The GHOSTS survey. II. The diversity of Halo Color and Metallicity
  Profiles of Massive Disk Galaxies
  We study the stellar halo color properties of six nearby massive highly
inclined disk galaxies using Hubble Space Telescope Advanced Camera for Surveys
and Wide Field Camera 3 observations in both F606W and F814W filters from the
GHOSTS survey. The observed fields, placed both along the minor and major axis
of each galaxy, probe the stellar outskirts out to projected distances of ~
50-70 kpc from their galactic centre along the minor axis. The 50% completeness
levels of the color magnitude diagrams are typically at two mag below the tip
of the red giant branch. We find that all galaxies have extended stellar halos
out to ~ 50 kpc and two out to ~ 70 kpc. We determined the halo color
distribution and color profile for each galaxy using the median colors of stars
in the RGB. Within each galaxy we find variations in the median colors as a
function of radius which likely indicates population variations, reflecting
that their outskirts were built from several small accreted objects. We find
that half of the galaxies (NGC 0891, NGC 4565, and NGC 7814) present a clear
negative color gradient, reflecting a declining metallicity in their halos; the
other have no significant color or population gradient. In addition,
notwithstanding the modest sample size of galaxies, there is no strong
correlation between their halo color/metallicity or gradient with galaxy's
properties such as rotational velocity or stellar mass. The diversity in halo
color profiles observed in the GHOSTS galaxies qualitatively supports the
predicted galaxy-to-galaxy scatter in halo stellar properties; a consequence of
the stochasticity inherent in the assembling history of galaxies.
