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Stellar populations in active galactic nuclei. II. Population synthesis

The relationship of an AGN to its host galaxy is one crucial question in the study of galaxy evolution. We present a method to estimate the stellar contribution in active galactic nuclei. We perform stellar population synthesis in the central regions of a sample of 12 galaxies of different levels of ac- tivity: normal galaxies, starburst galaxies, LINERs, Seyfert 2 and Seyfert 1 galaxies. Quantification of the stellar contribu- tion is carried out in the visible range (5000 to 8800 ˚ A) using the equivalent widths of the absorption features throughout the spectrum. The synthesis is done by a variant of the new GPG method (Pelat, 1997). This method, contrary to previous ones, gives a unique solution. We find quite different stellar populations for the differ- ent types of activity, which seems to be indicative of an age sequence. The starburst galaxies present the youngest popula- tions of the sample. The Seyfert 2 nuclei and NGC 1275, a Seyfert 1 with signs of interaction and where young stellar clus- ters have been found, also show the contribution of a young population, less intense than in the starburst galaxies but metal rich. NGC 3516, a typical Seyfert 1, has a normal population characteristic of galaxies of the same Hubble type and finally the LINERs show the oldest populations in the sample, metal rich, with little star formation still going on. It is found that a strong CaII triplet, even though these lines are sensitive to gravity, does not imply necessarily a stellar pop- ulation dominated by supergiant stars.