Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1980
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1980msngr..22...13v&link_type=abstract
The Messenger, No.22, P. 13, 1980
Astronomy and Astrophysics
Astrophysics
Scientific paper
One of the characteristics of Seyfert 1 nuclei and quasars is the presence in their spectrum of broad permitted Iines or broad wings to the permitted lines. The forbidden lines show no such wings. Because broad He I and He Illines appear in the spectra of quasars and Seyfert 1 galaxies, it seems very likely that ions such as 0+, 0++ or Ne" actually do exist in the broad-line region and that the forbidden lines are suppressed by collisional de-excitation in a region with electron densities Ne > 107 -10+8 cm-3 (Souffrin, 1969, Astronomy and Astrophysics, 1, 305; Anderson 1970, Astrophysical Journal, 162, 743). Some class 1 Seyfert 1 galaxies and low redshift quasars exhibit an anomalously strong Hel A 5876 Aline; this has been believed to show an unusually large helium-to-hydrogen abundance ratio; however, in a high-density nebula, the He I triplet line intensities are significantly enhanced by electron collisional excitation. Theoretical and observational evidence shows that the gas which gives rise to the broad He Ilines is characterized by Ne- 5 x 109 cm -3 and T - 15,000°Kwith normal abundance (Netzer 1978, Ap. J, 219,822; Feldman and MacAlpine 1978, Ap. J, 221, 486).
Veron M.-P.
Véron Pascal
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