Barth, A.J., Martini, P., Nelson, C.H. and Ho, L.C. 2003, ApJ, 594, L95
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 1549 and Mg II 2800 emission lines. The iron emission blend at rest wavelength 29003000 Å 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/ 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 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 (26) × 109 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 × 109 M by Willott, McLure, & Jarvis and supports their conclusion that the quasar is radiating close to its Eddington luminosity.
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