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

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 J(R) reveal arc-like segments. We found a high J(R) region centered at R approximate to 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 J(R) 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 J(R) area at a Galactocentric radii of similar to 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.

Automated summary

This study utilizes the astrometry provided by Gaia EDR3 and the velocities inferred from the RVS spectra of Gaia DR3 to estimate the action variables of a large volume of stars, aiming to locate structures in the Galactic disc. The results reveal arc-like segments in the maps of the percentiles of the radial action J(R), with a high J(R) region centered at R approximately 10.5 kpc. Spiral arms are also identified in the overdensities of the giant population. The dynamics of old stars show consistency with the spatial distribution of spiral arms traced by young populations, with potential contributions from moving groups.