Quasars & Active Galaxies

Quasar Emission Line Regions

Quasars and their less luminous cousins, the Seyfert 1 galaxies, are defined by the presence of broad emssion lines (BELs) in the rest-frame UV. The figure on this page shows some of these features in a composite UV spectrum of redshift >4 quasars. BELs form in dynamic, photoionized plasmas within a few pc of the central energy source, and they carry a wealth of encrypted information about these environments. We are primarily interested in diagnostics of the physical conditions, the shape of the ionizing spectrum and the elemental abundances. Major ongoing efforts, headed by Matthias Dietrich, include a study of BEL cloud structure (using ultra-high resolution spectroscopy) and and an investigation into the relationship of BEL properties (e.g. physical conditions and abundances) to the AGN redshift and luminosity.

Normalized mean spectrum of 13 quasars at redshift >4 (from Shields et al. 1997). Prominant BELs are labeled.

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Counting Broad-Line Region Clouds

A characteristic feature of active galactic nuclei (AGN) is their strong optical and ultraviolet emission-line spectrum. 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 massive black hole. But 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. Assuming that each cloud contributes in a comparable way to the total line flux, it can be expected that a finite number of clouds will introduce an intrinsic microstructure in the line profiles. These fluctuations will be most prominent in the profile wings due to the smaller number of clouds in comparison to the line core. This intrinsic scatter can be separated from the unavoidable observational noise using echelle spectroscopy with high S/N ratio and high spectral resolution Deltav ~ 10 km/s.

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 10^4 to 10^5 individual clouds. Recently, Arav et al. (1997,1998) estimated the number of BLR clouds to be at least of the order of 10^7.

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 10^8 based on the analysis of the H-alpha and H-beta emission line profile residuals.

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 10^5 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 acchived by non-dissipative magnetohydrodynamic waves as has been recently suggested (Bottorff & Ferland 2000).

H-alpha and H-beta line profiles of 3C273 (top panel) together with their corresponding residuals (bottom panel).

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This page created by Matthias Dietrich and Craig Warner
Questions or comments should be sent to: Matthias Dietrich

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