Observational Constraints on the Formation of the Milky Way's Thick Disk

Young Sun Lee

Michigan State University

The Milky Way's thick disk differs in its structure, kinematics, and chemical abundances from the thin disk. These discrepancies imply that the thick disk has experienced different formation and evolution history from the thin disk. The currently discussed mechanisms for the thick-disk formation are dynamical heating of pre-exising thin disk by satellite mergers, accretion of satellite galaxies, chaotic mergers of gas-rich systems, and radial migration of stars.

In this talk, using [alpha/Fe] ratios obtained for a large sample of low-resolution (R~2000) spectra of G-type dwarf stars from the Sloan Extension for Galactic Understanding and Exploration (SEGUE), I divide the sample into likely thin- and thick-disk populations, and explore the observed correlations of rotational velocity with metallicity, Galactocentric distance and distance from the Galactic plane, as well as the orbital eccentricity distributions for the individual populations. These correlations are also compared with the predictions from the formation scenarios mentioned above in order to provide observational constraints.