Astronomy and Astrophysics – Astronomy
Scientific paper
Oct 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991mnras.252..586l&link_type=abstract
Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 252, Oct. 15, 1991, p. 586-592.
Astronomy and Astrophysics
Astronomy
256
Active Galactic Nuclei, Radio Astronomy, Seyfert Galaxies, Spectral Line Width, Cosmic Dust, Forbidden Bands, Optical Thickness
Scientific paper
Evidence concerning the relative occurrence of narrow-lined and broad-lined AGN in optical, IR, X-ray, and radio-selected samples are discussed. Marginal evidence that narrow-lined AGN were more common in the past is found. Narrow-lined objects have weaker O III forbidden lines at a given radio power. These data are inconsistent with the simplest 'unified scheme', where a similar thick molecular torus surrounds all AGN, and with the simplest modification, that torus geometrical thickness is a function of source power. It is argued that both the typical geometrical thickness and the typical optical thickness of obscuring material vary with source power. It is proposed that the obscuring material, rather than being an orbiting torus of cool molecular matter, may be an expelled shell of gas, only the outer parts of which are dusty.
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