The thick disk 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 ± 50pc, with a local density of 5.6 ± 1% of the thin disc. The scale length is constrained to be 2.8 ± 0.8kpc, well in agreement with the disc scale length (2.5 ± 0.3kpc). The mean metallicity of the thick disc is found to be -0.7 ±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)