NEAR-INFRARED SPECTROSCOPY OF SDSS J0303 – 0019: A LOW-LUMINOSITY, HIGH-EDDINGTON-RATIO QUASAR ATz∼ 6

We present sensitive near-infrared (NIR) VLT ISAAC spectroscopic observations of the z = 6.08 quasar SDSS J030331.40-001912.9. This quasi-stellar object (QSO) is more than a magnitude fainter than other QSOs at z ~ 6 for which NIR spectroscopy has been obtained to date and is therefore presumably more representative of the QSO population at the end of cosmic reionization. Combining rest-frame UV continuum luminosity with the width measurements of the Mg II and C IV lines, we derive a black hole mass of 2+1.0 –0.5 × 108 M ☉, the lowest mass observed for z ~ 6 QSOs to date, and derive an Eddington ratio of 1.6+0.4 –0.6, among the highest value derived for QSOs at any redshift. The Spitzer 24 μm nondetection of this QSO does not leave space for a significant hot dust component in its optical/NIR spectral energy distribution, in common with one other faint QSO at z = 6, but in contrast to more than 20 more z = 6 QSOs and all known lower redshift QSOs with sufficiently deep multiwavelength photometry. We conclude that we have found evidence for differences in the intrinsic properties of at least one z ~ 6 QSO as compared to the lower redshift population.