Formation of permitted lines in the spectrum of type 1 Seyfert galaxies and quasars. II - Fe II lines and the low excitation region

Details Formation of permitted lines in the spectrum of type 1 Seyfert galaxies and quasars. II - Fe II lines and the low excitation region Formation of permitted lines in the spectrum of type 1 Seyfert galaxies and quasars. II - Fe II lines and the low excitation region

Mar 1980

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980a%26a....83..190c&link_type=abstract

Astronomy and Astrophysics, vol. 83, no. 1-2, Mar. 1980, p. 190-198.

Astronomy and Astrophysics

Astrophysics

73

Line Spectra, Quasars, Seyfert Galaxies, Stellar Mass Accretion, Ultraviolet Spectra, Astronomical Models, Collisional Plasmas, Emission Spectra, Gas Ionization, Iron, Light (Visible Radiation), Photoionization

Scientific paper

Following a previous study (Collin-Souffrin et al., 1979) we investigate the relative intensities of the visible and UV lines of the intense Fe II spectrum of type 1 Seyfert galaxies and quasars. A 9-level atom is used in the computation of the line intensities and relatively accurate collision strengths are computed as we devote particular attention to the collisional excitation mechanism. We confirm that the excitation mechanism is collisional: we show that, in addition to the drawbacks mentioned in Paper I, if the excitation was radiative, the line intensities would be too small compared to the observations. We find that relative intensities of the Fe II lines and of the Mg II 2798 line are well accounted for by an emission region with 1010 ≦ ne ≦ 1011 and 7500 ≦ Te ≦ 10,000 °K. The optical thickness in the UV lines of Fe II is large (∼105).
We examine also other low excitation lines and show that Hα is likely to be emitted at least partly by the same Fe II region, while Lα, Si II, O I, should be emitted by a hotter region and Ca II by a colder one.
The Fe II region is ionized by collisions from level 2 of hydrogen which is populated by the trapped Lα photons (τLα ∼109).
We discuss the geometry of this Fe II region, and find typical dimensions of R ∼ 1016 cm, and H (thickness) ∼ 1014-1016 cm.
Finally we examine the significance of this region and conclude that it is likely to be the outer part of an extended accretion disk completely shielded from the UV and X radiation of the central object. We discuss the reality of the photoionized models and, although we are not able to give a definite answer to this problem, we suggest that the collisional models could perhaps account for all the broad lines in quasars and Seyfert 1 galaxies.

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