Thin disc, Thick Disc and Halo in a Simulated Galaxy
  Within a cosmological hydrodynamical simulation, we form a disc galaxy with
sub- components which can be assigned to a thin stellar disc, thick disk, and a
low mass stellar halo via a chemical decomposition. The thin and thick disc
populations so selected are distinct in their ages, kinematics, and
metallicities. Thin disc stars are young (<6.6 Gyr), possess low velocity
dispersion ({\sigma}U,V,W = 41, 31, 25 km/s), high [Fe/H], and low [O/Fe]. The
thick disc stars are old (6.6<age<9.8 Gyrs), lag the thin disc by \sim21 km/s,
possess higher velocity dispersion ({\sigma}U,V,W = 49, 44, 35 km/s),
relatively low [Fe/H] and high [O/Fe]. The halo component comprises less than
4% of stars in the "solar annulus" of the simulation, has low metallicity, a
velocity ellipsoid defined by ({\sigma}U,V,W = 62, 46, 45 km/s) and is formed
primarily in-situ during an early merger epoch. Gas-rich mergers during this
epoch play a major role in fuelling the formation of the old disc stars (the
thick disc). This is consistent with studies which show that cold accretion is
the main source of a disc galaxy's baryons. Our simulation initially forms a
relatively short (scalelength \sim1.7 kpc at z=1) and kinematically hot disc,
primarily from gas accreted during the galaxy's merger epoch. Far from being a
competing formation scenario, migration is crucial for reconciling the short,
hot, discs which form at high redshift in {\Lambda}CDM, with the properties of
the thick disc at z=0. The thick disc, as defined by its abundances maintains
its relatively short scale-length at z = 0 (2.31 kpc) compared with the total
disc scale-length of 2.73 kpc. The inside-out nature of disc growth is
imprinted the evolution of abundances such that the metal poor {\alpha}-young
population has a larger scale-length (4.07 kpc) than the more chemically
evolved metal rich {\alpha}-young population (2.74 kpc).
