Density Profiles of CDM Microhalos and their Implications for
  Annihilation Boost Factors
  In a standard cold dark matter (CDM) cosmology, microhalos at the CDM cutoff
scale are the first and smallest objects expected to form in the universe. Here
we present results of high resolution simulations of three representative
roughly Earth-mass microhalos in order to determine their inner density
profile. We find that CDM microhalos in simulations without a cutoff in the
power spectrum roughly follow the NFW density profile, just like the much
larger CDM halos on galaxy and galaxy cluster scales. But having a cutoff in
the initial power spectrum at a typical neutralino free streaming scale of
$10^{-7} M_{\odot}$ makes their inner density profiles considerably steeper,
i.e. $\rho \propto r^{-(1.3-1.4)}$, in good agreement with the results from
Ishiyama et al. (2010). An extrapolation of the halo and subhalo mass functions
down to the cutoff scale indicates that microhalos are extremely abundant
throughout the present day dark matter distribution and might contribute
significantly to indirect dark matter detection signals. Assuming a transition
from a NFW to a steeper inner profile ($\rho \propto r^{-1.4}$) two orders of
magnitude above the cutoff scale, the total boost factor for a Milky Way sized
dark matter halo increases from about 3.5 to 4. We further find that CDM
microhalo concentrations are consistent with the Bullock et al. (2001) model
and clearly rule out simplistic power law models for the mass dependence of
concentrations and subhalo annihilation, which would erroneously lead to very
large boost factors (a few hundred for galaxy halos and over 1000 for
clusters).
