Stellar populations and dust extinction in non-active and Seyfert spiral galaxies

Abstract

Metallicity and age gradients of the stellar populations and dust extinction are studied for a sample of 32 non-active, seven type 1 Seyfert (S1) and 17 type 2 Seyfert (S2) spiral galaxies. The sample galaxies cover the whole range from face-on to edge-on view, and the variation of the optical and near-infrared colour gradients in the disc as a function of the inclination angle is investigated in order to separate colour changes caused by population gradients from those due to dust effects. The measurements show that the observed colour gradients in the discs of the non-active galaxies are significantly larger than those found for the S1 and S2 galaxies. In the near-infrared wavelengths, however, these differences disappear, and the colour gradients are the same for all three galaxy types. No systematic differences are found between the colour gradients of the discs of the S1 galaxies and those of the S2 galaxies. The data are compared to model images of dusty galaxies with a variety of age and metallicity gradients in the disc. The radial variations of the optical and near-infrared colours of the model galaxies are calculated from the radial changes of the ages and the metallicities of the stars, using broad-band colours of a single stellar population. The stellar content at a given position in the disc is determined by the average age, the metallicity and the star formation history. For the non-active galaxies, the observed colour gradients are represented best by a model with a metallicity gradient, with the inner regions of the stellar disc being more metal-rich than the outer regions. However, the presence of an age gradient, with the inner regions of the stellar disc being older than the outer regions, cannot be ruled out. For the S1 and S2 galaxies, the comparison between data and models indicates that the age and metallicity gradients in the stellar disc are small. As far as the internal dust extinction is concerned, the comparison between data and models indicates that both the non-active and the S2 galaxies show significant dust extinction, but they are not optically thick. Ke yw ords: dust, extinction ‐ galaxies: abundances ‐ galaxies: active ‐ galaxies: spiral ‐ galaxies: stellar content ‐ galaxies: structure.