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 norma l galaxies, and 73 stars at a resolution of 2.2 Angstrom over the wavelengt h region 3500-5300 Angstrom. Cross-correlation between the stellar spectra is performed to group the individual observations into 44 synthesis standar d spectra. The standard groups include a solar abundance sequence of spectr al 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 ste llar data with previously published spectra is performed both with the indi vidual spectra and the standard groups. For each galaxy, two distinct spati al regions are considered: the nucleus and the external bulge. Spectroscopi c variations from one galaxy to another and from the central to the externa l region are briefly discussed. It is found that the central region of a Se yfert 2 galaxy, after subtracting the bulge stellar population, always show s 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 a nd G band) (Sey2e); b) few emission lines, many absorption lines, and a red der 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 dist ributions of Seyfert 2 galaxies with Balmer lines detected in absorption an d 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 les s than 100 Myr) in their central region, clearly unveiled by the high order Balmer series seen in absorption.