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 and Bland-Hawthorn 2002) and have provided tight constraints on several Galaxy formation models (e.g.: Abadi et al. 2003; Bekki andChiba 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 MilkyWay 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 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.