The Formation of Stellar Halos in Late-Type Galaxies
  Near-field observations may provide tight constraints - i.e. "boundary
conditions" - on any model of structure formation in the Universe. Detailed
observational data have long been available for the Milky Way (e.g. Freeman
$\&$ Bland-Hawthorn 2002) and have provided tight constraints on several Galaxy
formation models (e.g. Abadi et al. 2003, Bekki $\&$ Chiba 2001). An implicit
assumption still remains unanswered though: is the Milky Way a "normal" spiral?
Searching for directions, it feels natural to look at our neighbour: Andromeda.
An intriguing piece of the puzzle is provided by contrasting its stellar halo
with that of our Galaxy, even more so since Mouhcine et al. (2005) have
suggested that a correlation between stellar halo metallicity and galactic
luminosity is in place and would leave the Milky Way halo as an outlier with
respect to other spirals of comparable luminosities. Further questions hence
arise: is there any stellar halo-galaxy formation symbiosis? Our first step has
been to contrast the chemical evolution of the Milky Way with that of Andromeda
by means of a semi-analytic model. We have then pursued a complementary
approach through the analysis of several semi-cosmological late-type galaxy
simulations which sample a wide variety of merging histories. We have focused
on the stellar halo properties in the simulations at redshift zero and shown
that - at any given galaxy luminosity - the metallicities of the stellar halos
in the simulations span a range in excess of $\sim$ 1 dex, a result which is
strengthened by the robustness tests we have performed. We suggest that the
underlying driver of the halo metallicity dispersion can be traced to the
diversity of galactic mass assembly histories inherent within the hierarchical
clustering paradigm.
