Spectral synthesis of the nuclear regions of Seyfert 2 and radio galaxies

Abstract

We present the results of an optical spectral synthesis analysis for the nuclei of 20 Seyfert 2 and four radio galaxies, using a base of stellar population templates of different ages and metallicities and a power-law continuum. Compared with the stellar population of elliptical galaxies, we find that Seyfert 2s usually have a smaller contribution from old metal-rich stars (age 10 Gyr, Z ≥ Z̛), and a larger contribution from stars with ages of 100 Myr. We also find that the contributions from stars with ages ≤ 10 Myr and from a power-law continuum are small, rarely exceeding 5 per cent. These results show that the general assumption of elliptical galaxies as stellar population templates for these objects is incorrect, also implying that the excess blue continuum frequently found in their nuclear spectra is probably due to this template mismatch.We find a considerable contribution from 100-Myr-old stars (~~5 per cent), which can be interpreted from the point of view of models where the fuelling of the AGN is carried out by interactions/mergers.