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The masses of the black holes powering quasars represent a fundamentalparameter of active galaxies. Estimates of quasar black hole masses usingsingle-epoch spectra are quite uncertain, and require quantitative improvement.We recently identified a correction for C IV $\lambda$1549-based scalingrelationships used to estimate quasar black hole masses that relies on thecontinuum-subtracted peak flux ratio of the ultraviolet emission-line blend SiIV + OIV] (the $\lambda$1400 feature) to that of C IV. This parametercorrelates with the suite of associated quasar spectral properties collectivelyknown as "Eigenvector 1" (EV1). Here we use a sample of 85 quasars withquasi-simultaneous optical-ultraviolet spectrophotometry to demonstrate howbiases in the average EV1 properties can create systematic biases in C IV-basedblack hole mass scaling relationships. This effect results in nearly an orderof 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 existingreverberation-mapped samples of quasars with ultraviolet spectra -- used tocalibrate C IV-based scaling relationships -- have significant EV1 biases thatresult in predictions of black hole masses nearly 50\% too high for the averagequasar. We offer corrections and suggestions to account for this bias.

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