Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in
  the Galaxy
  Based on the second Gaia data release (DR2), combined with the LAMOST and
APOGEE spectroscopic surveys, we study the kinematics and metallicity
distribution of the high-velocity stars that have a relative speed of at least
220 ${\rm km\ s^{-1}}$ with respect to the local standard of rest in the
Galaxy. The rotational velocity distribution of the high-velocity stars with
[Fe/H] $>-1.0$ dex can be well described by a two-Gaussian model, with peaks at
$V_{\phi}\sim +164.2\pm0.7$ and $V_{\phi}\sim +3.0\pm0.3$ ${\rm km\ s^{-1}}$,
associated with the thick disk and halo, respectively. This implies that there
should exist a high-velocity thick disk (HVTD) and a metal-rich stellar halo
(MRSH) in the Galaxy. The HVTD stars have the same position as the halo in the
Toomre diagram and but show the same rotational velocity and metallicity as the
canonical thick disk. The MRSH stars have basically the same rotational
velocity, orbital eccentricity, and position in the Lindblad and Toomre diagram
as the canonical halo stars, but they are more metal-rich. Furthermore, the
metallicity distribution function (MDF) of our sample stars are well fitted by
a four-Gaussian model, associated with the outer-halo, inner-halo, MRSH, and
HVTD, respectively. Chemical and kinematic properties and age imply that the
MRSH and HVTD stars may form in situ.
