Exploring the Galaxy's halo and very metal-weak thick disk with SkyMapper and Gaia DR2

In this work we combine spectroscopic information from the extit{SkyMapper survey for Extremely Metal-Poor stars} and astrometry from Gaia DR2 to investigate the kinematics of a sample of 475 stars with a metallicity range of $ -6.5 leq m [Fe/H] leq -2.05$ dex. Exploiting the action map, we identify 16 and 40 stars dynamically consistent with the extit{Gaia Sausage} and extit{Gaia Sequoia} accretion events, respectively. The most metal-poor of these candidates have metallicities of $ m [Fe/H]=-3.31$ and $ m [Fe/H]=-3.74$, respectively, helping to define the low-metallicity tail of the progenitors involved in the accretion events. We also find, consistent with other studies, that $sim$21% of the sample have orbits that remain confined to within 3~kpc of the Galactic plane, i.e., |Z$_{max}$| $leq$ 3~kpc. Of particular interest is a sub-sample ($sim$11% of the total) of low |Z$_{max}$| stars with low eccentricities and prograde motions. The lowest metallicity of these stars has [Fe/H] = --4.30 and the sub-sample is best interpreted as the very low-metallicity tail of the metal-weak thick disk population. The low |Z$_{max}$|, low eccentricity stars with retrograde orbits are likely accreted, while the low |Z$_{max}$|, high eccentricity pro- and retrograde stars are plausibly associated with the extit{Gaia Sausage} system. We find that a small fraction of our sample ($sim$4% of the total) is likely escaping from the Galaxy, and postulate that these stars have gained energy from gravitational interactions that occur when infalling dwarf galaxies are tidally disrupted.