Chemo-kinematics of the Gaia RR Lyrae: the halo and the disc

Abstract

ABSTRACT We present the results of a multicomponent kinematic model of a large sample of RR Lyrae detected by Gaia. By imposing a fourfold symmetry and employing Gaia proper motions, we are able to infer the behaviour of the velocity ellipsoid between ≈3 and ≈30 kpc from the centre of the Galaxy. We detect the presence of two distinct components: a dominant non-rotating halo-like population and a much smaller rotating disc-like population. We demonstrate that the halo RR Lyrae can be described as a superposition of an isotropic and radially biased parts. The radially biased portion of the halo is characterized by a high orbital anisotropy β ≈ 0.9 and contributes between 50 per cent and 80 per cent of the halo RR Lyrae at 5 < R(kpc)<25. In line with previous studies, we interpret this high-β component as the debris cloud of the ancient massive merger also known as the Gaia Sausage (GS) whose orbital extrema we constrain. The light-curve properties of the RR Lyrae support the kinematic decomposition: the GS stars are more metal-rich and boast higher fractions of Oosterhoff Type 1 and high-amplitude short period (HASP) variables compared to the isotropic halo component. The metallicity/HASP maps reveal that the inner 10 kpc of the halo is likely inhabited by the RR Lyrae born in situ. The mean azimuthal speed and the velocity dispersion of the disc RR Lyrae out to R ≈ 30 kpc are consistent with the behaviour of a young and metal-rich thin disc stellar population.