The Gas-phase Metallicity Profiles of Star-forming Galaxies in the Modified Accretion Disk Framework

TL;DR

This paper presents an analytic model for gas-phase metallicity profiles in star-forming galaxies based on a modified accretion disk framework, fitting observed profiles and revealing the role of radial inflow and inflow metallicity.

Contribution

The work introduces a simple analytic model with three parameters to describe metallicity gradients, fitting well to observed data and linking star formation and inflow properties.

Findings

Model accurately fits observed metallicity profiles.

Metallicity floor at outer disk matches inflow gas metallicity.

Inferred SFR scalelength agrees with independent measurements.

Abstract

Simulations indicate that the inflow of gas of star-forming galaxies is almost co-planar and co-rotating with the gas disk, and that the outflow of gas driven by stellar winds and/or supernova explosions is preferentially perpendicular to the disk. This indicates that the galactic gas disk can be treated as a modified accretion disk. In this work, we focus on the metal enhancement in galactic disks in this scenario of gas accretion. Assuming that the star formation rate surface density ($\Sigma_{\rm SFR}$) is of exponential form, we obtain the analytic solution of gas-phase metallicity with only three free parameters: the scalelength of $\Sigma_{\rm SFR}$ ($h_{\rm R}$), the metallicity of the inflowing gas and the mass-loading factor defined as the wind-driven outflow rate surface density per $\Sigma_{\rm SFR}$. According to this simple model, the negative gradient of gas-phase metallicity is a natural consequence of the radial inflow of cold gas which is continuously enriched by in-situ star formation as it moves towards the disk center. We fit the model to the observed metallicity profiles for six nearby galaxies chosen to have well-measured metallicity profiles extending to very large radii. Our model can well characterize the overall features of the observed metallicity profiles. The observed profiles usually show a floor at the outer regions of the disk, corresponding to the metallicity of inflow gas. Furthermore, we find the $h_{\rm R}$ of $\Sigma_{\rm SFR}$ inferred from these fits agree well with independent estimates from $\Sigma_{\rm SFR}$ profiles, supporting the basic model.