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

Abstract

Results from an analysis of low-resolution (R ~ 250)† near-infrared long-slit spectra covering simultaneously the I, J, H and K bands, for a sample of 15 Seyfert galaxies and the NGC5253 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 μm and [S III] λ 9532, followed by Paβ. In some cases, [Fe II] 1.26 and 1.64 μm are also seen. [Fe II] 1.26 μm and H2 v ˆ 1 2 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]/Paβ ratio as a measure of the gaseous excitation in Mrk 573, NGC1386 and NGC7582. 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 Paβ in NGC1365 and possibly nuclear Brγ 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 ~1000K)† dust emission dominates the nuclear continuum in NGC1365, NGC2110, NGC3281, NGC7582 and ESO 362-G18. In NGC1386, 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.