We present a simple metallicity estimator based on the logarithmic [N ii] λ6584/Hα ratio, hereafter N2, which we envisage will become very useful for ranking galaxies in a metallicity sequence from redshift survey‐quality data even for moderately low spectral resolution. We have calibrated the N2 estimator using a compilation of H ii galaxies having accurate oxygen abundances, plus photoionization models covering a wide range of abundances. The comparison of models and observations indicates that both primary and secondary nitrogen are important for the relevant range of metallicities. The N2 estimator follows a linear relation with log(O/H) that holds for the whole abundance range covered by the sample, from approximately 1/50th to twice the Solar value [7.2<12+log(O/H)<9.1]. We suggest that the ([S ii] λλ6717,6731/Hα) ratio (hereafter S2) can also be used as a rough metallicity indicator. Because of its large scatter the S2 estimator will be useful only in systems with very low metallicity, where [N ii] λ6584 is not detected or in low‐resolution spectra where [N ii] λ6584 is blended with Hα.

We present a study of the stellar populations in the central ~ 200 pc of a large and homogeneous sample comprising 79 nearby galaxies, most of which are type 2 Seyferts. The star-formation history of these nuclei is reconstructed by means of state-of-the art population synthesis modeling of their spectra in the 3500--5200 A interval. A QSO-like featureless continuum (FC) is added to the models to account for possible scattered light from a hidden AGN. We find that: (1) The star-formation history of Seyfert 2 nuclei is remarkably heterogeneous: young starbursts, intermediate age, and old stellar populations all appear in significant and widely varying proportions. (2) A significant fraction of the nuclei show a strong FC component, but this FC is not always an indication of a hidden AGN: it can also betray the presence of a young, dusty starburst. (3) We detect weak broad Hbeta emission in several Seyfert 2s after cleaning the observed spectrum by subtracting the synthesis model. These are most likely the weak scattered lines from the hidden Broad Line Region envisaged in the unified model, given that in most of these cases independent spectropolarimetry data finds a hidden Seyfert 1. (4) The FC strengths obtained by the spectral decomposition are substantially larger for the Seyfert 2s which present evidence of broad lines, implying that the scattered non-stellar continuum is also detected. (5) There is no correlation between the star-formation in the nucleus and either the central or overall morphology of the parent galaxies.Comment: 25 pages, 20 figs, MNRAS accepte

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We present measurements of the gas-phase abundance ratio C/O in six H II regions in the spiral galaxies M101 and NGC 2403, based on ultraviolet spectroscopy using the Faint Object Spectrograph on the Hubble Space T elescope. The ratios of C to O increase systematically with O/H in both galaxies, from log C/O B [0.8 at log O/H \ [4.0 to log C/O B [0.1 at log O/H \ [3.4. C/N shows no correlation with O/H. The rate of increase of C/O is somewhat uncertain because of uncertainty as to the appropriate UV reddening law and uncertainty in the metallicity dependence on grain depletions. However, the trend of increasing C/O with O/H is clear, conÐrming and extending the trend in C/O indicated previously from observations of irregular galaxies. Our data indicate that the radial gradients in C/H across spiral galaxies are steeper than the gradients in O/H. Comparing the data to chemicalevolution models for spiral galaxies shows that models in which the massive star yields do not vary with metallicity predict radial C/O gradients that are much Ñatter than the observed gradients. The most likely hypothesis at present is that stellar winds in massive stars have an important e †ect on the yields and thus on the evolution of carbon and oxygen abundances. C-to-O and N-to-O abundance ratios in the outer disks of spirals determined to date are very similar to those in dwarf irregular galaxies. This implies that the outer disks of spirals have average stellar-population ages much younger than those of the inner disks.

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