Our GHOSTS survey measures the stellar envelope properties of 14 nearby disk galaxies by imaging their resolved stellar populations with HST/ACS & WFPC2 .
Most of the massive galaxies in the sample ( V _ { rot } > 200 km/s ) have very extended stellar envelopes with Sérsic law profiles or \mu ( r ) \sim r ^ { -2.5 } power law profiles in the outer regions .
For these massive galaxies we can fit the central bulge light and the outer halo out to 30 kpc with one and the same Sérsic profile .
The stellar surface density of these profiles correlate with Hubble type and bulge-to-disk ratio , suggesting that the central bulges and inner halos are created in the same process .
Smaller galaxies ( V _ { rot } \sim 100 km/s ) have much smaller stellar envelopes , but depending on geometry they could still be more luminous than expected from satellite remnants in hierarchical galaxy formation models .
Alternatively , they could be created by disk heating through the bombardment of small dark matter sub-halos .
The halos we fit are highly flattened , with minor-over-major axis ratios c / a \simeq 0.4 .
The halos are somewhat more compact than hierarchical model predictions .
The halos show small metallicity gradients out to 30 kpc and the massive galaxies have typical [ Fe/H ] \sim -0.8 .
We find indications of halo substructure in many galaxies , but some halos seem remarkable smooth .