ON THE DIVERSITY OF THE BROAD EMISSION-LINE PROFILES IN ACTIVE GALACTIC NUCLEI

The profiles of the broad emission lines in active galactic nuclei (AGN) vary widely in both width and shape between objects. This diversity has important consequences for models of AGN. If the conventional view that the linewidths reflect bulk velocities of the emitting gas is correct, then differences in profile shape indicate that the structure and dynamics of the broad-line region may also differ among the AGN population, even within the same phenomenological class. In this paper, the nature and possible. origin of the pro file diversity are discussed in terms of a simple model based on a parametrization of some basic properties of the BLR - the radial depth, line emissivity distribution and velocity law - which govern the shape of the emitted line profile. For a given geometrical configuration, the profiles produced by this model can be characterized in terms of the relative width of the core and the curvature of the wings. It is proposed that variations in both of these properties are required to account for the diversity of the Ha and HP profiles in a sample of Seyfert galaxies. It is not possible to identify the underlying variables unambiguously, but in this interpretation the observed profile diversity implies variations in both the slope of the line emissivity distribution and either the radial depth or the velocity law. Variations in the former two properties imply in turn that the effective size of the BLR also varies among AGN, regardless of continuum luminosity. Several consequences which follow from this are briefly discussed.