Rest-frame Optical Spectra and Black Hole Masses of 3 <z<6 Quasars

We present the rest-frame optical spectral properties of 155 luminous quasars at 3.3 < z < 6.4 taken with the AKARI space telescope, including the first detection of the Ha emission line as far out as z similar to 6. We extend the scaling relation between the rest-frame optical continuum and the line luminosity of active galactic nuclei (AGNs) to the high-luminosity, high-redshift regime that has rarely been probed before. Remarkably, we find that a single log-linear relation can be applied to the 5100 angstrom and H alpha AGN luminosities over a wide range of luminosity (10(42) < L-5100 < 10(47) ergs s(-1)) or redshift (0 < z < 6), suggesting that the physical mechanism governing this relation is unchanged from z = 0 to. 6, over five decades in luminosity. Similar scaling relations are found between the optical and the UV continuum luminosities or line widths. Applying the scaling relations to the H beta black hole (BH) mass (MBH) estimator of local AGNs, we derive the MBH estimators based on the H alpha, Mg (II), and C (IV) lines, finding that the UV-line-based masses are overall consistent with the Balmer-line-based, but with a large intrinsic scatter of 0.40 dex for the C (IV) estimates. Our 43 MBH estimates from Ha confirm the existence of BHs as massive as similar to 10(10) M-circle dot out to z similar to 5. and provide a secure footing for previous results from Mg (II)-line-based studies that a rapid MBH growth has occurred in the early universe.