Quasar Emission Line Regions

Counting Broad-Line Region Clouds

Strong optical and ultraviolet emission-lines are a characteristic feature of active galactic nuclei (AGN). One major class of AGN models assumes that the line emitting material is confined to an ensemble of numerous discrete clouds moving around the central super-massive black hole. However, the number of these individual emitters is still unknown, as well as their origin and nature. The number of clouds is an important parameter to discriminate between different AGN models. Making the simple assumption that each cloud contributes similar to the total line flux, it can be expected that a finite number of clouds will introduce an intrinsic microstructure into the line profiles. These fluctuations will be most prominent in the profile wings due to the smaller number of emitters in comparison to the line core. This intrinsic scatter can be separated from observational noise employing data of high spectral resolution and high signal-to-noise ratio. Early estimates of the number of BLR clouds were presented by Capriotti et al. (1981) and Atwood et al. (1982). They studied the line profiles of Seyfert galaxies with high spectral resolution and derived a lower limit of ~10,000 to ~100,000 individual clouds. Recently, Arav et al. (1997,1998) estimated the number of BLR clouds to be at least of the order of 10,000,000. We observed the luminous quasar 3C273 and several Seyfert 1 galaxies to test the BLR cloud model with respect to signatures of a finite number of emitters (Dietrich et al. 1999). We find for the luminous quasar 3C273 that the number of BLR clouds will be of the order of at least 100,000,000 based on the analysis of Ha and Hb emission line profile. This is consistent with the result of Arav et al. but is in conflict with AGN models assuming a population of individual emitters with only 100,000 as predicted by detailed bloated star models (Alexander & Netzer 1994). However, the number of individual emitters can be significantly reduced if the line widths is not dominated by thermal broadening. Broad individual profiles of the order of FWHM=100 km/s and more can be achieved by non-dissipative magnetohydrodynamic waves, as has been recently suggested by Bottorff & Ferland (2000).

(Dietrich et al. 1999, Astron. & Astrophys. 351, 31)

Publications

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This page is under construction. Last update: April 30, 2005.
Questions or comments should be sent to: dietrich@astronomy.ohio-state.edu

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