Existence of the Metal-Rich Stellar Halo and High-velocity Thick Disk in the Galaxy

TL;DR

This study uses Gaia DR2, LAMOST, and APOGEE data to identify a high-velocity thick disk and a metal-rich stellar halo in the Milky Way, revealing their properties and possible in situ formation.

Contribution

It provides evidence for the existence of a high-velocity thick disk and a metal-rich stellar halo, with detailed kinematic and metallicity analysis based on combined spectroscopic and astrometric data.

Findings

Identification of a high-velocity thick disk with similar properties to the canonical thick disk.

Detection of a metal-rich stellar halo with properties akin to the canonical halo.

Metallicity distribution function fits reveal four distinct stellar components.

Abstract

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.