Thick Disc

Figure: An overview of Stellar Populations in the Galaxy. The Gaia-ESO Survey will quantify kinematic and elemental abundance distributions in all stellar populations. In addition to bulge, halo and thick disc, it will provide precise data for a representative sample of open clusters in the thin disc, covering its whole age range, and a detailed determination of the population structure within one disc scale length of the Solar neighbourhood.

Thick discs seem common in large spiral galaxies (Gilmore & Reid, 1983, MNRAS, 201, 73; Collins et al., 2011, MNRAS, 413, 1548; Yoachim & Delcanton, 2006, AJ, 131, 226; see Figure). Are they evidence that the last major merger event occurred very much longer ago than is expected in standard cosmologies? Are they artifacts of thin disc dynamical evolution? Are they both or neither of these (Qu et al., 2011, EAS, 45, 421; Schoenrich & Binney, 2009, MNRAS, 399, 1145)? How did the metallicity of the Inter-Stellar Medium (ISM) evolve at very early times? How does this vary with Galactocentric distance? Do major infall events occasionally depress the metallicity of the ISM (Chiappini, 2011, EAS, 45, 293)? We will determine quantitative kinematics and abundance patterns for large samples of thick disc FG stars over one outer radial and three vertical scale lengths to help elucidate these key questions in Galaxy formation and evolution. We supplement that with a survey of the rare but important very outer thin/thick disc K giant stars, extending to the warp, flare and Mon Stream (Casetti-Dinescu et al., 2010, AJ, 139, 1889) in the distant discs.