Gemini Near Infrared Spectrograph - Distant Quasar Survey: Prescriptions for Calibrating UV-Based Estimates of Supermassive Black Hole Masses in High-Redshift Quasars

Abstract

The most reliable single-epoch supermassive black hole mass ($M_{\mathrm{BH}}$) estimates in quasars are obtained by using the velocity widths of low-ionization emission lines, typically the H${\hat{I}}^2$ $\lambda 4861$ line. Unfortunately, this line is redshifted out of the optical band at $z\approx 1$, leaving $M_{\mathrm{BH}}$ estimates to rely on proxy rest-frame ultraviolet (UV) emission lines, such as C IV $\lambda 1549$ or Mg II $\lambda 2800$, which contain intrinsic challenges when measuring, resulting in uncertain $M_{\mathrm{BH}}$ estimates. In this work, we aim at correcting $M_{\mathrm{BH}}$ estimates derived from the C IV and Mg II emission lines based on estimates derived from the H${\hat{I}}^2$ emission line. We find that employing the equivalent width of C IV in deriving $M_{\mathrm{BH}}$ estimates based on Mg II and C IV provides values that are closest to those obtained from H${\hat{I}}^2$. We also provide prescriptions to estimate $M_{\mathrm{BH}}$ values when only C IV, only Mg II, and both C IV and Mg II are measurable. We find that utilizing both emission lines, where available, reduces the scatter of UV-based $M_{\mathrm{BH}}$ estimates by $\sim 15\%$ when compared to previous studies. Lastly, we discuss the potential of our prescriptions to provide more accurate and precise estimates of $M_{\mathrm{BH}}$ given a much larger sample of quasars at $3.20\lesssim z\lesssim 3.50$, where both Mg II and H${\hat{I}}^2$ can be measured in the same near-infrared spectrum.

19 pages (AASTeX 6.3.1), 9 figures, accepted for publication in ApJ