2002ApJ...565...78B

Recent studies of black holes in the nuclei of both active and normal galaxies have yielded relationships that permit a physical interpretation of the principal components of the spectra of QSOs. It is shown that principal component (or eigenvector) 1 (PC1) is driven predominantly by L/L_Edd, and principal component 2 (PC2) is driven by accretion rate. This results in a PC2 versus PC1 diagram in which lines of constant black hole mass are diagonal. Using a sample consisting of the low-redshift PG objects supplemented by 75 radio-loud QSOs, it is shown that such a diagram effectively distinguishes radio-loud from radio-quiet objects and demonstrates that both narrow-line Seyfert 1 galaxies and broad absorption line QSOs lie at the high L/L_Edd extreme, although these two types of objects are well separated in the PC2 direction. A simple picture that ties together physical parameters (black hole mass and Eddington ratio) and classification of active galactic nuclei is presented. Based on the location of core-dominated and lobe-dominated radio-loud QSOs, orientation can be modeled as a third parameter in this scheme, implying an enhancement in the radio flux of core-dominated objects.