Metallicity and Kinematics of the Galactic halo from the LAMOST sample stars

TL;DR

This study analyzes the metallicity and kinematic properties of distant giant stars in the Galactic halo from the LAMOST survey, revealing distinct components and their spatial distributions.

Contribution

It provides a detailed metallicity distribution and kinematic analysis of halo stars, identifying three Galactic components and their spatial variations, using a large spectroscopic sample.

Findings

Metallicity distribution shows three peaks corresponding to different Galactic components.

Inner-halo stars are more prevalent at greater distances from the Galactic plane.

Retrograde stars tend to be more metal-poor than prograde stars.

Abstract

We study the metallicity distribution and kinematic properties of 4,680 A/F/G/K-type giant stars with $|z|>$ 5 kpc selected from the LAMOST spectroscopic survey. The metallicity distribution of giant stars with 5 $<|z|\leqslant$ 15 kpc can be described by a three-peak Gaussian model with peaks at [Fe/H] $\sim-0.6\pm0.1$, $-1.2\pm0.3$ and $-2.0\pm0.2$, corresponding to the ratio of 19$\%$, 74$\%$ and 7$\%$, respectively. The $\rm{[\alpha/Fe]}$ is used to associate the three peaks with the thick disk, inner-halo and outer-halo components of the Galaxy. The metallicity distribution of these giant stars, which is fit with Gaussians corresponding to the three components, show a growing fraction of inner-halo component and declining fraction of the thick-disk component with increasing distance from the Galactic plane. Adopting a galaxy potential model, we also derive the orbital parameters of the sample stars, such as orbit eccentricity and rotation velocity. The peak values of derived orbital eccentricity for stars covering different metallicity regions maintain $e\sim$ 0.75, independent of height above the plane, within the range 5$<|z|<$ 15 kpc. By comparing the MDFs of stars in different rotation velocity intervals, we find that the majority of the retrograde stars are more metal-poor than the prograde stars.