Abstract

Short-wavelength IUE archival data for NGC 3783 were re-extracted and analysed to constrain numerical modelling parameters in detailed photoionization analyses. The He IIλ1640, C IVλ1549, and C IIIλ1909 line intensities and trends can be reasonably well reproduced by a two cloud component model. In order to produce satisfactory line intensities or trends for other higher ionization lines and lower ionization lines, still more gas components are necessary with gas density ranging from roughly 1011 cm−3 to 109 cm−3 or less. In going from the inner to outer clouds, the optical depth increases and the gas density decreases approximately as r−2.

The amount of dust obscuration along the line of sight, as required by the models, is consistent with reddening estimates from HeII line ratios, and CNO abundance ratios derived from intercombination line intensities suggest abundances of carbon, nitrogen and oxygen lower by a factor of about 2 relative to solar. The HeII line rest equivalent widths from the models suggest a gas covering factor of order 0.25.

Introduction

Although sophisticated numerical photoionization models have been employed for two decades in investigations of the broad–line–emitting regions (BLRs) of Seyfert 1 galaxies, some of the most fundamental BLR properties such as the intrinsic ionizing radiation field, the ionization parameter, emitting gas density, chemical abundances, and the “covering factor” (the fraction of solid angle occupied by optically thick gas around the ionizing radiation source) are still open to question.