GHOSTS - Bulges, Halos, and the Resolved Stellar Outskirts of Massive
  Disk 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 Sersic law profiles or mu(r)~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 Sersic profile and
the stellar surface density of the profiles correlate with Hubble type and
bulge- to-disk ratio. This suggests that the central bulges and inner halos are
created in the same process. 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. The halos we fit are highly
flattened, with minor- over-ma jor axis ratios c/a=0.6. The halos are somewhat
more compact than hierarchical model predictions. The halos show small
metallicity gradient out to 30 kpc and the massive galaxies have typical
[Fe/H]~-0.8. We find indications of halo substructure in many galaxies, but
some halos seem remarkable smooth.
