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 \textit{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 \rm [Fe/H] \leq -2.05$ dex. Exploiting the action map, we identify
16 and 40 stars dynamically consistent with the \textit{Gaia Sausage} and
\textit{Gaia Sequoia} accretion events, respectively. The most metal-poor of
these candidates have metallicities of $\rm [Fe/H]=-3.31$ and $\rm
[Fe/H]=-3.74$, 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}$|, high eccentricity pro- and
retrograde stars are plausibly associated with the \textit{Gaia Sausage}
system. We find that a small fraction of our sample ($\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.
