Iron Emission in the z = 6.4 Quasar SDSS J114816.64+525150.3

We present near-infrared J and K-band spectra of the z = 6.4 quasar SDSSJ114816.64+525150.3 obtained with the NIRSPEC spectrograph at the Keck-IItelescope, covering the rest-frame spectral regions surrounding the C IV 1549and Mg II 2800 emission lines. The iron emission blend at rest wavelength2900-3000 A is clearly detected and its strength appears nearlyindistinguishable from that of typical quasars at lower redshifts. The Fe II /Mg II ratio is also similar to values found for lower-redshift quasars,demonstrating that there is no strong evolution in Fe/alpha broad-line emissionratios even out to z=6.4. In the context of current models for iron enrichmentfrom Type Ia supernovae, this implies that the SN Ia progenitor stars formed atz > 10. We apply the scaling relations of Vestergaard and of McLure & Jarvis toestimate the black hole mass from the widths of the C IV and Mg II emissionlines and the ultraviolet continuum luminosity. The derived mass is in therange (2-6)x10^9 solar masses, with an additional uncertainty of a factor of 3due to the intrinsic scatter in the scaling relations. This result is inagreement with the previous mass estimate of 3x10^9 solar masses by Willott,McLure, & Jarvis, and supports their conclusion that the quasar is radiatingclose to its Eddington luminosity.Comment: To appear in ApJ Letter