Biases in the Quasar Mass-Luminosity Plane

TL;DR

The paper demonstrates that the previously reported sub-Eddington boundary in the quasar mass-luminosity plane is an artifact caused by biases in black hole mass measurement methods, and that using line dispersion reduces these biases.

Contribution

It shows that recalibrating mass estimates and using line dispersion instead of FWHM eliminates the apparent boundary, clarifying the true quasar mass-luminosity relationship.

Findings

Biases in FWHM-based mass estimates cause the perceived boundary.

Recalibration reduces the significance of the boundary.

Line dispersion estimates produce less biased black hole masses.

Abstract

We find that the recently reported departure from the Eddington luminosity limit for the highest quasar black hole masses at a given redshift is an artifact due to biases in black hole mass measurements. This sub-Eddington boundary (with non-unity slope) in the quasar mass-luminosity plane was initially reported by Steinhardt & Elvis (2010a) using the FWHM-based black hole mass catalogue of Shen et al. (2008). However, the significance of the boundary is reduced when the FWHM-based mass-scaling relationship is recalibrated following Wang et al. (2009) and using the most updated reverberation mapping estimates of black hole masses. Furthermore, this boundary is not seen using mass estimates based on the line dispersion of the same quasars' MgII emission lines. Thus, the initial report of a sub-Eddington boundary with non-unity slope was due to biases in estimating masses using the FWHM of a fit of one or two Gaussians to quasar MgII emission lines. We provide evidence that using the line dispersion of the MgII line produces less biased black hole mass estimates.