The thick disc of the Galaxy: Sequel of a merging event

Accurate characterization of thick disc properties from recent kinematic and photometric surveys provides converging evidences that this intermediate population is a sequel of the violent heating of early disc populations by a merging satellite galaxy. The thick disc population is revisited under the light of new data in a number of galactic sample fields. Various thick disc hypotheses are fitted to observational data through a maximum likelihood technique. The resulting characteristics of the thick disc are the following : a scale height of 760 $\pm$ 50 pc, with a local density of 5.6 $\pm$ 1 \% of the thin disc. The scale length is constrained to be 2.8 $\pm$ 0.8 kpc, well in agreement with the disc scale length (2.5 $\pm$ 0.3 kpc). The mean metallicity of the thick disc is found to be -0.7 $\pm$ 0.2 dex, with no {significant} metallicity gradients. These photometric constraints in combination with kinematic data give new constraints on the thick disc formation. We show that thick disc characteristics are {hardly compatible with a top-down formation scenario} but fully compatible with a violent merging event arising at the early thin disc life time as described by Quinn, Hernquist \& Fullagar (1993).