Constraints on Radial Gas Flows in the Milky Way Disk Revealed by Large Stellar Age Catalogs

Abstract

Disk galaxies like the Milky Way are expected to experience gas flows carrying matter toward their centers. This paper investigates the role of these radial gas flows in models of Galactic chemical evolution (GCE). We follow five different parameterizations of the Galactocentric radial velocity, $v_{r,g}$, of the interstellar medium (ISM). Relative to the $v_{r,g}=0$ limit, all models predict stellar metallicity to decline less significantly with age in the outer disk and more significantly in the inner disk. This outcome arises because radial flows cannot remove gas from one region of the Galaxy without placing it elsewhere, leading to opposing effects on enrichment timescales between the inner and outer Galaxy. This prediction is at odds with recent observational constraints, which indicate remarkably minimal decline in metallicity ($\lesssim$$0.1$ dex) between young ($\sim$$0-2$ Gyr) and old populations ($\sim$$8-10$ Gyr) across the \textit{entire} Galactic disk. Radial gas flows cannot be the sole explanation of this result at all Galactocentric radii. Our models reproduce this result at $R\gtrsim6$ kpc if the flow velocity is relatively constant in both radius and time near $v_{r,g}\approx-1$ km/s. In agreement with previous GCE models, all of our flow prescriptions lead to lower metallicities and steeper radial gradients relative to static models. Exploiting this universal outcome, we identify mixing effects and the relative rates of star formation and metal-poor accretion as the processes that establish the ISM metallicity at low redshift. We provide a suite of analytic formulae describing radial metallicity gradient evolution based on this connection.