Spiral Arms are Metal Freeways: Azimuthal Gas-Phase Metallicity Variations in Simulated Cosmological Zoom-in Flocculent Disks

TL;DR

This study uses high-resolution cosmological simulations to analyze azimuthal metallicity variations in spiral galaxy disks, revealing that spiral arms act as channels for metal-rich and metal-poor gas, independent of local star formation.

Contribution

It demonstrates that azimuthal metallicity patterns in flocculent spirals arise from large-scale gas flows rather than local enrichment, a novel insight into galaxy metallicity distribution.

Findings

Spiral arms alternate between metal-rich and metal-poor regions.

Patterns persist despite variations in metal mixing models.

Arms channel gas radially, influencing metallicity distribution.

Abstract

We examine the azimuthal variations in gas-phase metallicity profiles in simulated Milky Way mass disk galaxies from the Feedback in Realistic Environments (FIRE-2) cosmological zoom-in simulation suite, which includes a sub-grid turbulent metal mixing model. We produce spatially resolved maps of the disks at $z \approx 0$ with pixel sizes ranging from 250 to 750~pc, analogous to modern integral field unit (IFU) galaxy surveys, mapping the gas-phase metallicities in both the cold & dense gas and the ionized gas correlated with HII regions. We report that the spiral arms alternate in a pattern of metal rich and metal poor relative to the median metallicity on the order of $\lesssim 0.1$~dex, appearing generally in this sample of flocculent spirals. The pattern persists even in a simulation with different strengths of metal mixing, indicating that the pattern emerges from physics above the sub-grid scale. Local enrichment does not appear to be the dominant source of the azimuthal metallicity variations at $z \approx 0$: there is no correlation with local star formation on these spatial scales. Rather, the arms are moving inwards and outwards relative to each other, carrying their local metallicity gradients with them radially before mixing into the larger-scale interstellar medium. We propose that the arms act as freeways channeling relatively metal poor gas radially inwards, and relatively enriched gas radially outwards.