Size and properties of the NLR in the Seyfert-2 galaxy NGC 1386

¹ Astronomisches Institut Ruhr-Universität Bochum, Universitätsstrasse 150, 44780 Bochum, Germany e-mail: [nbennert;haas;chini]@astro.rub.de
² Astronomical Institute, Academy of Sciences of the Czech Republic, Boční II 1401, 141 31 Prague 4, Czech Republic e-mail: bruno@ig.cas.cz
³ CRAL-Observatoire de Lyon, 9 avenue Charles André, 69561 Saint-Genis-Laval Cedex, France
⁴ Max-Planck Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany e-mail: skomossa@xray.mpe.mpg.de

Received: 3 June 2005
Accepted: 3 October 2005

Abstract

We study the narrow-line region (NLR) of the Seyfert-2 galaxy NGC 1386 by means of long-slit spectroscopy obtained with FORS1 at the VLT. We use the galaxy itself for subtracting the stellar template, applying reddening corrections to fit the stellar template to the spectra of the NLR. The continuum gets steadily redder towards the nucleus. The spatial distribution of the reddening derived from the Balmer decrement differs from the continuum reddening, indicating dust within the NLR with a varying column density along the line of sight. Using spatially resolved spectral diagnostics, we find a transition between central line ratios falling into the AGN regime and outer ones in the -region regime, occuring at a radius of ~6´´ (310 pc) in all three diagnostic diagrams. Applying CLOUDY photoionisation models, we show that the observed distinction between -like and AGN-like ratios in NGC 1386 represents a true difference in ionisation source and cannot be explained by variations of physical parameters such as ionisation parameter, electron density or metallicity. We interpret it as a real border between the NLR, i.e. the central AGN-photoionised region and surrounding regions. We derive surface brightness, electron density, and ionisation parameter as a function of distance from the nucleus. Both the electron density and the ionisation parameter decrease with radius. We discuss the consequences of these observations for the interpretation of the empirical NLR size-luminosity relation. In the outer part of the NLR, we find evidence for shocks, resulting in a secondary peak of the electron-density and ionisation-parameter distribution north of the nucleus. We compare the NLR velocity curve with the stellar one and discuss the differences.