Black hole mass estimates in quasars

A comparative analysis of high- and low-ionization lines

¹ INAF, Osservatorio Astronomico di Padova, vicolo dell’ Osservatorio 5, 35122 Padova, Italy
e-mail: paola.marziani@oapd.inaf.it
² Instituto de Astrofisíca de Andalucía, IAA-CSIC, Glorieta de la Astronomia s/n, 18008 Granada, Spain
e-mail: chony@iaa.es
³ Departamento de Matemática Aplicada and IUMA, Universidad de Zaragoza, 50009 Zaragoza, Spain
e-mail: gelimc@unizar.es
⁴ Center for Theoretical Physics, Polish Academy of Science, 02-668 Warsaw, Poland
⁵ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, 40129 Bologna, Italy
⁶ Instituto de Astronomía, UNAM, Mexico D.F. 04510, Mexico
⁷ Dipartimento di Fisica & Astronomia “Galileo Galilei”, Università di Padova, Vicolo ell’Osservatorio 3, 35122 Padova, Italy
⁸ Astronomical Observatory, Belgrade, Serbia

Received: 11 February 2019
Accepted: 1 May 2019

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 IVλ1549 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 C IVλ1549 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 C IVλ1549 and Hβ 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 IVλ1549 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(C IVλ1549) for Eddington ratio (using the C IVλ1549 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 IVλ1549 line width. However, once a corrected FWHM C IVλ1549 is used, a C IVλ1549-based scaling law can yield unbiased M_BH values with respect to the ones based on Hβ with sample standard deviation ≈0.3 dex.