Collimating the Ionizing Radiation in AGN

Details Collimating the Ionizing Radiation in AGN Collimating the Ionizing Radiation in AGN

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.8401k&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #84.01; Bulletin of the American Astronomical Society, Vol. 24, p.1257

Astronomy and Astrophysics

Astronomy

1

Scientific paper

The most popular `unified models' of AGN invoke an optically thick torus surrounding the nucleus and the broad-line clouds which both collimates the ionizing radiation and either reveals or obscures the central regions to the observer, depending on the line of sight. The simplest version of the obscuring torus (infinite optical depth, sharp edges), however, cannot explain the whole set of available data. Several Seyfert 1 galaxies (e.g., NGC 4151, NGC 3227, NGC 3516, NGC 3783) exhibit biconical emission line regions, intrinsic UV absorption lines, and high column densities in the soft X-ray. In NGC 4151 in particular, simple geometric arguments based on the analysis of HST images place our line of sight in the shadowed region up to 30(deg) away from the cone edge. This contradicts predictions for a type 1 object, where our line of sight is expected to be within the opening angle of the cone. In our revised picture of the unified model for AGN, the infinitely optically thick torus is surrounded by an atmosphere of lower density neutral gas. Columns of N_H = 10(20) cm(-2) will be optically thick at all wavelengths past the Lyman edge up to the soft X-ray. Above the Lyman limit, and for normal dust-to-gas ratios, extinction will not have much impact until gas column densities exceed several times 10(21) cm(-2) . This rather simple revision has a very important consequence -- the gas in the atmosphere can collimate the ionizing radiation in a well defined conical structure with an opening angle much smaller than that defined by the shadow of the opaque torus. Lines of sight falling within the cone defined by the opaque torus, but exterior to the ionization cone, will have a relatively clear view of the central regions of the active nucleus at wavelengths longward of the Lyman edge of neutral hydrogen. This collimation concept has several testable predictions which we will discuss. skip=.5 This work is supported by NASA-GSFC grant NAG 5-1630 to the FOS team and NASA contract NAS 5-27000 to the Johns Hopkins University.

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