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

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.