Extended gas in Seyfert galaxies: near-infrared observations of 15 active nuclei

Results from an analysis of low-resolution (R similar to 250) near-infrared long-slit spectra covering simultaneously the I, J, H and K bands, for a sample of 15 Seyfert galaxies and the NGC 5253 starburst nucleus, are presented. The Seyfert galaxies were selected as presenting 'linear' or cone-like high-excitation emission lines in the optical, most probably arising from the collimation of the radiation of the central source by a dusty molecular torus. Our goal was to look for signatures of this torus, and to investigate the gaseous distribution, excitation and reddening. The strongest emission lines detected are usually He I 1.083 mu m and [S III] lambda 9532, followed by Pa beta. In some cases, [Fe II] 1.26 and 1.64 mu m are also seen. [Fe II] 1.26 mu m and H-2 upsilon = 1 - 0 S(1) are detected in some of the higher resolution spectra obtained for five galaxies. The emission lines are spatially extended in most cases, and we have used the [Fe II]/Pa beta ratio as a measure of the gaseous excitation in Mrk 573, NGC 1386 and NGC 7582, Values for this ratio between 1.5 and 6 are found: suggesting excitation of [Fe II] by X-rays or shock waves in some regions. Broad permitted lines are observed in three Seyfert 1 galaxies, Nuclear Pa beta in NGC 1365 and possibly nuclear Br gamma in Mrk 573 are also broad. From analysis of the spatial distribution of the continuum (J - H) and (H - K) colours derived from our spectra, we find redder colours for the nucleus than for the nearby bulge in most of the Seyfert 2s observed. Comparison with models including emission from dust and stars shows that hot (T similar to 1000 K) dust emission dominates the nuclear continuum in NGC 1365, NGC 2110, NGC 3281, NGC 7582 and ESO 362-G18. In NGC 1386, 5643 and 5728 the main contributor is the underlying stellar population, combined with some foreground reddening and/or cooler dust emission. In a few cases, the (J - H) colours on opposite sides of the nucleus differ by 0.3-0.8 mag, an effect that we interpret as partly resulting from differences in the local stellar population, and possibly extinction gradients.