Black hole mass estimates in quasars

Abstract

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 C IV λ 1549 and H β 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 C IV λ 1549 could be correlated with H β 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 C IV λ 1549 and H β over a broad range of luminosity log L ∼ 44 − 48.5 [erg s −1 ] and redshift (0 − 3), following an approach based on the quasar main sequence. Results . The present analysis indicates that the line width of C IV λ 1549 is not immediately offering a virial broadening estimator equivalent to H β . 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(C IV λ 1549) for Eddington ratio (using the C IV λ 1549 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 C IV λ 1549 line width. However, once a corrected FWHM C IV λ 1549 is used, a C IV λ 1549-based scaling law can yield unbiased M BH values with respect to the ones based on H β with sample standard deviation ≈0.3 dex.