Estimating Black Hole Masses in Quasars Using Broad Optical and UV Emission Lines

TL;DR

This paper reviews methods for estimating black hole masses in quasars using broad optical and UV emission lines, discussing their accuracy, limitations, and applicability across different redshifts.

Contribution

It provides a comprehensive review of existing virial mass estimation techniques and introduces new insights into their reliability and potential improvements.

Findings

FWHM H-beta is the most widely used virial estimator at low redshift.

FWHM MgII 2800 can be used as a surrogate for H-beta up to z ~ 6.5.

CIV 1549 is affected by non-virial broadening and is less reliable.

Abstract

We review past work using broad emission lines as virial estimators of black hole masses in quasars. Basically one requires estimates of the emitting region radius and virial velocity dispersion to obtain black hole masses. The three major ways to estimate the broad-line emitting region (BLR) radius involve: (1) direct reverberation mapping, (2) derivation of BLR radius for larger samples using the radius-luminosity correlation derived from reverberation measures, and (3) estimates of BLR radius using the definition of the ionization parameter solved for BLR radius (photoionization method). At low redshift (z < 0.7) FWHM H-beta serves as the most widely used estimator of virial velocity dispersion. FWHM H-beta can provide estimates for tens of thousands of quasars out to z ~ 3.8 (IR spectroscopy beyond z ~ 1). A new photoionization method also shows promise for providing many reasonable estimates of BLR radius via high S/N IR spectroscopy of the UV region 1300 -- 2000 A. FWHM MgII 2800 can serve as a surrogate for FWHM H-beta in the range 0.4 < z < 6.5 while CIV 1549 is affected by broadening due to non-virial motions and best avoided (i.e. there is no clear conversion factor between FWHM H-beta and FWHM CIV 1549). Most quasars yield black hole mass estimates in the range 7 < log M< 9.7. There is no strong evidence for values above 10.0 and there may be evidence for a turnover in the maximum black hole mass near z ~ 5.