Near-infrared spectroscopy of quasars at z ∼ 3 and estimates of their supermassive black hole masses

Abstract

Abstract We present the results of new infrared spectroscopic observations of 37 quasars at z ∼ 3, selected based on the optical r ′-band magnitude and the availability of nearby bright stars for future imaging follow-up with an adaptive optics system. The supermassive black hole (SMBH) masses (MBH) were successfully estimated in 28 out of 37 observed objects from the combination of the Hβ emission linewidth and continuum luminosity at rest-frame 5100 Å. Comparing these results with those from previous studies of quasars with similar redshift, our sample exhibited slightly lower Eddington ratios (∼−0.11 dex in median), and the SMBH masses are slightly higher (∼0.38 dex in median). The SMBH growth time, tgrow, was calculated by dividing the estimated SMBH mass by the mass accretion rate measured using optical luminosity. We found, given reasonable assumptions, that tgrow was smaller than the age of the universe at the redshift of individual quasars for a large fraction of observed sources, suggesting that the SMBHs in many of our observed quasars are in the growing phase with high accretion rates. A comparison of the SMBH masses derived from our Hβ data and archived C iv data indicated considerable scattering, as indicated in previous studies. All quasars with measured SMBH masses have at least one nearby bright star, such that they are suitable targets for adaptive optics observations to study the mass relationship between SMBHs and host galaxies’ stellar components at high redshift.