Journal Article

Linewidth distributions as evidence for axisymmetry in the broad-line regions of active galaxies

C. M. Rudge and D. J. Raine

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 297, issue 1, pages L1-L4
Published in print June 1998 | ISSN: 0035-8711
Published online June 1998 | e-ISSN: 1365-2966 | DOI:
Linewidth distributions as evidence for axisymmetry in the broad-line regions of active galaxies

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The nuclei of a wide class of active galaxies emit broad emission lines with widths at half-maximum (FWHM) in the range 103–104 km s−1. This spread of widths is not solely a consequence of the range of luminosity of these sources, since a plot of width versus luminosity shows a large scatter. We propose that the broad-line emission region is axially symmetric and that this scatter in linewidth arises from an additional dependence on the angle of the line of sight to the axis of the emission region. Such a relation is natural in unified models of active nuclei which link a variety of observed properties to viewing angle. Adopting a simple form for the linewidth as a function of luminosity and angle, and convolving this with the observed luminosity function, allows us to predict a linewidth distribution consistent with the available data. Furthermore, we use the relation between the equivalent width of a line and the luminosity in the continuum (the ‘Baldwin effect’) to predict an observed correlation between linewidth and equivalent width. The scatter on this correlation is again provided by angular dependence. The results have applications as diagnostics of models of the broad-line emission region and in cosmology.

Keywords: galaxies: active; quasars: emission lines; galaxies: Seyfert; cosmology: theory

Journal Article.  0 words. 

Subjects: Astronomy and Astrophysics

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