  Indirect detection of high-energy particles from dark matter interactions is
a promising avenue for learning more about dark matter, but is hampered by the
frequent coincidence of high-energy astrophysical sources of such particles
with putative high-density regions of dark matter. We calculate the boost
factor and gamma-ray flux from dark matter associated with two shell-like
caustics of luminous tidal debris recently discovered around the Andromeda
galaxy, under the assumption that dark matter is its own supersymmetric
antiparticle. These shell features could be a good candidate for indirect
detection of dark matter via gamma rays because they are located far from the
primary confusion sources at the galaxy's center, and because the shapes of the
shells indicate that most of the mass has piled up near apocenter. Using a
numerical estimator specifically calibrated to estimate densities in N-body
representations with sharp features and a previously determined N-body model of
the shells, we find that the largest boost factors do occur in the shells but
are only a few percent. We also find that the gamma-ray flux is an order of
magnitude too low to be detected with Fermi for likely dark matter parameters,
and about 2 orders of magnitude less than the signal that would have come from
the dwarf galaxy that produces the shells in the N-body model. We further show
that the radial density profiles and relative radial spacing of the shells, in
either dark or luminous matter, is relatively insensitive to the details of the
potential of the host galaxy but depends in a predictable way on the velocity
dispersion of the progenitor galaxy.
