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

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.

15 pages, 14 figures, MNRAS accepted

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Cited by in corpus (8)

• Photometric metallicity and distance estimates for $\sim$ 136,000 RR Lyrae stars from Gaia DR3
• Mapping the anisotropic Galactic stellar halo with Blue Horizontal Branch stars
• Probing the Galactic halo with RR Lyrae stars -- IV. On the Oosterhoff dichotomy of RR Lyrae stars
• Photometric-Metallicity and Distance Estimates for $\sim$70,000 RR Lyrae Stars from the Zwicky Transient Facility
• HALO7D III: Chemical Abundances of Milky Way Halo Stars from Medium Resolution Spectra
• The Pristine Survey -- XXVII. Journey to the Galactic outskirts -- Mapping the outer halo red giant stars down to the very metal-poor end
• Determining the Scale Length and Height of the Milky Way's Thick Disc Using RR~Lyrae
• Reconsidering The Bailey Diagrams of ab-type RR Lyrae Stars, Metallicity-Mediated Evolution as the Direct Cause of the Oosterhoff Phenomenon