Gaia DR2 reveals a star formation burst in the disc 2-3 Gyr ago

We use Gaia data release 2 (DR2) magnitudes, colours, and parallaxes for stars with G < 12 to explore a parameter space with 15 dimensions that simultaneously includes the initial mass function (IMF) and a non-parametric star formation history (SFH) for the Galactic disc. This inference is performed by combining the Besançon Galaxy Model fast approximate simulations (BGM FASt) and an approximate Bayesian computation algorithm. We find in Gaia DR2 data an imprint of a star formation burst 2-3 Gyr ago in the Galactic thin disc domain, and a present star formation rate (SFR) of ≈1 M_⊙/yr. Our results show a decreasing trend of the SFR from 9-10 Gyr to 6-7 Gyr ago. This is consistent with the cosmological star formation quenching observed at redshifts z < 1.8. This decreasing trend is followed by a SFR enhancement starting at ∼5 Gyr ago and continuing until ∼1 Gyr ago which is detected with high statistical significance by discarding the null hypothesis of an exponential SFH with a p-value = 0.002. We estimate, from our best fit model, that about 50% of the mass used to generate stars, along the thin disc life, was expended in the period from 5 to 1 Gyr ago. The timescale and the amount of stellar mass generated during the SFR enhancement event lead us to hypothesise that its origin, currently under investigation, is not intrinsic to the disc. Thus, an external perturbation is needed for its explanation. Additionally, for the thin disc we find a slope of the IMF of α₃ ≈ 2 for masses M > 1.53 M_⊙ and α₂ ≈ 1.3 for the mass range between 0.5 and 1.53 M_⊙. This is the first time that we consider a non-parametric SFH for the thin disc in the Besançon Galaxy Model. This new step, together with the capabilities of the Gaia DR2 parallaxes to break degeneracies between different stellar populations, allow us to better constrain the SFH and the IMF.