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

TL;DR

This study demonstrates that longitude-velocity diagrams effectively trace the Milky Way's metallicity gradients and azimuthal variations, offering a distance-independent method to analyze galactic chemical structure using both observations and simulations.

Contribution

It introduces the use of $ ell$-$v$ diagrams as a novel, distance-independent diagnostic for metallicity structure, comparing observational data with FIRE-2 simulations to identify discrepancies.

Findings

$ ell$-$v$ diagrams reveal radial metallicity gradients in the MW and simulated galaxies.

Current HII region data lack the precision to detect azimuthal variations.

Simulated galaxies show anomalous gas motions with higher velocity tails than the MW.

Abstract

We investigate longitude-velocity ($\ell$-$v$) 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 $\ell$-$v$ diagrams trace radial metallicity gradients and non-axisymmetric azimuthal metallicity variations. Because they do not rely on distance measurements, $\ell$-$v$ diagrams complement face-on maps for studying metallicity structure. In the MW, we detect the radial metallicity gradient in $\ell$-$v$ 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 $\ell$-$v$ 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 $\ell$-$v$ 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.