Age-chemical abundance structure of the Galaxy I: Evidence for a late
  accretion event in the outer disc at z ~ 0.6
  We investigate the age-chemical abundance structure of the outer Galactic
disc at a galactocentric distance of r > 10 kpc as recently revealed by the
SDSS/APOGEE survey. Two sequences are present in the [alpha/Fe]-[Fe/H] plane
with systematically different stellar ages. Surprisingly, the young sequence is
less metal-rich, suggesting a recent dilution process by additional gas
accretion. As the stars with the lowest iron abundance in the younger sequence
also show an enhancement in alpha-element abundance, the gas accretion event
must have involved a burst of star formation. In order to explain these
observations, we construct a chemical evolution model. In this model we include
a relatively short episode of gas accretion at late times on top of an
underlying secular accretion over long timescales. Our model is successful at
reproducing the observed distribution of stars in the three dimensional space
of [alpha/Fe]-[Fe/H]-Age in the outer disc. We find that a late-time accretion
with a delay of 8.2 Gyr and a timescale of 0.7 Gyr best fits the observed data,
in particular the presence of the young, metal-poor sequence. Our best-fit
model further implies that the amount of accreted gas in the late-time
accretion event needs to be about three times the local gas reservoir in the
outer disc at the time of accretion in order to sufficiently dilute the metal
abundance. Given this large fraction, we interpret the late-time accretion
event as a minor merger presumably with a gas-rich dwarf galaxy with a mass M_*
< 10^9 M_Sun and a gas fraction of ~ 75 per cent.
