Techniques for profile binning and analysis of eigenvector composite spectra: Comparing Hββ and Mgiiλλ2800 as virial estimators

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 Hββ, are the lines of choice. For redshifts greater than 0.7 – 0.8 one can follow Hββ into the IR windows or find an Hββ surrogate. We explain why UV Civλλ1549 is not a safe virial estimator and how Mgiiλλ2800 serves as the best virial surrogate for Hββ up to the highest redshift quasar known at z≈7z≈7. We show how spectral binning in a parameter space context (4DE1) makes possible a more effective comparison of Hββ and Mgii. It also helps to derive more accurate mass estimates from appropriately binned spectra and, finally, to map the dispersion in MBHMBH and Eddington ratio across the quasar population. FWHM MgII is about 20% smaller than FWHM Hββ in the majority of type 1 AGN requiring correction when comparing MBHMBH estimates from these two lines. The 20% of sources showing narrowest FWHM Hββ (<4000<4000 km s−1) and strongest FeII (RFe≳1.0RFe≳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 Hββ is the safer virial estimator for these extreme Eddington emitters.