2019A&A...627A..88M

Context. The inter-line comparison between high- and low-ionization emission lines has yielded a wealth of information on the structure and dynamics of the quasar broad line region (BLR), including perhaps the earliest unambiguous evidence in favor of a disk + wind structure in radio-quiet quasars. Aims. We carried out an analysis of the CIVλ1549 and Hβ line profiles of 28 Hamburg-ESO high-luminosity quasars and of 48 low-z, low-luminosity sources in order to test whether the width of the high-ionization line CIVλ1549 could be correlated with Hβ and be used as a virial broadening estimator. Methods. We analyze intermediate- to high-S/N, moderate-resolution optical and near-infrared (NIR) spectra covering the redshifted CIVλ1549 and Hβ over a broad range of luminosity log L∼44-48.5[erg/s] and redshift (0-3), following an approach based on the quasar main sequence. Results. The present analysis indicates that the line width of CIVλ1549 is not immediately offering a virial broadening estimator equivalent to Hβ. At the same time a virialized part of the BLR appears to be preserved even at the highest luminosities. We suggest a correction to FWHM(CIVλ1549) for Eddington ratio (using the CIVλ1549 blueshift as a proxy) and luminosity effects that can be applied over more than four dex in luminosity. Conclusions. Great care should be used in estimating high-L black hole masses M_BH from CIVλ1549 line width. However, once a corrected FWHM CIVλ1549 is used, a CIVλ1549-based scaling law can yield unbiased M_BH values with respect to the ones based on Hβ with sample standard deviation ~=0.3dex.