CIV emission line properties and uncertainties in black hole mass estimates of z ~ 3.5 quasars

Abstract

Using a high luminosity ($L_{\mathrm{bol}}\sim 10^{47.5}-10^{48.3}$ 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 CIV-based black hole mass estimates ($M_{\mathrm{BH}}$). The median logarithm of the CIV- and H$\beta$-based $M_{\mathrm{BH}}$ ratios is 0.110 dex with the scatter of 0.647 dex. The CIV-to-H$\beta$ BH mass differences are significantly correlated with the CIV FWHMs, blueshifts and asymmetries. Corrections of the CIV FWHM using the blueshift and asymmetry reduce the scatter of the mass differences by $\sim$ 0.04-0.2 dex. Quasars in our sample accrete at the Eddington ratio $R_{\mathrm{EDD}}>0.3$ and cover a considerable range of blueshifts, with 18/19 of the quasars showing CIV blueshifts (with the median value of 1126 km s${}^{-1}$) and 14/19 of the quasars showing CIV 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 CIV rest-frame equivalent width (REW) and the continuum luminosity at 1350 $\overset{\lower.5em\circ}{\mathrm{A}}$ 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 CIV blueshift, and detect a higher ratio of [OIII] quasars with REW${}_{[\mathrm{OIII}]}>5$ $\overset{\lower.5em\circ}{\mathrm{A}}$ in the subsample with lower CIV blueshift. It is more likely that they are caused by the combination of the Eddington ratio and the orientation effect.

17 pages, 12 figures with additional 6 pages and 2 figures in the appendix. Accepted for publication in ApJ

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