Other
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21915424s&link_type=abstract
American Astronomical Society, AAS Meeting #219, #154.24
Other
Scientific paper
Active galactic nuclei show a wide range in the intensity of Fe II emission from the broad line region (BLR). We propose that this results from differing degrees of depletion of iron into grains in the low ionization of region of the BLR (Shields et al. 2010, ApJ, 721, 1835). This can occur naturally in a model in which gas flows inward from the "dusty torus" commonly postulated in AGN. As the gas reaches the inner edge of the torus and becomes exposed to the ionizing radiation from the central source, the grain temperature rises. Photoionization models show that, for conventional BLR parameters, the grains reach their sublimation temperature at an intermediate depth in the the Fe II emitting zone. This makes plausible the idea that Fe is restored to the gas phase deep in the partially ionized zone in some objects, giving an extensive zone of strong Fe II emission. In other AGN, the gas traverses most of the partially ionized zone before the grains are destroyed, giving a low gas-phase abundance and weak Fe II. Such a model explains why the BLR radius appears to be smaller than the nominal sublimation radius, because the grains are shielded by the dusty torus as they approach the central continuum source.
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