Chemo-dynamics of prograde and retrograde Milky Way stars

Context: The accretion history of the Milky Way is still unknown, despite the recent discovery of stellar systems that stand out in energy-angular momentum space, such as Gaia-Enceladus. In particular, it is still unclear how these groups are linked and to what extent they are well mixed. Aims: We investigate the similarities and differences in the properties between the prograde and retrograde (counter-rotating) stars, and put those results in context with the properties of Gaia-Enceladus, Thamnos and other suggested accreted populations. Methods: We use the stellar metallicities of the major large spectroscopic surveys (APOGEE, Gaia-ESO, GALAH, LAMOST, RAVE, SEGUE) in combination with astrometric and photometric data coming from Gaia's second data-release. We investigate the presence of radial and vertical metallicity gradients as well as the possible correlations between the azimuthal velocity $v_\phi$ and metallicity [M/H] as qualitative indicators of the presence of mixed populations. Results: We find that (1) a handful of super metal-rich stars exist on retrograde orbits at various distances from the Galactic center and the Galactic plane. (2) The counter-rotating stars appear to be a well mixed population, showing radial and vertical metallicity gradients of the order of $\sim -0.04$ dex/kpc and $-0.06$ dex/kpc, respectively, with little, if any, variation when probing different regions of the Galaxy. (3) The prograde stars show a $v_\phi-[M/H]$ relation that flattens, and perhaps even reverses as a function of distance from the plane. (5) Thamnos and Gaia-Enceladus stars seem to be different populations yet very much linked, as they follow the same trend in the eccentricity versus metallicity space.