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 .