Testing galaxy formation models with the GHOSTS survey: The color
  profile of M81's stellar halo
  We study the properties of the stellar populations in M81's outermost part,
which hereafter we will term the stellar halo, using HST ACS/WFC observations
of 19 fields from the GHOSTS survey. The observed fields probe the stellar halo
out to a projected distance of ~ 50 kpc from the galactic center. Each field
was observed in both F606W and F814W filters. The 50% completeness levels of
the color magnitude diagrams (CMDs) are typically at 2 mag below the tip of the
red giant branch (TRGB). Fields at distances closer than 15 kpc show evidence
of disk-dominated populations whereas fields at larger distances are mostly
populated by halo stars. The RGB of the M81's halo CMDs is well matched with
isochrones of ~ 10 Gyr and metallicities [Fe/H] ~ -1.2 dex, suggesting that the
dominant stellar population of M81's halo has a similar age and metallicity.
The halo of M81 is characterized by a color distribution of width ~ 0.4 mag and
an approximately constant median value of (F606W - F814W) ~ 1 mag measured
using stars within the magnitude range 23.7 < F814W < 25.5. When considering
only fields located at galactocentric radius R > 15 kpc, we detect no color
gradient in the stellar halo of M81. We place a limit of 0.03+/-0.11 mag
difference between the median color of RGB M81 halo stars at ~ 15 and at 50
kpc, corresponding to a metallicity difference of 0.08+/-0.35 dex over that
radial range for an assumed constant age of 10 Gyr. We compare these results
with model predictions for the colors of stellar halos formed purely via
accretion of satellite galaxies. When we analyze the cosmologically motivated
models in the same way as the HST data, we find that they predict no color
gradient for the stellar halos, in good agreement with the observations.
