IRON EMISSION IN THE z = 6.4 QUASAR SDSS J114816.64+525150.3

摘要

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 λ1549 and Mg Ⅱ λ2800 emission lines. The iron emission blend at rest wavelength 2900-3000 A 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/α 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 approx> 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 solar mass, 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 solar mass by Willott, McLure, & Jarvis and supports their conclusion that the quasar is radiating close to its Eddington luminosity.