Characteristics of the two sequences seen in the high-velocity
  Hertzsprung-Russell diagram in Gaia DR2
  In this study we use a sample of about 9 million SkyMapper stars with
metallicities to investigate the properties of the two stellar populations seen
in the high-velocity ($V_{\rm T} > 200$ km/s) Gaia DR2 Hertzsprung-Russell
diagram. Based on 10,000 red giant branch (RGB) stars (out of 75,000 with high
velocity), we find that the two sequences have different metallicity
distribution functions; one peaks at $-1.4$ dex (blue sequence) and the other
at $-0.7$ dex (red sequence). Isochrones with ages in the range $11$-$13.5$
Gyr, and metallicities chosen to match the observations for each sequence, fit
the turnoffs and broad RGBs well, indicating that the two populations formed at
comparable times within the uncertainties. We find that the mean tangential
velocity of disk stars increases steadily with decreasing metallicity, and that
the red sequence is made up of the high-velocity stars at the lowest
metallicities of the thick-disk population. Using relative number densities, we
further find that the red-sequence stars are more centrally concentrated in the
Galaxy, and we estimate the radial scale length of this population to be on the
order of $2$-$3$ kpc. The blue-sequence stars, on the other hand, follow a
nearly flat radial density profile. These findings tighten the link between the
red-sequence stars and the chemically defined thick disk.
