Cosmological Simulations of Decaying Dark Matter: Implications for
  Small-scale Structure of Dark Matter Halos
  We present a set of N-body simulations of a class of models in which an
unstable dark matter particle decays into a stable non-interacting dark matter
particle, with decay lifetime comparable to the Hubble time. We study the
effects of the kinematic recoil velocity received by the stable dark matter on
the structures of dark matter halos ranging from galaxy-cluster to Milky Way
mass scales. For Milky Way-mass halos, we use high-resolution, zoom-in
simulations to explore the effects of decays on Galactic substructure. In
general, halos with circular velocities comparable to the magnitude of kick
velocity are most strongly affected by decays. We show that decaying dark
matter models with lifetimes comparable to Hubble time and recoil speeds about
20-40 km/s can significantly reduce both the abundance of Galactic subhalos and
the internal densities of the subhalos. We also compare subhalo circular
velocity profiles with observational constraints on the Milky Way dwarf
satellite galaxies. Interestingly, we find that decaying dark matter models
that do not violate current astrophysical constraints, can significantly
mitigate both the well-documented "missing satellites problem" and the more
recent "too big to fail problem" associated with the abundances and densities
of Local Group dwarf satellite galaxies. A relatively unique feature of late
decaying dark matter models is that they predict significant evolution of halos
as a function of time. This is an important consideration because at high
redshifts, prior to decays, decaying models exhibit the same sequence of
structure formation as cold dark matter. We conclude that models of decaying
dark matter make predictions that are relevant for the interpretation of
observations of small galaxies in the Local Group and can be tested or
constrained by the kinematics of Local Group dwarf galaxies as well as by
forthcoming large-scale surveys.
