Recent 2000 ;Ferrarese & Merritt 2000). Although the

Recent studies have derived black hole masses for normaland active galactic nuclei (AGNs) and have attempted torelate them to observable properties. Perhaps the moststriking of these relations is the discovery that the nuclearblack hole mass in normal galaxies is tightly correlated withpn, where p is the velocity dispersion of the bulge of the hostgalaxy and n is in the range 3.75È4.8 (Gebhardt et al. 2000 ;Ferrarese & Merritt 2000). Although the distances to eventhe nearest QSOs preclude the spatially resolved spectros-copy that is the basis for determination of black hole massesfor normal galaxies, other techniques (e.g., reverberationmapping), as well as the presumed extension of the relationbetween black hole mass and host galaxy bulges, permit thecomparison of physical parameters with observable proper-ties for AGNs.Boroson and Green (1992, hereafter BG92) showed thatmost of the variance in the measured optical emission-lineproperties and a broad range of continuum properties(radio through X-ray) in a complete sample of low-redshiftQSOs was contained in two sets of correlations, eigen-vectors of the correlation matrix. Principal component 1(PC1) links the strength of Fe II emission, O III emission,and Hb line asymmetry. Principal component 2 (PC2) pro-jects most strongly on optical luminosity and the strengthof He II j4686 emission. Subsequent studies (see Sulentic etal. 2000 and references therein) have added observedproperties to the list and have, in general, conÐrmed thereality of the correlations.BG92 and others since have tried to understand therelationship between the principal components and thephysical parameters that govern the energy-producing andradiation-emitting processes. As the summary presentationin a conference titled “” Structure and Kinematics of QuasarBroad Line Regions,ÏÏ Gaskell (1999) polled the conferenceattendees on the question, “” What drives Boroson-GreenEigenvector 1 ? ÏÏ The overwhelming consensus was “” donÏtknow,ÏÏ which received 68% of the votes.

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