2001ApJ...558..553M

We recently discussed an Eigenvector 1 (E1) parameter space that provides optimal discrimination between the principal classes of broad-line active galactic nuclei (AGNs). In this paper we begin a search for the most important physical parameters that are likely to govern correlations and data point distribution in E1 space. We focus on the principal optical parameter plane involving the width of the Hβ broad component [FWHM(Hβ_BC)] and the equivalent width ratio between the Fe II blend at 4570 Å and Hβ_BC. We show that the observed correlation for radio-quiet sources can be accounted for if it is primarily driven by the ratio of AGN luminosity to black hole mass (L/M∝Eddington ratio) convolved with source orientation. L/M apparently drives the radio-quiet correlation only for FWHM(Hβ)≲4000 km s^–1, which includes narrow-line Seyfert 1 galaxies and can be said to define an AGN ``main sequence.'' Source orientation plays an increasingly important role as FWHM(Hβ_BC) increases. We also argue that AGNs lying outside the radio-quiet main sequence, and specifically those with optical Fe II much stronger than expected for a given FWHM(Hβ_BC), may all be broad absorption line QSOs.