The Galactic Thick Disc represents a ’snap frozen’ relic of the state of a (heated) early disc, containing fossil information about the main epoch of dissipation (or soon after). What signatures of a merger-origin for the thick disc might remain observable today ?
During the merging, orbital energy is deposited in the internal degrees of freedom of both the thin disc and the satellite, and acts to disrupt the satellite and heat the disc. In general one expects that the final ‘thick disc’ will be a mix of heated thin disc and satellite debris. In ΛCDM cosmological models the typical satellite/subhalo initial orbital angular momentum is close to half that of a circular orbit of the same energy (e.g. Benson 2005; Zentner et al. 2005) so one would expect an azimuthal streaming velocity of around 100 km/s for satellite debris in a system like the Milky Way with a flat rotation (circular velocity) curve, with amplitude ∼ 220 km/s.
A component with a mean azimuthal streaming velocity of around 100 km/s is indeed seen in radial velocity measurements of faint (V ≥ 18) F/G dwarfs of Gilmore, Wyse & Norris (2002) and Wyse et al. (2006), in widely separated lines-of-sight, and in data sets taken with different multi-object spectrographs and telescopes. Is this the manifestation of a tidal debris from the last significant merger suffered by the Milky Way causing the formation of the galactic thick disc?
An international programme, for which we are responsible and that involves colleagues from France, UK, USA and Australia, is already in course with the multi-fiber spectrograph FLAMES of the ESO Very Large Telescope. The goal of our project is to explore the nature of this lower angular momentum sub-population of the thick disc. A PhD thesis, centered on this project, is in course within our group.