Searching for velocity-dependent dark matter annihilation signals from
  extragalactic halos
  We consider gamma-ray signals of dark matter annihilation in extragalactic
halos in the case where dark matter annihilates from a $p$-wave or $d$-wave
state. In these scenarios, signals from extragalactic halos are enhanced
relative to other targets, such as the Galactic Center or dwarf spheroidal
galaxies, because the typical relative speed of the dark matter is larger in
extragalactic halos. We perform a mock data analysis of gamma rays produced by
dark matter annihilation in halos detected by the Sloan Digital Sky Survey. We
include a model for uncorrelated galactic and extragalactic gamma ray
backgrounds, as well as a simple model for backgrounds due to astrophysical
processes in the extragalactic halos detected by the survey. We find that, for
models which are still allowed by other gamma ray searches, searches of
extragalactic halos with the current Fermi exposure can produce evidence for
dark matter annihilation, though it is difficult to distinguish the $p$-wave
and $d$-wave scenarios. With a factor $10\times$ larger exposure, though,
discrimination of the velocity-dependence is possible.
