Searching for the physical drivers of the Eigenvector 1 correlation space

We recently discussed an Eigenvector 1 (E1) parameter space that provides o ptimal discrimination between the principal classes of broad-line active ga lactic nuclei (AGNs). In this paper we begin a search for the most importan t physical parameters that are likely to govern correlations and data point distribution in E1 space. We focus on the principal optical parameter plan e involving the width of the H beta broad component [FWHM(H beta (BC))] and the equivalent width ratio between the Fe II blend at 4570 Angstrom and H beta (BC) We show that the observed correlation for radio-quiet sources can be accounted for if it is Hb BC. primarily driven by the ratio of AGN lumi nosity to black hole mass (L/M proportional to Eddington ratio) convolved w ith source orientation. L/M apparently drives the radio-quiet correlation o nly for FWHM(H beta) less than or similar to 4000 km s(-1), which includes narrow-line Seyfert 1 galaxies and can be said to define an AGN "main seque nce." Source orientation plays an increasingly important role as FWHM(H bet a (BC)) increases. We also argue that AGNs lying outside the radio-quiet ma in sequence, and specifically those with optical Fe II much stronger than e xpected for a given (H beta (BC)), may all be broad absorption line QSOs.