Spatially Resolved Gas-phase Metallicity in FIRE-2 Dwarfs: Affecting Metallicity Relations with Star Formation and Mergers

We present an analysis of spatially resolved metallicity relations in five dwarf galaxies (M_halo ≈ 10¹¹ M_☉, M_* ≈ 10^8.8–10^9.6 M_☉) from the FIRE-2 (Feedback in Realistic Environments) cosmological zoom-in simulation suite near z ≈ 0, over a period of 1.5 Gyr, and compare our findings with observations. While these dwarf galaxies represent a diverse sample, we find that all simulated galaxies match the observed mass-metallicity (MZR) and mass-metallicity gradient (MZGR) relations. We note that in all five galaxies, the metallicities are effectively identical between phases of the interstellar medium (ISM), with 95% being within ±.1 dex between various ISM phases, including the cold and dense gas (T < 500 K and n_H > 1 cm^-3), ionized gas (near the Hα T ≈ 10⁴ K ridge-line), and nebular regions (ionized gas pixels where the 10 Myr-averaged star formation rate is non-zero). We find that most of the scatter in relative metallicity between cold and dense gas and ionized gas/nebular regions can be attributed to either local starburst events or metal-poor inflows. We also note the presence of a major merger in one of our galaxies, m11e, finding that metal enrichment and mixing of the outer ionized gas halos between the primary and companion galaxies occurs relatively quickly and triggers a starburst in the main galaxy.