We investigate the detectability of starburst signatures in the nuclear spectrum of Seyfert 2 galaxies by constructing spectral models in the wavelength range jj3500È4100, combining the spectrum of a bulge population (of age B10 Gyr) with that of younger stellar populations, spanning ages from B3 Myr to 1 Gyr. The major constraints in the analysis are (1) the continuum ratio jj3660/4020, which efficiently discriminates between models combining a bulge spectrum with a stellar population younger than B50 Myr and those with older stellar populations ; (2) the presence of the Balmer lines H8, H9, and H10 in absorption, which are unambiguous signatures of stellar populations with ages in the range 10 MyrÈ1 Gyr for the relevant metallicities. Their detectability depends both on the age of the young component and on its contribution to the total Ñux relative to that of the bulge. We also construct models combining the bulge template with a power-law (PL) continuum, which is observed in some Seyfert 2 galaxies in polarized light, contributing with typically 10%È40% of the Ñux at 4020We conclude that such con-A . tinuum cannot be distinguished from that of a very young stellar population (age ¹ 10 Myr), contributing with less than B0.02% of the mass of the bulge. The models are compared with nuclear spectraÈcorresponding to a radius of 200È300 pc at the galaxyÈof 20 Seyfert 2 galaxies, in which we speciÐcally look for the signatures above of young-to intermediate-aged stellar populations. We Ðnd them in 10 galaxies, thus 50% of the sample. But only in six cases (30% of the sample) can they be attributed to young stars (age \ 500 Myr) : Mrk 1210, ESO 362-G8, NGC 5135, NGC 5643, NGC 7130, and NGC 7582. In the remaining four cases, the signatures are caused by intermediate-aged stars (B1 Gyr). We Ðnd a tendency for the young stars to be found more frequently among the late-type Seyfert galaxies, a well-known e †ect in the nuclei of normal galaxies. This tendency is supported by a comparison between the equivalent widths (W ) of absorption lines of the nuclear spectra of the Seyfert 2 galaxies with those of normal galaxies of the same Hubble type. For the late-type galaxies, the W values of the Seyfert galaxies are within the observed range of the normal galaxies, suggesting a similar stellar population. On the other hand, the W values are lower than those of the normal galaxies for seven out of the 11 Seyferts in early-type galaxies.

We study the gas emission of galaxies with active star formation, consisting mostly of H ii and starburst galaxies, as well as some Seyfert 2 galaxies, and determine chemical and physical parameters. The data consist of 19 high signal‐to‐noise ratio optical templates, a result of grouping 185 emission‐line galaxy spectra. Underlying stellar population models (from Raimann et al.) were subtracted from the templates in order to isolate the pure emission component. We analyse the distribution of these improved signal‐to‐noise ratio emission spectra in diagnostic diagrams and find that the H ii templates show a smaller spread in log([O iii]/Hβ) values than the individual galaxies, apparently as a result of the population subtraction and a better signal‐to‐noise ratio. We thus suggest the template sequence as a fiducial observational locus for H ii galaxies which can be used as reference for models. The sequence of line ratios presented by the H ii galaxies in the diagram log([O iii]λ5007/Hβ) versus log([N ii]λ6584/Hα) is primarily owing to the gas metallicity, of which the log([N ii]/Hα) ratio is a direct estimator. We also study the properties of the starburst galaxies and those intermediate between H ii and starburst galaxies, which are more metal rich and sit on more massive galaxies. We discuss the present results in the frame of a recently proposed equivalent‐width diagnostic diagram for emission‐line galaxies (by Rola et al.) and conclude that the observed ranges in W([O ii])/W(Hβ) and W(Hβ) are mostly owing to the non‐ionizing stellar population contribution. We propose that W(Hβ) be used as an estimator of this contribution to the continuum, and briefly discuss implications to the cosmological use of H ii galaxies.

We use high signal‐to‐noise ratio long‐slit spectra in the λλ3600–4700 range of the 20 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 the equivalent width, W, 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 of the sample has Ca iiK and G band W values smaller at the nucleus than at 1 kpc from it, owing 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 ≤100 Myr and/or of an FC, this proportion decreases to 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies have a contribution >10 per cent of the 3‐Myr/FC component (a degenerate component in which one cannot separate what is caused by an FC or by a 3‐Myr stellar population) at the nucleus, but only 25 per cent of them have this contribution at 3 kpc. As a 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 1 Gyr 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.

We investigate the nature of the continuum emission and stellar populations in the inner 1-3 kiloparsecs of a complete sample of twenty-four southern radio galaxies, and compare the results with a control sample of eighteen non-active early-type galaxies. Twelve of the radio galaxies are classified as Fanaroff-Riley type I (FRI), eight as FRII and four as intermediate or undefined type (FRx). Optical long-slit spectra are used to perform spectral synthesis as a function of distance from the nucleus at an average sampling of 0.5-1.0kpc and quantify the relative contributions of a blue featureless continuum and stellar population components of different ages. Our main finding is a systematic difference between the stellar populations of the radio and control sample galaxies: the former have a larger contribution from an intermediate age (1Gyr) component, suggesting a connection between the present radio activity and a starburst which occurred about 1Gyr ago. In addition, we find a correlation between the contribution of the 1Gyr component and the radio power, suggesting that more massive starbursts have led to more powerful radio emission. A similar relation is found between the radio power and the mean age of the stellar population, in the sense that stronger nuclear activity is found in younger galaxies.Comment: 16 pages, 10 figures, MNRAS in pres