MaGICC Thick Disk I: Comparing a Simulated Disk Formed with Stellar
  Feedback to the Milky Way
  We analyse the structure and chemical enrichment of a Milky Way-like galaxy
with a stellar mass of 2 10^{10} M_sun, formed in a cosmological hydrodynamical
simulation. It is disk-dominated with a flat rotation curve, and has a disk
scale length similar to the Milky Way's, but a velocity dispersion that is ~50%
higher. Examining stars in narrow [Fe/H] and [\alpha/Fe] abundance ranges, we
find remarkable qualitative agreement between this simulation and observations:
a) The old stars lie in a thickened distribution with a short scale length,
while the young stars form a thinner disk, with scale lengths decreasing, as
[Fe/H] increases. b) Consequently, there is a distinct outward metallicity
gradient. c) Mono-abundance populations exist with a continuous distribution of
scale heights (from thin to thick). However, the simulated galaxy has a
distinct and substantive very thick disk (h_z~1.5 kpc), not seen in the Milky
Way. The broad agreement between simulations and observations allows us to test
the validity of observational proxies used in the literature: we find in the
simulation that mono-abundance populations are good proxies for single age
populations (<1 Gyr) for most abundances.
