Chemical signature of Galactic spiral arms

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 {phi}, the slope gradually varies from d[M/H]/dR~-0.054dex/kpc at {phi}~-20{deg} to ~-0.036dex/kpc at {phi}~20{deg}. 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 ~0.05-0.1dex, making the assumption of a linear radial decrease not applicable for this sample.