Discovery of Excess O I Absorption toward the z=6.42 QSO SDSS J1148+5251

We present a search for O I in the spectra of nine 4.9 , z sub(QSO) , 6.4 QSOs taken with Keck HIRES. We detect six systems with N super(OI) > 10 super(13.7) cm super(-2) in the redshift intervals where OI l1302 falls redward of the Lya forest. Four of these lie toward SDSS J1148+5251 (z sub(QSO) = 6.42). This imbalance is unlikely to arise from variations in sensitivity among our data or from a statistical fluctuation. The excess O I occurs over a redshift interval that also contains transmission in Lya and Lyb. Therefore, if these O I systems represent pockets of neutral gas, then they must occur within or near regions of the IGM that are highly ionized. In contrast, no O I is detected toward SDSS J1030+0524 (z sub(QSD) = 6.30), whose spectrum shows complete absorption in Lya and Lyb over z 6 0.2. Assuming no ionization corrections, we measure mean abundance ratios = -0.04 c 0.06, = -0.31 c 0.09, and = -0.34 c 0.07 (2 s), which are consistent with enrichment dominated by Type II supernovae. The O/Si ratio limits the fraction of silicon in these systems contributed by metal-free very massive stars to 30%, a result that is insensitive to ionization corrections. The ionic comoving mass densities along the z sub(QSO) > 6.2 sight lines, including only the detected systems, are super(O I) = (7.0 c 0.6) x 10 super(-8), super(Si II) = (9.6 c 0.9) x 10 super(-9), and super(C II) = (1.5 c 0.2) x 10 super(-8).