THE THICK DISC OF THE GALAXY - SEQUEL OF A MERGING EVENT

Accurate characterization of thick disc properties from recent kinemat ic and photometric surveys provides converging evidences that this int ermediate population is a sequel of the violent heating of early disc populations by a merging satellite galaxy. The thick disc population i s revisited under the light of new data in a number of galactic sample fields. Various thick disc hypotheses are fitted to observational dat a through a maximum likelihood technique. The resulting characteristic s of the thick disc are the following: a scale height of 760 +/- 50 pc , with a local density of 5.6 +/- 1 % of the thin disc. The scale leng th is constrained to be 2.8 +/- 0.8 kpc, well in agreement with the di sc scale length (2.5 +/- 0.3 kpc). The mean metallicity of the thick d isc is found to be -0.7 +/- 0.2 dex, with no significant metallicity g radients. These photometric constraints in combination with kinematic data give new constraints on the thick disc formation. We show that th ick disc characteristics are hardly compatible with a top-down formati on scenario but fully compatible with a violent merging event arising at the early thin disc life time as described by Quinn, Hernquist & Fu llagar (1993).