2013A&A...555A..89M

Broad MgIIλ2800 and Hβ lines have emerged as the most reliable virial estimators of the black hole mass in quasars. Which is more reliable? Part of the challenge centers on comparing MgIIλ2800 and Hβ line profiles in order to improve the ±1dex M_BH uncertainties inherent in single-epoch FWHM measures from noisy spectra. We compare MgIIλ2800 and Hβ profile measures in the same sources and especially FWHM measures that provide the virial broadening estimator. We identify 680 bright Sloan Digital Sky Survey Data Release 7 quasars with spectra showing both MgIIλ2800 and Hβ 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 high S/N composite spectra. We confirm that MgIIλ2800 shows a profile that is ≃20% narrower as suggested in some previous studies. FWHM measures for Population B sources (i.e., with FWHM of Hβ larger than 4000km/s) are uncertain because they show complex profiles with at least two broad-line components involving a nearly unshifted broad and redshifted very-broad component. Only the broad component is likely to be a valid virial estimator. If Hβ and MgIIλ2800 are not corrected for the very broad component then black hole mass M<sub>BH</sub> values for Population B sources will be systematically overestimated by up to ΔlogM<sub>BH</sub>≃0.3-0.4dex. We suggest a simple correction that can be applied to the majority of sources. MgIIλ2800 is the safer virial estimator for Population B sources because the centroid shifts with respect to rest frame are lower than for Hβ. In the broad and very broad component profile interpretation, this is a consequence of the lower very broad to broad component intensity ratio for MgIIλ2800. Eigenvector-based studies show that effective discrimination of black hole mass and Eddington ratio at fixed redshift is not achieved via luminosity binning but rather by binning in a ``4D eigenvector 1'' context that reflects different broad line region geometry/kinematics probably driven by the Eddington ratio.