Black Hole Masses of Intermediate-Redshift Quasars: Near Infrared Spectroscopy

Abstract

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.