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

TL;DR

This study analyzes the optical spectra of 155 high-redshift quasars, extending scaling relations for black hole mass estimation to z~6, and confirms the presence of supermassive black holes in the early universe.

Contribution

It extends optical spectral scaling relations to high luminosity and redshift, enabling more accurate black hole mass estimates for distant quasars.

Findings

A single scaling relation applies across 0<z<6 and 10^42< L_{5100}<10^47 erg/s.

UV-line based black hole masses are consistent with Balmer-line estimates, with some scatter.

Supermassive black holes up to 10^10 M_sun are confirmed at z~5.

Abstract

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 H$\alpha$ emission line as far out as z~6. We extend the scaling relation between the rest-frame optical continuum and 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${\rm \AA}$ and H$\alpha$ AGN luminosities over a wide range of luminosity (10$^{42}$<$L_{5100}$<10$^{47}$ergs/s) 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 mass ($M_{\rm BH}$) estimator of local AGNs, we derive the $M_{\rm BH}$ estimators based on H$\alpha$, MgII, and CIV lines, finding that the UV-line based masses are overall consistent with the Balmer-line based, but with a large intrinsic scatter of 0.40dex for the CIV estimates. Our 43 $M_{\rm BH}$ estimates from H$\alpha$ confirm the existence of BHs as massive as ~10$^{10}M_{\odot}$ out to z~5, and provide a secure footing for previous MgII-line based studies that a rapid $M_{\rm BH}$ growth has occurred in the early universe.