Rehabilitating C IV-based black hole mass estimates in quasars

Abstract

Currently, the ability to produce black hole mass estimates using the C IV λ1549 line that are consistent with Hβ mass estimates is uncertain, due in large part to disagreement between velocity line width measurements for the two lines. This discrepancy between Hβ and C IV arises from the fact that both line profiles are treated the same way in single-epoch scaling relationships based on the assumption that the broad-line region is virialized, even though a non-virialized emission component is often significant in the C IV line and absent or weak in the Hβ line. Using quasi-simultaneous optical and ultraviolet spectra for a sample of 85 optically-bright quasars with redshifts in the range z = 0.03-1.4, we present a significant step along the path to rehabilitating the C IV line for black hole mass estimates. We show that the residuals of velocity line width between C IV and Hβ are significantly correlated with the peak flux ratio of Si IV+O IV] λ1400 to C IV. Using this relationship, we develop a prescription for estimating black hole masses from the ultraviolet spectrum that agree better with Hβ-based masses than the traditional C IV masses. The scatter between Hβ and C IV masses is initially 0.43 dex in our sample and is reduced to 0.33 dex when using our prescription. The peak flux ratio of Si IV+O IV] λ1400 to C IV is an ultraviolet indicator of the suite of spectral properties commonly known as `Eigenvector 1'; thus, the reduction in scatter between C IV and Hβ black hole masses is essentially due to removing an Eigenvector 1 bias in C IV-based masses.