  In hierarchical galaxy formation the stellar halos of galaxies are formed by
the accretion of minor satellites and therefore contain valuable information
about the (early) assembly process of galaxies. 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 (Vrot>200 km/s) have very extended stellar envelopes with mu(r) ~
r^{-2.5} power law profiles in the outer regions. For these massive galaxies
there is some evidence that the stellar surface density of the profiles
correlates with Hubble type and bulge-to-disk ratio, begging the question
whether these envelopes are more related to bulges than to a Milky Way-type
stellar halo. Smaller galaxies (Vrot 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. We find that galaxies show varying amounts of halo
substructure.
