The chemical signature of the Galactic spiral arms revealed by Gaia DR3

Abstract

Taking advantage of the recent Gaia Data Release 3 (DR3), we mapped chemical inhomogeneities in the Milky Way’s disc out to a distance of ∼4 kpc from the Sun, using different samples of bright giant stars. The samples were selected using effective temperatures and surface gravities from the GSP-Spec module, and they are expected to trace stellar populations of a different typical age. The cool (old) giants exhibit a relatively smooth radial metallicity gradient with an azimuthal dependence. Binning in Galactic azimuth ϕ, the slope gradually varies from d[M/H]/dR ∼ −0.054 dex kpc−1 at ϕ ∼ −20° to ∼ − 0.036 dex kpc−1 at ϕ ∼ 20°. On the other hand, the relatively hotter (and younger) stars present remarkable inhomogeneities, which are apparent as three (possibly four) metal-rich elongated features in correspondence with the spiral arms’ locations in the Galactic disc. When projected onto the Galactic radius, those features manifest themselves as statistically significant bumps on top of the observed radial metallicity gradients with amplitudes up to ∼0.05–0.1 dex, making the assumption of a linear radial decrease not applicable for this sample. The strong correlation between the spiral structure of the Galaxy and the observed chemical pattern in the young sample indicates that the spiral arms might be at the origin of the detected chemical inhomogeneities. In this scenario, the spiral arms would leave a strong signature in the younger stars which progressively disappears when cooler (and older) giants are considered.