Disc scalelengths of non-active and active spiral galaxies

Disc scalelengths r(D) are determined for a sample of 32 non-active and 28 active spiral galaxies from optical CCD images. For 21 of the 32 non-active galaxies and 20 of the 28 active galaxies B, V, R and I data have been obtained, while for the remaining galaxies only B and I images have been taken. For 18 of the 21 non-active galaxies, which are measured in all four passbands, rD decreases systematically from B to I, whereas such a decrease is found for only four of the 20 active galaxies with BVRI data. For the non-active galaxies, the ratios r(D)(B)/r(D)(I), r(D)(V)/r(D)(I) and r(D)(R)/r(D)(I) increase systematically with increasing apparent ellipticity epsilon of the galaxies. For the active galaxies, no systematic variation of any of the ratios with increasing epsilon is found. The variation of r(D)(B)/r(D)(I) with epsilon is compared with model calculations. For the non-active galaxies, the data are represented best by a model with a stellar disc that has an intrinsic colour gradient and with a central optical depth in the B band for face-on view of tau (B)(0) = 3. For the active galaxies, the best agreement between data and models is found for models with a stellar disc with no intrinsic colour gradient and no dust. The best-fitting model for the non-active galaxies does not reproduce the data of the active galaxies. The main conclusion of this work is that structural differences seem to exist between the discs of non-active and active galaxies. The non-active galaxies show significant colour gradients within their discs, whereas the active galaxies do not. These gradients are probably caused by a combination of an intrinsic colour gradient within the stellar disc, and dust extinction. Furthermore, the measurements indicate that the non-active galaxies show significant dust extinction in the centre, but they are optically thin in the outer regions. The active galaxies do not seem to have intrinsic colour gradients within the stellar disc and they are optically thin throughout the disc.