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.