Metallicity Structure in Galactic Longitude-Velocity Diagrams of the Milky Way Disk and FIRE-2 Simulations
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arXiv
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| Natura: | Preprint |
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2026
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| author | Liu, Victor Balser, Dana S. Wenger, Trey V. |
| author_facet | Liu, Victor Balser, Dana S. Wenger, Trey V. |
| contents | 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_20278 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | Metallicity Structure in Galactic Longitude-Velocity Diagrams of the Milky Way Disk and FIRE-2 Simulations Liu, Victor Balser, Dana S. Wenger, Trey V. Astrophysics of Galaxies 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. |
| title | Metallicity Structure in Galactic Longitude-Velocity Diagrams of the Milky Way Disk and FIRE-2 Simulations |
| topic | Astrophysics of Galaxies |
| url | https://arxiv.org/abs/2601.20278 |