Investigation of disc scalelengths of Sb galaxies

For a sample of 21 Sb galaxies, disc scalelengths r(D) in B and I are investigated in order to measure the dust content. The ratios r(D)(B)/r(D)(I) increase from 1.0 for face-on galaxies to 1.5 for edge-on galaxies. B and I images of model galaxies are calculated for a variety of geometries, optical depths and inclination angles. The best agreement between data and simulations is found for model galaxies for which the scalelength r(0) of the stellar disc is 12.5% larger in the B passband than in the I passband, and for which the scalelength of the dust disc is 50% larger than r(0)(I). This indicates that the observed scalelength ratios are produced by dust extinction as well as intrinsic colour gradients within the stellar disc. The central optical depth in B for face-on view for the best-fitting model is tau(0)(B) = 3.