Is MgIIλ2800 a reliable virial broadening estimator for quasars

Context. Broad Mgii�2800 and Hlines have emerged as the most reliable virial estimators of black hole mass in quasars. Which is more reliable? Part of the challenge centers on comparing Mgii�2800 and Hline profiles in order to improve the ± 1 dex MBH uncertainties inherent in single-epoch FWHM measures from noisy spectra. Aims. Comparison of Mgii�2800 and Hprofile measures in the same sources and especially FWHM measures that provide the virial broadening estimator. Methods. Identification of 680 bright Sloan Digital Sky Survey DataRelease 7 quasars with spectra showing both Mgii�2800 and Hlines, 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. Results. We confirm that Mgii�2800 shows a profile that is � 20% narrower as sug- gested in some previous studies. FWHM measures for Population B sources (i.e., with FWHM of Hlarger than 4000 km s 1 ) are uncertain because they show complex pro- files 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 Hand Mgii�2800 are not corrected for the very broad compo- nent then black hole mass MBH values for Population B sources will be systematically overestimated by up tologMBH � 0.3 0.4 dex. We suggest a simple correction that can be applied to the majority of sources. Mgii�2800 is the safer virial estimator for Population B sources because the centroid shifts with respect to rest frame are lower than for H�. In the broad and very broad component profile interpretation this is a consequence of the lower very broad to broad component intensity ratio for Mgii�2800. 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 likely driven by Eddington ratio.