The dynamically selected stellar halo of the Galaxy with Gaia and the tilt of the velocity ellipsoid

TL;DR

This study uses Gaia DR1 and RAVE data to dynamically identify halo stars near the Sun, analyze their velocity ellipsoid, and infer that the Galaxy's gravitational potential is likely spherical in the local halo region.

Contribution

It introduces a new dynamical criterion based on action integrals for selecting halo stars, improving the understanding of the local stellar halo's properties.

Findings

The local velocity ellipsoid is aligned in spherical coordinates.

The gravitational potential in the local halo region is approximately spherical.

Dynamical selection impacts the observed velocity distributions.

Abstract

We study the dynamical properties of halo stars located in the Solar Neighbourhood. Our goal is to explore how the properties of the halo depend on the selection criteria used for defining a sample of halo stars. Once this is understood we proceed to measure the shape and orientation of the halo's velocity ellipsoid and we use this information to put constraints on the gravitational potential of the Galaxy. We use the recently released Gaia DR1 catalogue cross-matched to the RAVE dataset for our analysis. We develop a dynamical criterion based on the distribution function of stars in various Galactic components, using action integrals to identify halo members, and compare this to metallicity and to kinematically selected samples. With this new method, we find 1156 stars in the Solar Neighbourhood to be likely members of the stellar halo. Our dynamically selected sample consists mainly of distant giants on elongated orbits. Their metallicity distribution is rather broad, with roughly half of the stars having [M/H] $\ge -1$ dex. The use of different selection criteria has an important impact on the characteristics of the velocity distributions obtained. Nonetheless, for our dynamically selected and for the metallicity selected samples, we find the local velocity ellipsoid to be aligned in spherical coordinates in a Galactocentric reference frame: this suggests that the total gravitational potential is rather spherical in the region spanned by the orbits of the halo stars in these samples.