Dust-to-gas mass surface density ratio Σ_dust/Σ_gas in apertures with R_a = 30 arcsec as a function of galactocentric radius R. Hexagons are color coded according to the atomic gas surface density Σ_HI, given by the colorbars, The blue line shows the linear fit to the data as given by χ² minimization. After radial binning, the median values and their dispersion are shown with filled blue circles and vertical blue bars, respectively. The dashed lines indicate the density of data points.