Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
  We have examined the resolved stellar populations at large galactocentric
distances along the minor axis (from 10 kpc up to between 40 and 75 kpc), with
limited major axis coverage, of six nearby highly-inclined Milky Way-mass disc
galaxies using HST data from the GHOSTS survey. We select red giant branch
stars to derive stellar halo density profiles. The projected minor axis density
profiles can be approximated by power laws with projected slopes of between
$-2$ and $-3.7$ and a diversity of stellar halo masses of $1-6\times
10^{9}M_{\odot}$, or $2-14\%$ of the total galaxy stellar masses. The typical
intrinsic scatter around a smooth power law fit is $0.05-0.1$ dex owing to
substructure. By comparing the minor and major axis profiles, we infer
projected axis ratios $c/a$ at $\sim 25$ kpc between $0.4-0.75$. The GHOSTS
stellar haloes are diverse, lying between the extremes charted out by the
(rather atypical) haloes of the Milky Way and M31. We find a strong correlation
between the stellar halo metallicities and the stellar halo masses. We compare
our results with cosmological models, finding good agreement between our
observations and accretion-only models where the stellar haloes are formed by
the disruption of dwarf satellites. In particular, the strong observed
correlation between stellar halo metallicity and mass is naturally reproduced.
Low-resolution hydrodynamical models have unrealistically high stellar halo
masses. Current high-resolution hydrodynamical models appear to predict stellar
halo masses somewhat higher than observed but with reasonable metallicities,
metallicity gradients and density profiles.
