2007ApJ...669...32K

We present sensitive near-infrared spectroscopic observations for a sample of five z∼6 quasars. These quasars are among the most distant, currently known quasars in the universe. The spectra have been obtained using ISAAC at the VLT and include the C IV, Mg II, and Fe II lines. We measure the Fe II/Mg II line ratio, as an observational proxy for the Fe/α-element ratio. We derive a ratio of 2.7±0.8 for our sample, which is similar to that found for lower redshift quasars; i.e., we provide additional evidence for the lack of evolution in the Fe II/Mg II line ratio of quasars up to the highest redshifts. This result demonstrates that star formation must have commenced at z≥8 in the quasar hosts. The line widths of the Mg II and C IV lines give two estimates for the black hole masses. A third estimate is given by assuming that the quasars emit at their Eddington luminosity. The derived masses using these three methods agree well, implying that the quasars are not likely to be strongly lensed. We derive central black hole masses of (0.3-5.2)x10⁹ M_☉. We use the difference between the redshift of Mg II (a proxy for the systemic redshift of the quasar) and the onset of the Gunn-Peterson trough to derive the extent of the ionized Strömgren spheres around our target quasars. The derived physical radii are about 5 Mpc. Using a simple ionization model, the emission of the central quasars would need of order 10⁶-10⁸ yr to create these cavities. As the e-folding timescale for the central accreting black hole is on the order of a few times 10⁷ yr, it can grow by one e-folding or less within this time span.