GaiaDR2 reveals a star formation burst in the disc 2–3 Gyr ago

Abstract

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 ≈ 1M /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 ∼ 5Gyr ago and continuing until ∼ 1Gyr 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 α 3 ≈ 2 for masses M > 1.53M and α 2 ≈ 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.

The Canada–France Imaging Survey: Reconstructing the Milky Way Star Formation History from Its White Dwarf Population

“…This is consistent with the results of Rowell (2013) and Vergely et al (2002), as shown in the left-hand panel of Figure 10, who see an increase in star formation between 2 and 3 Gyr and a resulting decrease in the vicinity of 6 Gyr. This result is also in line with the recent work by Mor et al (2019), who compared simulations of the Besançon Galactic model with the colors, magnitudes, and parallaxes from Gaia DR2, which shows a burst of star formation between 2 and 3 Gyr in the past. The slight bimodality suggested by Figure 11 is, however, not seen in the GCE models of Snaith et al (2014) andToyouchi &Chiba (2018), as seen in the right-hand panel of Figure 10.…”

White dwarfs as advanced physics laboratories. The axion case

“…The excesses around M bol ∼ 9 -10 could be due to recent bursts of star formation not resolved by the present binning of the massive luminosity function and, as mentioned before, deserves a detailed analysis. Figure 9 also shows that values of g ae > ∼ 3 × 10 -13 are not compatible with the results obtained by Mor et al (2019).…”

“….48 × 10 -13 , respectively. The red line corresponds to the star formation rate obtained by Mor et al (2019).…”

“…The dashed line corresponds to a case with no axions and an SFR constant plus an exponential tail as in (Tremblay et al 2019). Right panel: in black, the corresponding star formation rate (Isern 2019), in red, the Galactic disc star formation (Mor et al 2019). .48 × 10 -13 , respectively.…”

“…The existing degeneracy between galactic properties and evolutionary models imply that different stellar models can lead to different star formation histories, for which reason it is necessary to compare these results with others obtained independently. Mor et al (2019) computed the star formation history of the Galactic disk using main sequence stars from the Gaia DR2 and the Besançon Galaxy Model. Since this function is expressed in stars per unit of disk surface it has been divided by an arbitrary and constant height scale above the galactic plane (red line, Fig.…”