Metallicity Structure in Galactic Longitude-Velocity Diagrams of the Milky Way Disk and FIRE-2 Simulations

We investigate longitude-velocity ($\mathrm{\ell }$-$v$) 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 HII regions across the MW disk, together with MW-mass galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological simulations, we show that $\mathrm{\ell }$-$v$ diagrams trace radial metallicity gradients and non-axisymmetric azimuthal metallicity variations. Because they do not rely on distance measurements, $\mathrm{\ell }$-$v$ diagrams complement face-on maps for studying metallicity structure. In the MW, we detect the radial metallicity gradient in $\mathrm{\ell }$-$v$ space, but current HII region oxygen abundance errors are too high to reveal azimuthal variations. In the FIRE-2 MW-mass galaxies, the radial gradient is evident in $\mathrm{\ell }$-$v$ 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 (LSR) velocity tail 3 times larger in the FIRE-2 simulations compared to the MW. Our results highlight $\mathrm{\ell }$-$v$ 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.