Age-chemical abundance structure of the Galaxy I: Evidence for a late accretion event in the outer disc at z ~ 0.6

Understand

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.