Systematic Problems With Using Dark Matter Simulations to Model Stellar
  Halos
  The limits of available computing power have forced models for the structure
of stellar halos to adopt one or both of the following simplifying assumptions:
(1) stellar mass can be "painted" onto dark matter particles in progenitor
satellites; (2) pure dark matter simulations that do not form a luminous galaxy
can be used. We estimate the magnitude of the systematic errors introduced by
these assumptions using a controlled set of stellar halo models where we
independently vary whether we look at star particles or painted dark matter
particles, and whether we use a simulation in which a baryonic disk galaxy
forms or a matching pure dark matter simulation that does not form a baryonic
disk. We find that the "painting" simplification reduces the halo concentration
and internal structure, predominantly because painted dark matter particles
have different kinematics than star particles even when both are buried deep in
the potential well of the satellite. The simplification of using pure dark
matter simulations reduces the concentration further, but increases the
internal structure, and results in a more prolate stellar halo. These
differences can be a factor of 1.5-7 in concentration (as measured by the
half-mass radius) and 2-7 in internal density structure. Given this level of
systematic uncertainty, one should be wary of overinterpreting differences
between observations and the current generation of stellar halo models based on
dark matter only simulations when such differences are less than an order of
magnitude.
