Probing the Galactic halo with RR Lyrae stars $-$ III. The chemical and kinematic properties of the stellar halo

TL;DR

This study analyzes a large sample of RR Lyrae stars to reveal the chemical and kinematic structure of the Galactic halo, identifying distinct inner and outer components with different metallicity and velocity properties.

Contribution

It provides a detailed chemical and kinematic profile of the Galactic halo, highlighting the dual nature and the influence of Gaia-Enceladus-Sausage merger events.

Findings

Inner halo is metal-rich with radial anisotropy (~0.9).

Outer halo has broad MDF with multiple peaks and moderate anisotropy (~0.5).

Inner halo likely formed from Gaia-Enceladus-Sausage merger.

Abstract

Based on a large spectroscopic sample of $\sim$ 4,300 RR Lyrae stars with metallicity, systemic radial velocity and distance measurements, we present a detailed analysis of the chemical and kinematic properties of the Galactic halo. Using this sample, the metallicity distribution function (MDF) as a function of $r$ and the velocity anisotropy parameter $\beta$ profiles (for different metallicity populations) are derived for the stellar halo. Both the chemical and kinematic results suggest that the Galactic halo is composed of two distinct parts, the inner halo and outer halo. The cutoff radius ($\sim$ 30 kpc) is similar to the previous break radius found in the density distribution of the stellar halo. We find that the inner part is dominated by a metal-rich population with extremely radial anisotropy ($\beta \sim 0.9$). These features are in accordance with those of ``{\it Gaia}-Enceladus-Sausage'' (GES) and we attribute this inner halo component as being dominantly composed of stars deposited from this ancient merged satellite. We find that GES probably has a slightly negative metallicity gradient. The metal-poor populations in the inner halo are characterized as a long-tail in MDF with an anisotropy of $\beta \sim 0.5$, which is similar to that of the outer part. The MDF for the outer halo is very broad with several weak peaks and the value of $\beta$ is around 0.5 for all metallicities.