Eigenvector 1: An optimal correlation space for active galactic nuclei

We identify a correlation space involving optical and UV emission-line para meters as well as the soft X-ray spectral index that provides optimal discr imination between all principal classes of active galactic nuclei (AGNs). M ost of the sources in our three high-quality data samples show a strong int ercorrelation with narrow-line Seyfert 1 (NLSy1) galaxies and steep-spectru m radio galaxies occupying opposite extrema in the space. NLSy1 sources sho w a clear continuity with broader line sources, indicating that they are no t a disjoint class of AGN as is sometimes suggested. We interpret the princ ipal intercorrelation in the parameter space as being driven by the AGN lum inosity-to-black hole mass ratio (L/M is proportional to the Eddington rati o). Source orientation no doubt also plays an important role, but it is not yet clear whether FWHM H beta or C IV lambda 1549 line shift is the better indicator. We tentatively identify two radio-quiet populations: an almost pure radio-quiet population A, with FWHM less than or equal to 4000, and po pulation B, which occupies the same parameter domain as the flat-spectrum r adio-loud sources. A possible interpretation sees population A/NLSy1 as low er mass/high accretion rate sources and population B/radio-loud sources as the opposite.