Techniques for profile binning and analysis of eigenvector composite spectra: Comparing Hβ and Mgiiλ2800 as virial estimators☆

Abstract We review the basic techniques for extracting information about quasar structure and kinematics from the broad emission lines in quasars. We consider which lines can most effectively serve as virial estimators of black hole mass. At low redshift the Balmer lines, particularly broad H β , are the lines of choice. For redshifts greater than 0.7 – 0.8 one can follow H β into the IR windows or find an H β surrogate. We explain why UV C iv λ 1549 is not a safe virial estimator and how Mg ii λ 2800 serves as the best virial surrogate for H β up to the highest redshift quasar known at z ≈ 7 . We show how spectral binning in a parameter space context (4DE1) makes possible a more effective comparison of H β and Mg ii . It also helps to derive more accurate mass estimates from appropriately binned spectra and, finally, to map the dispersion in M BH and Eddington ratio across the quasar population. FWHM MgII is about 20% smaller than FWHM H β in the majority of type 1 AGN requiring correction when comparing M BH estimates from these two lines. The 20% of sources showing narrowest FWHM H β ( 4000  km s −1 ) and strongest FeII ( R Fe ≳ 1.0 ) emission (we call them bin A3-4 sources) do not show this FWHM difference and a blueshift detected in MgII for these sources suggests that FWHM H β is the safer virial estimator for these extreme Eddington emitters.