Dust-to-gas mass ratio and metallicity radial profiles with the conversion factor X_CO 0.1 − 0.9  ×  10²⁰ cm^-2 (K km s (see text for the X_COadopted for each galaxy-orange solid curves), and with constant X_CO = 1.8  ×  10²⁰ cm^-2 (red dot-dashed curves) and X_CO = 4.0  ×  10²⁰ cm^-2 (green dotted curves). The magenta circles are the oxygen abundance converted to the dust-to-gas scale with Eq. (3). The continuous black lines show the fit to the abundance data.