Stellar population gradients in Seyfert 2 galaxies . Northern sample

We use high signal-to-noise ratio long-slit spectra in the λλ3600-4700 Å range of the twenty brightest northern Seyfert 2 galaxies to study the variation of the stellar population properties as a function of distance from the nucleus. In order to characterize the stellar population and other continuum sources (e.g. featureless continuum FC) we have measured equivalent widths Ws of six absorption features, four continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. About half the sample has CaIIK and G-band W values smaller at the nucleus than at 1 kpc from it, due to a younger population and/or FC. The stellar population synthesis shows that, while at the nucleus, 75 per cent of the galaxies present contribution >20 per cent of ages ≤100Myr and/or of a FC, this proportion decreases to 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies have contribution >10 per cent of the 3Myr/FC component (a degenerate component in which one cannot separate what is due to a FC or to a 3Myr stellar population) at the nucleus, but only 25 per cent of them have this contribution at 3 kpc. As reference, the stellar population of 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert (from S0 to Sc) was also studied. A comparison between the stellar population of the Seyferts and that of the non-Seyferts shows systematic differences: the contribution of ages younger than 1Gyr is in most cases larger in the Seyfert galaxies than in non-Seyferts, not only at the nucleus but up to 1 kpc from it.