Systematically smaller single-epoch quasar black hole masses using a radius-luminosity relationship corrected for spectral bias

TL;DR

This study shows that correcting the radius-luminosity relationship for spectral bias leads to systematically smaller single-epoch quasar black hole mass estimates, especially for highly accreting quasars, improving accuracy in understanding quasar properties.

Contribution

The paper introduces a Fe ii to Hβ flux ratio correction to the radius-luminosity relationship, reducing overestimated black hole masses in single-epoch quasar measurements.

Findings

Fe-corrected relationship reduces mass overestimates by about a factor of two.

High accretion rate quasars are more affected by spectral bias corrections.

Results reinforce the link between Fe ii emission and accretion processes.

Abstract

Determining black hole masses and accretion rates with better accuracy and precision is crucial for understanding quasars as a population. These are fundamental physical properties that underpin models of active galactic nuclei. A primary technique to measure the black hole mass employs the reverberation mapping of low-redshift quasars, which is then extended via the radius-luminosity relationship for the broad-line region to estimate masses based on single-epoch spectra. An updated radius-luminosity relationship incorporates the flux ratio of optical Fe ii to H$\beta$ ($\equiv \mathcal{R}_{\rm Fe}$) to correct for a bias in which more highly accreting systems have smaller line-emitting regions than previously realized. In this current work, we demonstrate and quantify the effect of using this Fe-corrected radius-luminosity relationship on mass estimation by employing archival data sets possessing rest-frame optical spectra over a wide range of redshifts. We find that failure to use a Fe-corrected radius predictor results in overestimated single-epoch black hole masses for the most highly accreting quasars. Their accretion rate measures ($L_{\rm Bol}/ L_{\rm Edd}$ and $\dot{\mathscr{M}}$), are similarly underestimated. The strongest Fe-emitting quasars belong to two classes: high-z quasars with rest-frame optical spectra, which given their extremely high luminosities, require high accretion rates, and their low-z analogs, which given their low black holes masses, must have high accretion rates to meet survey flux limits. These classes have mass corrections downward of about a factor of two, on average. These results strengthen the association of the dominant Eigenvector 1 parameter $\mathcal{R}_{\rm Fe}$ with the accretion process.