2026ApJ...999..268L

We investigate longitude-velocity (ℓ-v) diagrams as a diagnostic tool to study the metallicity structure of the Milky Way (MW) disk. The present-day metallicity structure encodes the imprint of the Galaxy's formation, assembly, and secular evolution. Using oxygen abundances from H II regions across the MW disk, together with MW-mass galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological simulations, we show that ℓ-v diagrams trace radial metallicity gradients and nonaxisymmetric azimuthal metallicity variations. Because they do not rely on distance measurements, ℓ-v diagrams complement face-on maps for studying metallicity structure. In the MW, we detect the radial metallicity gradient in the ℓ-v space, but current H II region oxygen abundance errors are too high to reveal azimuthal variations. In the FIRE-2 MW-mass galaxies, the radial gradient is evident in ℓ-v diagrams regardless of observer location, but anomalous gas kinematics can mimic azimuthal metallicity variations. We term these “anomalous motions,” which have an excess local standard of rest velocity tail 3 times larger in the FIRE-2 simulations compared to the MW. Our results highlight ℓ-v diagrams as a largely unexplored tool for probing metallicity structure without requiring distances, and underscore discrepancies between the gas kinematics in the FIRE-2 simulations and those in the MW.