Chemical signatures of formation processes in the stellar populations of
  simulated galaxies
  We study the chemical properties of the stellar populations in eight
simulations of the formation of Milky-Way mass galaxies in a LCDM Universe. Our
simulations include metal-dependent cooling and an explicitly multiphase
treatment of the effects on the gas of cooling, enrichment and supernova
feedback. We search for correlations between formation history and chemical
abundance patterns. Differing contributions to spheroids and discs from in situ
star formation and from accreted populations are reflected in differing
chemical properties. Discs have younger stellar populations, with most stars
forming in situ and with low alpha-enhancement from gas which never
participated in a galactic outflow. Up to 15 per cent of disc stars can come
from accreted satellites. These tend to be alpha-enhanced, older and to have
larger velocity dispersions than the in situ population. Inner spheroids have
old, metal-rich and alpha-enhanced stars which formed primarily in situ, more
than 40 per cent from material recycled through earlier galactic winds. Few
accreted stars are found in the inner spheroid unless a major merger occurred
recently. Such stars are older, more metal-poor and more alpha-enhanced than
the in situ population. Stellar haloes tend to have low metallicity and high
alpha-enhancement. The outer haloes are made primarily of accreted stars. Their
mean metallicity and alpha-enhancement reflect the masses of the disrupted
satellites where they formed: more massive satellites typically have higher
[Fe/H] and lower [alpha/Fe]. Surviving satellites have distinctive chemical
patterns which reflect their extended, bursty star formation histories. These
produce lower alpha-enhancement at given metallicity than in the main galaxy,
in agreement with observed trends in the Milky Way.
