Spherical Cows in Dark Matter Indirect Detection
  Dark matter (DM) halos have long been known to be triaxial, but in studies of
possible annihilation and decay signals they are often treated as approximately
spherical. In this work, we examine the asymmetry of potential indirect
detection signals of DM annihilation and decay, exploiting the large statistics
of the hydrodynamic simulation Illustris. We carefully investigate the effects
of the baryons on the sphericity of annihilation and decay signals for both the
case where the observer is at 8.5 kpc from the center of the halo (exemplified
in the case of Milky Way-like halos), and for an observer situated well outside
the halo. In the case of Galactic signals, we find that both annihilation and
decay signals are expected to be quite symmetric, with axis ratios very
different from 1 occurring rarely. In the case of extragalactic signals, while
decay signals are still preferentially spherical, the axis ratio for
annihilation signals has a much flatter distribution, with elongated profiles
appearing frequently. Many of these elongated profiles are due to large
subhalos and/or recent mergers. Comparing to gamma-ray emission from the Milky
Way and X-ray maps of clusters, we find that the gamma-ray background appears
less spherical/more elongated than the expected DM signal from the large
majority of halos, and the Galactic gamma ray excess appears very spherical,
while the X- ray data would be difficult to distinguish from a DM signal by
elongation/sphericity measurements alone.
