Seyfert Galaxy Ultraviolet Emission-Line Intensities and Variability: A Self-consistent Photoionization Analysis Applied to Broad-Line--emitting Gas in NGC 3783

Abstract

Both strong and weak ultraviolet emission lines in SeYfert 1 galaxies have been accurately measured from reextracted short-wavelength camera data in the IUE satellite data archives. Line ratios, equivalent widths, and trends as functions of continuum variability were used to constrain numerical modeling parameters in detailed photoionization analyses for NGC 3783. All the data for He II λ1640, C IV λ1549, and C III] λ1909 can be reasonably well reproduced by two cloud components. One has a source-cloud distance of 24 lt-days, gas density around 3x10^10^ cm^-3^, ionization parameter range (as the continuum varies) of 4x10^-2^ to 2 x 10^-1^, and cloud thickness such that ~50% of the carbon is doubly ionized and ~50% is triply ionized. The other component is located approximately 96 lt-days from the source, is shielded from the source by the inner cloud, has a density around 3x10^9^ cm^-3^, is characterized by an ionization parameter range 10^-3^ to 2x10^-2^, and the cloud thickness is such that ~45% carbon is doubly ionized and ~55% is singly ionized. In going from the inner to outer clouds, the optical depth increases and the gas density decreases approximately as r^-2^. In order to produce satisfactory line intensities or trends for other higher ionization lines (e.g., O VI λ1034, N V λ1240) and lower ionization lines (e.g., Mg II λ2798), still more gas components are necessary with gas density ranging from roughly 10^11^ cm^-3^ to 10^9^ cm^-3^ or less. For the multiicomponent models the effective cloud ionization parameters would cover a large range from at least several times 10^-1^ to several times 10^-4^ (including allowance for continuum The amount of dust obscuration along the line of sight, as required by the models, is consistent with reddening estimates from He II 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 He II line rest equivalent widths from the models suggest a gas covering factor of order 0.25.