Spiral-like features in the disc revealed by Gaia DR3 radial actions

Abstract

Context. The so-called action variables are specific functions of the positions and velocities that remain constant along the stellar orbit. The astrometry provided by Gaia Early Data Release 3 (EDR3), combined with the velocities inferred from the Radial Velocity Spectrograph (RVS) spectra of Gaia DR3, allows for the estimation of these actions for the largest volume of stars to date. Aims. We explore such actions with the aim of locating structures in the Galactic disc. Methods. We computed the actions and the orbital parameters of the Gaia DR3 stars, assuming an axisymmetric model for the Milky Way. Using Gaia DR3 photometric data, we also selected a subset of giant stars with better astrometry as a control sample. Results. We find that the maps of the percentiles of the radial action JR reveal arc-like segments. We found a high JR region centered at R ≈ 10.5 kpc of 1 kpc width, as well as three arc-shape regions dominated by circular orbits at inner radii. We also identified the spiral arms in the overdensities of the giant population. Conclusions. For Galactic coordinates (X, Y, Z), we find good agreement with the literature in the innermost region for the Scutum-Sagittarius spiral arms. At larger radii, the low JR structure tracks the Local arm at negative X, while for the Perseus arm, the agreement is restricted to the X < 2 kpc region, with a displacement with respect to the literature at more negative longitudes. We detected a high JR area at a Galactocentric radii of ∼10.5 kpc, consistent with some estimations of the Outer Lindblad Resonance location. We conclude that the pattern in the dynamics of the old stars is consistent in several places with the spatial distribution of the spiral arms traced by young populations, with small potential contributions from the moving groups.