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 SDSS J114816.64 + 525150.3 obtained with the NIRSPEC spectrograph at the Keck II telescope, covering the rest-frame spectral regions surrounding the C IV lambda1549 and Mg II lambda2800 emission lines. The iron emission blend at rest wavelength 2900 - 3000 Angstrom is clearly detected, and its strength appears nearly indistinguishable 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 emission ratios even out to z = 6.4. In the context of current models for iron enrichment from Type Ia supernovae ( SNe Ia), this implies that the SN Ia progenitor stars formed at z greater than or similar to 10. We apply the scaling relations of Vestergaard and of McLure & Jarvis to estimate the black hole mass from the widths of the C IV and Mg II emission lines and the ultraviolet continuum luminosity. The derived mass is in the range (2-6) x 10(9) M-., with an additional uncertainty of a factor of 3 due to the intrinsic scatter in the scaling relations. This result is in agreement with the previous mass estimate of 3 x 10(9) M-. by Willott, McLure, & Jarvis and supports their conclusion that the quasar is radiating close to its Eddington luminosity.