C IV Emission-line Properties and Uncertainties in Black Hole Mass Estimates of z ∼ 3.5 Quasars

Using a high-luminosity (L_bol ∼ 10^47.5-10^48.3 erg s^-1), high-redshift (3.2 < z < 3.8) quasar sample of 19 quasars with optical and near-infrared spectroscopy, we investigate the reliability of the C IV-based black hole mass estimates (M_BH). The median logarithm of the C IV- and Hβ-based M_BH ratios is 0.110 dex, with a scatter of 0.647 dex. The C IV-to-Hβ BH mass differences are significantly correlated with the C IV FWHMs, blueshifts, and asymmetries. Corrections of the C IV FWHM using the blueshift and asymmetry reduce the scatter of the mass differences by ∼0.04-0.2 dex. Quasars in our sample accrete at the Eddington ratio R_EDD > 0.3 and cover a considerable range of blueshifts, with 18/19 of the quasars showing C IV blueshifts (with the median value of 1126 km s^-1) and 14/19 of the quasars showing C IV blueshifts larger than 500 km s^-1. It suggests that not all quasars with high Eddington ratios show large blueshifts. The Baldwin effect between the C IV rest-frame equivalent width (REW) and the continuum luminosity at 1350 Å is not seen, likely due to the limited luminosity range of our sample. We find a lack of flux in the red wing of the composite spectrum with larger C IV blueshift and detect a higher ratio of [O III] quasars with REW_{[O III]} > 5 Å in the subsample with lower C IV blueshift. It is more likely that they are caused by the combination of the Eddington ratio and the orientation effect.