Near-Infrared Spectroscopy of Quasars at z~3 and Estimates of Their
  Supermassive Black Hole Masses
  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 Adaptive
Optics system. The supermassive black hole (SMBH) masses (M_BH) were
successfully estimated in 28 out of 37 observed objects from the combination of
the H_beta emission linewidth and continuum luminosity at rest-frame 5100A.
Comparing these results with those from previous studies of quasars with
similar redshift, our sample exhibited slightly lower (~ -0.11 dex in median)
Eddington ratios; and, the SMBH masses are slightly (~ 0.38 dex in median)
higher. The SMBH growth time, t_grow, was calculated by dividing the estimated
SMBH mass by the mass accretion rate measured using optical luminosity. We
found, given reasonable assumptions, that t_grow 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 growing phase with high accretion rates. A comparison of the SMBH masses
derived from our H_beta data and archived CIV 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 component at high redshift.
