Evidence of high redshift Galactic alpha-poor disc

Our new data confirm the known bi-modality in the [Fe/H] - [alpha/Fe] space, which is often viewed as the manifestation of the chemical thin and thick discs. The over-densities significantly overlap in metallicity, age, and kinematics, and none of these is a sufficient criterion for distinguishing between the two disc populations. Different from previous studies, we find that the alpha-poor disc population has a very extended [Fe/H] distribution and contains ~ 20% old stars with ages of up to ~11Gyr.

Our results suggest that the Galactic thin disc was in place early, at look-back times corresponding to redshifts z~2 or more. At ages ~9 to 11Gyr, the two disc structures shared a period of co-evolution. Our data can be understood within the clumpy disc formation scenario that does not require a pre-existing thick disc to initiate a formation of the thin disc.

We anticipate that a similar evolution can be realised in cosmological simulations of galaxy formation.