Résumé :
Were the extended stellar halos the first stellar populations to form in
spiral galaxies, or have they been accreted over time from dwarf galaxies?
What are the systematic relations between the disk, the bulge and the
halo? Does stellar mass of the halo scale with the mass of the bulge or
the mass of the entire galaxy? To begin to address such questions we
observed a sample of nearby spiral galaxies with the HST and resolved the
top few magnitudes of the red-giant branch in the halo population. These
observations allow the first systematic study of the spatial
distributions, metallicities and metallicity distributions of halo stars
in normal spiral galaxies outside of the Local Group. I will present the
analysis of the colour-magnitude diagrams, including estimates of the mean
metallicities and metallicity distributions, and first systematic trends
within the sample of galaxies. A major result is that the metal-poor
stellar halo of the Milky Way may be peculiar for a luminous spiral
galaxy. I will also present new constraints on the halo galaxy assembly
and the protogalactic galaxy population that has formed the galactic halos
from the modelling of the new stellar halo metallicity-parent galaxy
luminosity.
