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

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 CIV λ1549-based scaling relationships used to estimate quasar black hole masses that relies on the continuumsubtracted peak flux ratio of the ultraviolet emission-line blend Si IV + OIV] (the λ1400 feature) to that of CIV. 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 CIV-based black hole mass scaling relationships. This effect results in nearly an order of magnitude moving from objects with small 〈peak λ1400/C IV〉, which have overestimated black hole masses, to objects with large 〈peak λ1400/C IV〉, which have underestimated values. We show that existing reverberationmapped samples of quasars with ultraviolet spectra - used to calibrate CIV-based scaling relationships - have significant EV1 biases that result in predictions of black hole masses nearly 50 per cent too high for the average quasar. We offer corrections and suggestions to account for this bias.