Ultraviolet Continuum, Physical Conditions, and Filling Factor in Active Galactic Nuclei

Abstract

The narrow-line region of active galaxies is formed by gas clouds surrounded by a diluted gas. Standard one-dimensional photoionization models are usually used to model this region in order to reproduce the observed emission lines. Since the narrow-line region is not homogeneous, two major types of models are used: (1) those assuming a homogeneous gas distribution and a filling factor less than unity to mimic the presence of the emitting clouds and (2) those based on a composition of single-cloud models combined in order to obtain the observed spectra. The first method is largely used but may induce misleading conclusions, as shown in this paper. The second one is more appropriate but requires a large number of observed lines in order to limit the number of single models used. After discussing the case of an extragalactic H II region, for which the ionizing radiation spectrum is better known, we show that one-dimensional models for the narrow-line region with a filling factor less than unity do not properly mimic the clumpiness but just simulate an overall lower density. Multicloud models lead to more reliable results. Both models are tested in this paper, using the emission-line spectra of two well-known Seyfert galaxies, NGC 4151 and NGC 1068. It is shown that ionizing radiation spectra with a blue bump cannot be excluded by multicloud models, although excluded by Alexander et al. using homogeneous models with a filling factor less than unity.