Radial distribution of 12 + log(O/H). Upper panel: comparisons of the present-day radial distribution of 12 + log(O/H) predicted by our best-fitting model (solid black line) with the observational data. Different symbols denote the observed metallicity data from different works as filled yellow asterisks (Bibby & Crowther 2010), filled green circles (Pilyugin et al. 2014), and filled red diamonds (Stanghellini et al. 2015). The linear best fit to the observed data and 1σ uncertainty are plotted as the solid and dotted blue lines, respectively. Lower panel: filled black circles show the deviations of the observations from our best-fitting model predictions, derived by subtracting the solid black line from the observed data in the upper panel, and the open blue circles show those obtained from subtracting the linear best fit from the observational data in the upper panel.