  We present near-infrared spectra of ten luminous, intermediate redshift
quasars observed with SofI at the NTT of ESO/La Silla. With these rest-frame
optical spectra we probe the Hb -[OIII] emission line region. Using the
standard scaling relation involving the width of the Hb line and the continuum
luminosity, we measure black hole masses in the range of ~2x10^9 to 10^10 Msol.
We also used SDSS spectra to probe MgII2798 and CIV1549 emission lines and used
these for black hole mass measurements as well. The massive black holes we
observe could not have grown by simple radiatively efficient accretion at the
observed accretion rate starting from seeds of up to thousand solar masses.
About 10% of the observed black hole mass must have been accumulated by earlier
merger events and radiatively inefficient accretion. Radiatively efficient
accretion would further grow these BHs to masses of several 10^9 Msol in 2 to 3
e-folding times i.e. in several 10^8yrs. The Hb-based Eddington luminosity
ratios are in the range of ~0.2 to ~0.7, with an average of (Lbol/Ledd) =
0.39+-0.05. The Lbol/Ledd ratio distribution follows a log-normal distribution
which is consistent with prior studies of quasars with comparable luminosity.
The Lbol/Ledd ratio distribution of less luminous quasars tend to be shifted to
lower Eddington ratios. We also find that the gas metallicity of the broad-line
region is ~3Z/Zsol, using NIII]/OIII] and NV/CIV emission line ratios. We find
no correlation of the gas metallicity with the optical FeII emission line
strength.
