Size and properties of the narrow-line region in Seyfert-1 galaxies from spatially-resolved optical spectroscopy

¹ Astronomisches Institut Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany e-mail: [haas;chini]@astro.rub.de
² Department of Physics and Astronomy, Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521, USA e-mail: nicola.bennert@ucr.edu
³ Astronomical Institute, Academy of Sciences of the Czech Republic, Boční II 1401, 141 31 Prague 4, Czech Republic e-mail: bruno@ig.cas.cz
⁴ CRAL-Observatoire de Lyon, 9 avenue Charles André, 69561 Saint-Genis-Laval Cedex, France
⁵ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany e-mail: skomossa@xray.mpe.mpg.de

Received: 22 April 2006
Accepted: 12 July 2006

Abstract

Context.Spatially resolved emission-line spectroscopy is a powerful tool for determining the physical conditions in the narrow-line region (NLR) of active galactic nuclei (AGNs). We recently used optical long-slit spectroscopy to study the NLRs of a sample of six Seyfert-2 galaxies. We have shown that such an approach, in comparison to the commonly used [] narrow-band imaging alone, allows us to probe the size of the NLR in terms of AGN photoionisation. Moreover, several physical parameters of the NLR can be directly accessed.

Aims.We here apply the same methods to study the NLR of six Seyfert-1 galaxies and compare our results to those of Seyfert-2 galaxies.

Methods.We employ diagnostically valuable emission-line ratios to determine the physical properties of the NLR, including the core values and radial dependencies of density, ionisation parameter, and reddening. Tracking the radial change of emission-line ratios in diagnostic diagrams allows us to measure the transition between AGN-like and -like line excitation, and thus to measure the size of the NLR.

Results.In the diagnostic diagrams, we find a transition between line ratios falling in the AGN regime and those typical of regions in two Seyfert-1 galaxies, thereby determining the size of the NLR. The central electron temperature and ionisation parameter are, in general, higher in type-1 Seyferts than in type 2s. In almost all cases, both electron density and ionisation parameter decrease with radius and the decrease is faster in Seyfert-1 galaxies than in Sy 2s. In several objects, the gaseous velocity distribution is characteristic of rotational motion in an (inclined) emission-line disk in the centre. We give estimates of the black-hole masses and discuss our findings in detail for each object.