Bias in C IV-based quasar black hole mass scaling relationships from reverberation mapped samples

TL;DR

This paper investigates how biases in quasar spectral properties, specifically EV1, affect the accuracy of black hole mass estimates derived from C IV emission lines, revealing systematic over- or underestimations.

Contribution

It identifies EV1-related biases in existing reverberation-mapped quasar samples that impact C IV-based mass scaling relationships and proposes corrections to improve accuracy.

Findings

Bias in EV1 properties causes systematic errors in mass estimates.

Reverberation-mapped samples have significant EV1 biases affecting calibration.

Correcting for EV1 biases reduces mass estimate errors by nearly 50%.

Abstract

The masses of the black holes powering quasars represent a fundamental parameter of active galaxies. Estimates of quasar black hole masses using single-epoch spectra are quite uncertain, and require quantitative improvement. We recently identified a correction for C IV $\lambda$1549-based scaling relationships used to estimate quasar black hole masses that relies on the continuum-subtracted peak flux ratio of the ultraviolet emission-line blend Si IV + OIV] (the $\lambda$1400 feature) to that of C IV. This parameter correlates with the suite of associated quasar spectral properties collectively known as "Eigenvector 1" (EV1). Here we use a sample of 85 quasars with quasi-simultaneous optical-ultraviolet spectrophotometry to demonstrate how biases in the average EV1 properties can create systematic biases in C IV-based black hole mass scaling relationships. This effect results in nearly an order of magnitude moving from objects with small $<$ peak $\lambda$1400/C IV $>$, which have overestimated black hole masses, to objects with large $<$ peak $\lambda1400$/C IV $>$, which have underestimated values. We show that existing reverberation-mapped samples of quasars with ultraviolet spectra -- used to calibrate C IV-based scaling relationships -- have significant EV1 biases that result in predictions of black hole masses nearly 50\% too high for the average quasar. We offer corrections and suggestions to account for this bias.