Contrasting the chemical evolution of the Milky Way and Andromeda   galaxies
  The chemical evolution history of a galaxy hides clues about how it formed and has been changing through time. We have studied the chemical evolution history of the Milky Way (MW) and Andromeda (M31) to find which are common features in the chemical evolution of disc galaxies as well as which are galaxy-dependent. We use a semi-analytic multi-zone chemical evolution model. Such models have succeeded in explaining the mean trends of the observed chemical properties in these two Local Group spiral galaxies with similar mass and morphology. Our results suggest that while the evolution of the MW and M31 shares general similarities, differences in the formation history are required to explain the observations in detail. In particular, we found that the observed higher metallicity in the M31 halo can be explained by either a) a higher halo star formation efficiency or b) a larger reservoir of infalling halo gas with a longer halo formation phase. These two different pictures would lead to a) a higher [O/Fe] at low metallicities or b) younger stellar populations in the M31 halo, respectively. Both pictures result in a more massive stellar halo in M31, which suggests a possible correlation between the halo metallicity and its stellar mass. 
Agostino Renda, Daisuke Kawata, Yeshe Fenner, Brad K. Gibson   (Swinburne University)