Multi‐density models of the Broad Line Region in Active Galactic Nuclei: a comparative study with single‐cloud models

Abstract

AbstractNearly ten years ago Kwan and Krolik (1979, 1981) published the firstsuccessful photoionization model of the Broad Line Region of Active Galactic Nuclei, the so‐called ‘Standard Model’. Since then several efforts have been made to obtain better results using more sophisticated models. Anopen issue is that photoionization models are generally computed startingfrom the assumption that the emission line spectrum is produced by a single‐slab cloud with a ‘standard’ hydrogen density nH = 109.5 cm‐3, but it seems more likely that a range of densities is present in the Broad Line Region.Purpose of this paper is to review the results given by single‐cloud models using the most recent photoionization code, Cloudy 84, and to investigate if the addition of one or more components with different densities does affect the line ratios. To this aim we compute the emission line ratios produced by photoionized single‐slab clouds for a wide range of hydrogen densities (nH = 109.5−1013 cm−3), ionization parameters (U = 10−4− 10−0.5) and column densities (NH = 1023−1025 cm−2). Two or more populations of clouds are then combined and the resulting emission line spectrum is compared with a sample of mean observed line ratios. We find that the addition to the standard component of clouds having different densities and located at different positions from the central source introduces many changes in the line ratios, and that these changes are in the direction of a better agreement with the observed emission line ratios.