Aims:
We develop a new theoretical framework to generate Besançon Galaxy Model Fast Approximate Simulations (BGM FASt) to address fundamental questions of the Galactic structure and evolution performing multi-parameter inference. In this thesis we are focused in a first application of our strategy to simultaneously infer the Initial Mass Function (IMF) and the Star Formation History (SFH) of the Milky Way (MW) disc.

Method:
BGM FASt is based on a reweighing scheme, that uses a specific pre-sampled simulation. We use it together with an approximate Bayesian computation algorithm to obtain the posterior probability distribution function of the inferred parameters. Our full strategy is codified to run on Apache Spark and Hadoop. We use Gaia data-release 2 with G<12 to explore a parameter space with 15 dimensions. This includes simultaneously the IMF and, for the first time, a non-parametric SFH for the Galactic disc.

Results and conclusions:
The set of tests applied show a very good agreement between equivalent simulations performed with BGM FASt and standard BGM. Gaia DR2 we find an imprint of a star formation burst 2-3 Gyr ago in the Galactic thin disc domain. Our results show a decreasing trend followed by a Star Formation Rate (SFR) enhancement starting at about 5 Gyr ago and continuing until about 1 Gyr ago. This enhancement is detected with high statistical significance by discarding the null hypothesis of an exponential SFH. The timescale and the amount of stellar mass generated during this SFR enhancement event lead us to hypothesise that its origin, currently under investigation, is not intrinsic to the disc.

The good performance of our whole strategy opens very promising perspectives, among them, the possibility to study whether the IMF variates with the time or not.

Tribunal Members

President: Dr. Pavel Kroupa

Secretary: Dra. Carme Gallart Gallart

Vocal: Dr. Andreas Just