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Simulations indicate that the inflow of gas of star-forming galaxies isalmost co-planar and co-rotating with the gas disk, and that the outflow of gasdriven by stellar winds and/or supernova explosions is preferentiallyperpendicular to the disk. This indicates that the galactic gas disk can betreated as a modified accretion disk. In this work, we focus on the metalenhancement in galactic disks in this scenario of gas accretion. Assuming thatthe star formation rate surface density ($\Sigma_{\rm SFR}$) is of exponentialform, we obtain the analytic solution of gas-phase metallicity with only threefree parameters: the scalelength of $\Sigma_{\rm SFR}$ ($h_{\rm R}$), themetallicity of the inflowing gas and the mass-loading factor defined as thewind-driven outflow rate surface density per $\Sigma_{\rm SFR}$. According tothis simple model, the negative gradient of gas-phase metallicity is a naturalconsequence of the radial inflow of cold gas which is continuously enriched byin-situ star formation as it moves towards the disk center. We fit the model tothe observed metallicity profiles for six nearby galaxies chosen to havewell-measured metallicity profiles extending to very large radii. Our model canwell 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 withindependent estimates from $\Sigma_{\rm SFR}$ profiles, supporting the basicmodel.

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