Stellar population gradients in normal and active galaxies

Abstract

We use high signal-to-noise ratio long-slit spectra in the λλ3500–7000-Å range of three early-type (Hubble type S0) normal, two low-ionization nuclear emission region (LINER) and three Seyfert 2 galaxies to make a comparative study of 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) we have measured equivalent widths (Ws) of six absorption features, continuum colours and their radial variations, and performed spectral population synthesis as a function of distance from the nucleus. The Ws for the normal galaxies are strongest at the nucleus and decrease outwards. The two LINER galaxies show a similar behaviour, but with the nuclear Ws in the blue (λ < 4500 ņ) ~~ 10–20 per cent smaller. Both normal galaxies and LINERs present a redder nucleus than their surroundings. The three Seyfert 2 galaxies present the opposite behaviour, with most Ws in the blue being smaller at the nucleus, which is also bluer than outside. By combining the results of the present work with those of a previous study, we conclude that, in a sample of 12 Seyfert 2s of Hubble type S0, 10 (~~ 80 per cent) present nuclear Ws smaller than those of normal galaxies, while at ~~ 1 kpc from the nucleus this number decreases to 4 (~~30 per cent). Sampling regions of ~~ 100–200 pc, we conclude that the normal galaxies have nuclear stellar populations dominated by the components with an age of 10 Gyr and metallicity solar or above. The contribution of this component decreases, while that of 1 Gyr increases, with increasing distance from the nucleus. The LINERs present a similar behaviour but have a ~~ 10–20 per cent larger contribution of the 1-Gyr component at the nucleus. For the Seyfert 2 galaxies, there are additional components: in the three cases studied here, there is a significant contribution from a 100-Myr-old population at the nucleus plus featureless continuum in two cases and also of a 3-Myr-old component in Mrk463E. Outside the nucleus, the stellar population is similar to that of normal galaxies in ESO138-G1, but it has an excess of old metal-poor components in NGC7743 and of young components in Mrk463E, when compared with normal galaxies. The reddening of the continuum is lowest in the normal galaxy nuclei ‰[E (B-V)†i ~~ 0.00Š] and highest in the Seyfert [E (B-V†)i ~~ 0.20]Š nuclei.