Bowen fluoresence and He II lines in active galaxies and gaseous nebulae

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The Astrophysical Journal, ISSN: 0004-637X, Vol: 299, Issue: 2, Page: 752-762

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Netzer, Hagai; Elitzur, Moshe; Ferland, Gary J.
IOP Publishing
Active Galaxies; Emission Spectra; Fluorescence; Galactic Nuclei; Helium Ions; Lyman Alpha Radiation; Nebulae; Active Galactic Nuclei; Doppler Effect; Monte Carlo Method; Radiative Transfer; Astrophysics and Astronomy; Physics
article description
We have calculated the intensity of He II and the O III and N III Bowen lines under a variety of astrophysical conditions. Our line-transfer method is based on the escape probability formalism and is especially suitable for combining line fluorescence and photoionization calculations in a simple, straightforward way. It reproduces the results of the more sophisticated line-transfer calculations quite accurately and is much better for producing realistic ionization and thermal structures. An extensive grid of models is presented and discussed, with special emphasis on the case of active galactic nuclei (AGNs). Many O III and N III Bowen lines are predicted to be strong and detectable in Seyfert galaxies and quasars. In particular, O III λ3133 is calculated to be up to 5 times stronger than He II λ4686, and the N III λ4640 band should be marginally detected in many objects. O III and He II lines, at 374 and 304 Å, are the strongest predicted far-ultraviolet lines, with the intensity of the latter approaching that of H Lyα. We study the He II λ1640/λ4686 line ratio, taking into account the interaction of He+ with the H Lyα radiation field. The process is found to be insignificant except for cases of large internal (microturbulent) motion, combined with extreme densities (Ne > 1010 cm-3) and/or ionization parameters. Our best estimate for AGN broad-line clouds is He II λ1640/λ4686 ≈ 10, although values as small as 7 cannot be excluded. This line ratio is a good reddening indicator.