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R.F.G. Wyse (JHU), G. Gilmore (IoA, Cambridge), J.E. Norris, K.C. Freeman (MSSSO)
Stellar populations are the fossil record of galactic evolution. Interpretation of this record for the Milky Way allows one to determine reliably the dominant physics controlling the evolution of a representative of those galaxies which dominate the luminosity in the local Universe, and is an essential prerequisite to understanding necessarily limited but more direct data at high redshifts. In our Galaxy, the key issues are the places and times of formation of the oldest stellar populations : the halo, thick disk and bulge - and their overlaps and evolutionary relationships, if any. Analysis of these stellar populations will allow us to determine the history of merging and accretion in a typical galaxy, of great importance in determining galactic evolution, and constraining cosmological theories of galaxy formation.
Is the thick disk a merger remnant? How does it overlap the halo? What are the systems that merge, how frequently does this happen over a Hubble time, and with what consequence? Could we identify stars from the intruder, and from our own early disk? We are addressing these questions for the Milky Way, through a comprehensive statistical study of the kinematics and metallicity distributions of stars in the interface between the thick disk and stellar halo, those stellar components for which mergers are most often implicated. We are using the two-degree-field multi-object spectrograph on the AAT, which provides 400 spectra simultaneously, to measure the radial velocities and chemical abundances for F/G main sequence stars at distances from the Sun of 3-7kpc (dependent on metallicity and magnitude), beyond significant contamination by the thin disk, down several key lines-of-sight. Preliminary results, for around 1,500 stars, will be presented here.