[X/Fe] Marks the Spot: Mapping Chemical Azimuthal Variations in the Galactic Disk with APOGEE

Abstract

Chemical cartography of the Galactic disk provides insights into its structure and assembly history over cosmic time. In this work, we use chemical cartography to explore chemical gradients and azimuthal substructure in the Milky Way disk with giant stars from Apache Point Observatory Galactic Evolution Experiment (APOGEE) DR17. We confirm the existence of a radial metallicity gradient in the disk of Δ[Fe/H]/Δ R ∼ –0.0678 ± 0.0004 dex kpc ^−1 and a vertical metallicity gradient of Δ[Fe/H]/Δ Z ∼ −0.164 ± 0.001. We find azimuthal variations (±0.1 dex) on top of the radial metallicity gradient that have been previously established with other surveys. The APOGEE giants show strong correlations with stellar age and the intensity of azimuthal variations in [Fe/H]; young populations and intermediate-aged populations both show significant deviations from the radial metallicity gradient, while older stellar populations show the largest deviations from the radial metallicity gradient. Beyond iron, we show that other elements (e.g., Mg, O) display azimuthal variations at the ±0.05 dex level across the Galactic disk. We illustrate that moving into the orbit-space could help constrain the mechanisms producing these azimuthal metallicity variations in the future. These results suggest that dynamical processes play an important role in the formation of azimuthal metallicity variations.

Keywords

• Galactic archaeology
• Milky Way disk
• Stellar abundances