2000ApJ...536L...5S

We identify a correlation space involving optical and UV emission-line parameters as well as the soft X-ray spectral index that provides optimal discrimination between all principal classes of active galactic nuclei (AGNs). Most of the sources in our three high-quality data samples show a strong intercorrelation with narrow-line Seyfert 1 (NLSy1) galaxies and steep-spectrum radio galaxies occupying opposite extrema in the space. NLSy1 sources show a clear continuity with broader line sources, indicating that they are not a disjoint class of AGN as is sometimes suggested. We interpret the principal intercorrelation in the parameter space as being driven by the AGN luminosity-to-black hole mass ratio (L/M is proportional to the Eddington ratio). Source orientation no doubt also plays an important role, but it is not yet clear whether FWHM Hβ or C IV λ1549 line shift is the better indicator. We tentatively identify two radio-quiet populations: an almost pure radio-quiet population A, with FWHM≤4000, and population B, which occupies the same parameter domain as the flat-spectrum radio-loud sources. A possible interpretation sees population A/NLSy1 as lower mass/high accretion rate sources and population B/radio-loud sources as the opposite.