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B.M. Peterson (Ohio State University), A. Wandel (UCLA and Hebrew University)
Emission-line variability data on bright AGNs argue strongly for the existence of massive dark objects in the centers of these sources. The time-delayed response of the emission lines to continuum variations is used to infer the size of the line-emitting region, and these determinations are combined with measurements of the Doppler widths of the variable line components to estimate a virial mass. In the case of the well-studied Seyfert galaxy NGC 5548, data for several different emission lines spanning an order of magnitude in distance from the central source show the expected virial signature, i.e., that the square of the Doppler line width is inversely proportional to the emission-line time delay; all the data are consistent with a single value for the mass of the central object. We also show that although the emission-line time delays vary with time (apparently due to changes in the mean luminosity of the central source), the Doppler width of the variable part of the emission line also changes in the opposite sense, again consistent with the virial prediction.
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