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 coplanar and corotating 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(SFR)) is of exponential form, we obtain the analytic solution of gas-phase metallicity with only three free parameters: the scale length of sigma(SFR) (h (R)), the metallicity of the inflowing gas, and the mass-loading factor defined as the wind-driven outflow rate surface density per sigma(SFR). According to this simple model, the negative gradient of gas-phase metallicity is a natural consequence of the radial inflow of cold gas that is continuously enriched by in situ star formation as it moves toward 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 (R) of sigma(SFR) inferred from these fits agree well with independent estimates from sigma(SFR) profiles, supporting the basic model.