Inside-out versus Upside-down: The Origin and Evolution of Metallicity Radial Gradients in FIRE Simulations of Milky Way-mass Galaxies and the Essential Role of Gas Mixing

Within the Milky Way (MW), younger stellar populations exhibit steeper (more negative) metallicity radial gradients; the origin of this trend remains debated. The FIRE-2 cosmological simulations of MW-mass galaxies show the same trend as the MW, which in FIRE-2 arises because the metallicity gradient of the interstellar medium (ISM), and thus of stars at birth, became steeper over time. We seek to understand this evolution in the context of inside-out radial growth of galaxies. Most FIRE-2 galaxies grew radially inside-out in both gas and stars; specifically, their surface density profiles, $\Sigma(R)$, became shallower over time. Combined with a realized superlinear (Kennicutt-Schmidt-like) relation between star formation and total gas density, the profile of the ratio $\Sigma_{\rm star}(R)/\Sigma_{\rm gas}(R)$ became shallower (flatter) over time. Thus, if metals stayed where they were injected into the ISM from stars, the metallicity gradient would become shallower over time, as some models predict. However, metallicity gradients in FIRE-2 became steeper over time, because of the additional effects of (radial) mixing of metals in the ISM. Specifically, the velocity dispersion and net radial advection of gas declined over time, as ISM turbulence decreased and the disk settled, leading to upside-down vertical growth. In FIRE-2, this evolution in metal mixing of gas associated with upside-down growth dominates over inside-out radial growth, causing the metallicity radial gradient of the ISM and of stars at birth to become steeper over time. We argue that this reflects the ISM history of the MW and of typical MW-mass galaxies.