A prediction about the age of thick discs as a function of the stellar mass of the host galaxy

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-star(z=0) < 10(10) M- have thick disc stars as young as 4-6 Gyr, whereas the youngest stars in the thick discs of Milky-Way-like galaxies are similar to 10 Gyr old. I tested this prediction against the scarcely available thick disc age estimates, all of them are from galaxies with M-star(z=0) greater than or similar to 10(10) M-circle dot, 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-star(z=0) similar to 10(9.5) M-circle dot and by checking whether they contain similar to 5 Gyr-old stars.

Automated summary

Thick discs may have formed quickly in situ from turbulent clumpy discs, with thin discs forming slowly within them. Observational tests suggest a positive correlation between the age of the youngest stars in thick discs and the stellar mass of the host galaxy. While field spiral galaxies seem to align with this prediction, lenticular galaxies exhibit younger thick disc ages than expected, indicating a fast early evolution for S0 galaxies.