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

TL;DR

This study assesses whether ell-v diagrams can trace present-day metallicity structure in the Milky Way and FIRE-2 MW-mass galaxies. It combines Te-based oxygen abundances from H II regions with FIRE-2 simulations to probe radial metallicity gradients and azimuthal variations in velocity space without relying on distance measurements. The radial gradient is clearly imprinted in ell-v space for both MW observations and FIRE-2 galaxies, while azimuthal variations in the MW remain challenging to detect due to abundance uncertainties; in FIRE-2, anomalous gas motions can mimic azimuthal variations and produce an excess LSR velocity tail larger than in the MW. Overall, ell-v diagrams emerge as a powerful distance-free tool for studying metallicity structure, and the work highlights notable discrepancies between the gas kinematics in FIRE-2 simulations and the Milky Way.

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.

Figures (1)

• Figure 1: The $\ell$-$v$ diagram for all known MW H ii regions from the WISE Catalog. There are 737 H ii regions with derived electron temperatures and 1295 H ii regions with no derived electron temperature, colored blue and brown respectively. Of the 737 sources with electron temperatures, 686 have accurately determined electron temperatures (see text). There are significantly more H ii regions with derived electron temperatures in the Galactic Southern Hemisphere compared to the Northern Hemisphere. Superimposed is the CO(J=1$\rightarrow$0) brightness temperature from Dame_2001_CO integrated over $|b| \leq3\degree$. The CO emission has a typical rms of 1.15 K degrees, or 0.06 K degrees in ${\rm log}_{10}$ units. The color bar range is 0.3$-$1.5 K degrees in ${\rm log}_{10}$ units. The red curve is the terminal velocity for the MW using $R_{\rm 0,MW}$ = 8.20 kpc for the solar Galactocentric radius and $\Theta_{\rm 0,MW}$ = 238 km $\rm s^{-1}$ for the circular rotation speed at the Sun's position.