Stellar populations in active galactic nuclei - II. Population synthesis

The relationship of an AGN to its host galaxy is one crucial question in th e study of galaxy evolution. We present a method to estimate the stellar co ntribution in active galactic nuclei. We perform stellar population synthes is in the central regions of a sample of 12 galaxies of different levels of activity: normal galaxies, starburst galaxies, LINERs, Seyfert 2 and Seyfe rt 1 galaxies. Quantification of the stellar contribution is carried out in the visible range (5000 to 8800 Angstrom) using the equivalent widths of t he absorption features throughout the spectrum. The synthesis is done by a variant of the new GPG method (Pelat, 1997). This method, contrary to previ ous ones, gives a unique solution. We find quite different stellar populations for the different types of acti vity, which seems to be indicative of an age sequence. The starburst galaxi es present the youngest populations of the sample. The Seyfert 2 nuclei and NGC 1275, a Seyfert 1 with signs of interaction and where young stellar cl usters have been found, also show the contribution of a young population, l ess intense than in the starburst galaxies but metal rich. NGC 3516, a typi cal Seyfert 1, has a normal population characteristic of galaxies of the sa me Hubble type and finally the LINERs show the oldest populations in the sa mple, metal rich, with little star formation still going on. It is found that a strong CaII triplet, even though these lines are sensiti ve to gravity, does not imply necessarily a stellar population dominated by supergiant stars.