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[Abridged] Broad MgII 2800 and Hbeta lines have emerged as the most reliablevirial estimators of black hole mass in quasars. Which is more reliable?Comparison of MgII 2800 and Hbeta profile measures in the same sources andespecially FWHM measures that provide the virial broadening estimator.Identification of 680 bright Sloan Digital Sky Survey DataRelease 7 quasarswith spectra showing both MgII 2800 and Hbeta lines, at redshift 0.4 < z <0.75. The s/n of these spectra are high enough to allow binning in the"four-dimensional (4D) eigenvector 1'' optical plane and construction of highs/n composite spectra. We confirm that MgII 2800 shows a profile that is ~ 20%narrower as suggested in some previous studies. FWHM measures for Population Bsources (i.e., with FWHM of Hbeta larger than 4000 km/s) are uncertain becausethey show complex profiles with at least two broad-line components involving anearly unshifted broad and redshifted very-broad component. Only the broadcomponent is likely to be a valid virial estimator. If Hbeta and MgII 2800 arenot corrected for the very broad component then black hole mass values forPopulation B sources will be systematically overestimated by up to logM ~0.3-0.4 dex. We suggest a simple correction that can be applied to the majorityof sources. MgII 2800 is the safer virial estimator for Population B sourcesbecause the centroid shifts with respect to rest frame are lower than forHbeta. In the broad and very broad component profile interpretation this is aconsequence of the lower very broad to broad component intensity ratio for MgII2800. Effective discrimination of black hole mass and Eddington ratio at fixedredshift is not achieved via luminosity binning but rather by binning in a "4Deigenvector 1'' context that reflects different broad line regiongeometry/kinematics likely driven by Eddington ratio.

Is Mg II 2800 a Reliable Virial Broadening Estimator for Quasars?