Characterizing the velocity anisotropy of the Milky Way's stellar halo

TL;DR

This study maps the velocity anisotropy of the Milky Way's stellar halo up to 70 kpc using over 10,000 stars, revealing a decrease in anisotropy with radius and its relation to stellar properties.

Contribution

It provides the first detailed profile of velocity anisotropy extending beyond 40 kpc, incorporating a large dataset and analyzing correlations with metallicity and age.

Findings

Anisotropy decreases with radius, becoming tangentially dominated beyond 30 kpc.

Removing accreted structures results in a radially-dominated anisotropy profile.

Older stars are dynamically colder with less radial orbits than younger stars.

Abstract

Modeling the Milky Way stellar halo requires well-determined density and velocity anisotropy profiles. However, it has been challenging to gather a large sample of stars with six-dimensional data that extend beyond 40 kpc to map the outer halo. Our work investigates the velocity anisotropy in the Milky Way stellar halo with more than 10,000 blue horizontal-branch stars, combining Gaia astrometric data and spectroscopic data from SEGUE, DESI and LAMOST. This large sample allows us to obtain a detailed profile of up to $\sim$70 kpc. Radial velocities are predominant in the inner halo ($< 30$ kpc), and the anisotropy presents a smooth decrease before rapidly dropping to negative values, becoming dominated by tangential dispersion velocities. Removing the main known accreted structures of the Milky Way, makes the anisotropy profile radially-dominated at all radii. Our profile clearly shows an increase in the anisotropy from the center of the Galaxy, in accordance to the simulations. We also investigate the correlation of anisotropy with metallicity and with color. The lack of correlation between metallicity and anisotropy in our clean sample reinforces that this relation is driven by merger events. The initial exploration with color indicates a relation between kinematics and age, showing that older stars are dynamically colder and present less radial orbits than younger stars in the inner halo.