  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.
