Orientation and quasar black hole mass estimation

TL;DR

This study investigates how the orientation of radio-loud quasars affects black hole mass estimates, revealing biases in virial methods and proposing a new orientation indicator based on mass ratios.

Contribution

It demonstrates that H-beta based mass estimates are orientation-biased, while [O III] 5007 based estimates are not, and introduces a potential new orientation estimator using mass ratios.

Findings

H-beta FWHM correlates with radio core dominance

[O III] 5007 FWHM is unaffected by orientation

Mass ratio serves as an orientation indicator

Abstract

We have constructed a sample of 386 radio-loud quasars with z < 0.75 from the Sloan Digital Sky Survey in order to investigate orientation effects on black hole mass estimates. Orientation is estimated using radio core dominance measurements based on FIRST survey maps. Black hole masses are estimated from virial-based scaling relationships using H-beta, and compared to the stellar velocity dispersion (sigma_*), predicted using the Full Width at Half Maximum (FWHM) of [O III] 5007, which tracks mass via the M-sigma_* relation. We find that the FWHM of Hbeta correlates significantly with radio core dominance and biases black hole mass determinations that use it, but that this is not the case for sigma_* based on [O III] 5007. The ratio of black hole masses predicted using orientation-biased and unbiased estimates, which can be determined for radio-quiet as well as radio-loud quasars, is significantly correlated with radio core dominance. Although there is significant scatter, this mass ratio calculated in this way may in fact serve as an orientation estimator. We additionally note the existence of a small population radio core-dominated quasars with extremely broad H-beta emission lines that we hypothesise may represent recent black hole mergers.