VINTERGATAN I: The origins of chemically, kinematically and structurally
  distinct discs in a simulated Milky Way-mass galaxy
  Spectroscopic surveys of the Milky Way's stars have revealed spatial,
chemical and kinematical structures that encode its history. In this work, we
study their origins using a cosmological zoom simulation, VINTERGATAN, of a
Milky Way-mass disc galaxy. We find that in connection to the last major merger
at $z\sim 1.5$, cosmological accretion leads to the rapid formation of an
outer, metal-poor, low-[$\alpha$/Fe] gas disc around the inner, metal-rich
galaxy containing the old high-[$\alpha$/Fe] stars. This event leads to a
bimodality in [$\alpha$/Fe] over a range of [Fe/H]. A detailed analysis of how
the galaxy evolves since $z\sim 1$ is presented. We demonstrate the way in
which inside-out growth shapes the radial surface density and metallicity
profile and how radial migration preferentially relocates stars from the inner
to the outer disc. Secular disc heating is found to give rise to increasing
velocity dispersions and scaleheights with stellar age, which together with
disc flaring explains several trends observed in the Milky Way, including
shallower radial [Fe/H]-profiles above the midplane. We show how the galaxy
formation scenario imprints non-trivial mappings between structural
associations (i.e. thick and thin discs), velocity dispersions,
$\alpha$-enhancements, and ages of stars, e.g. the most metal-poor stars in the
low-[$\alpha$/Fe] sequence are found to have a scaleheight comparable to old
high-[$\alpha$/Fe] stars. Finally, we illustrate how at low spatial resolution,
comparable to the thickness of the galaxy, the proposed pathway to distinct
sequences in [$\alpha$/Fe]-[Fe/H] cannot be captured.
