraction of blank
sky locations with TS>8.7 by more than a factor of two. Taking such
multi-wavelength information into account can enable experiments such as Fermi
to better characterize their backgrounds and increase their sensitivity to dark
matter in dwarf galaxies, the most important of which remain largely
uncontaminated by unresolved point sources. We also note that for the range of
dark matter masses and annihilation cross sections currently being tested by
studies of dwarf spheroidal galaxies, simulations predict that Fermi should be
able to detect a significant number of dark matter subhalos. These subhalos
constitute a population of sub-threshold gamma-ray point sources and represent
an irreducible background for searches for dark matter annihilation in dwarf
galaxies.
