2021A&A...645L..13C

One of the suggested thick disc formation mechanisms is that they were born quickly and in situ from a turbulent clumpy disc. Subsequently, thin discs formed slowly within them from leftovers of the turbulent phase and from material accreted through cold flows and minor mergers. In this Letter, I propose an observational test to verify this hypothesis. By combining thick disc and total stellar masses of edge-on galaxies with galaxy stellar mass functions calculated in the redshift range of z≤3.0, I derived a positive correlation between the age of the youngest stars in thick discs and the stellar mass of the host galaxy; galaxies with a present-day stellar mass of M*(z=0)<10¹⁰M_☉ have thick disc stars as young as 4-6Gyr, whereas the youngest stars in the thick discs of Milky-Way-like galaxies are ∼10Gyr old. I tested this prediction against the scarcely available thick disc age estimates, all of them are from galaxies with M*(z=0)≥10¹⁰M_☉, and I find that field spiral galaxies seem to follow the expectation. On the other hand, my derivation predicts ages that are too low for the thick discs in lenticular galaxies, indicating a fast early evolution for S0 galaxies. I propose the idea of conclusively testing whether thick discs formed quickly and in situ by obtaining the ages of thick discs in field galaxies with masses of M*(z=0)∼10^9.5M_☉ and by checking whether they contain ∼5Gyr-old stars.