Stellar populations in Seyfert 2 galaxies - I. Atlas of near-UV spectra

We have carried out a uniform spectroscopic survey of Seyfert 2 galaxies to study the stellar populations of the host galaxies. New spectra have been obtained for 79 Southern galaxies classified as Seyfert 2 galaxies, 7 normal galaxies, and 73 stars at a resolution of 2.2 Angstrom over the wavelength region 3500-5300 Angstrom. Cross-correlation between the stellar spectra is performed to group the individual observations into 44 synthesis standard spectra. The standard groups include a solar abundance sequence of spectral types from O5 to M3 for dwarfs, giants, and supergiants. Metal-rich and metal-weak F-K giants and dwarfs are also included. A comparison of the stellar data with previously published spectra is performed both with the individual spectra and the standard groups. For each galaxy, two distinct spatial regions are considered: the nucleus and the external bulge. Spectroscopic variations from one galaxy to another and from the central to the external region are briefly discussed. It is found that the central region of a Seyfert 2 galaxy, after subtracting the bulge stellar population, always shows a near-UV spectrum similar to one of three representative categories: a) many strong emission lines and only two visible absorption lines (Ca II K and G band) (Sey2e); b) few emission lines, many absorption lines, and a redder continuum than the previous category (Sey2a); c) an almost at continuum and high-order Balmer lines seen in absorption (Sey2b). The proportion of Seyfert 2 galaxies belonging to each class is found to be 22%, 28%, and 50% respectively. We find no significative differences between morphology distributions of Seyfert 2 galaxies with Balmer lines detected in absorption and the rest of the sample. This quick look through the atlas indicates that half of Seyfert 2 galaxies harbour a young stellar population (about or less than 100 Myr) in their central region, clearly unveiled by the high order Balmer series seen in absorption.