2006ApJ...640...69B

We present a search for O I in the spectra of nine 4.9≤z_QSO≤6.4 QSOs taken with Keck HIRES. We detect six systems with N_OI_>10^13.7/cm2 in the redshift intervals where O I λ1302 falls redward of the Lyα forest. Four of these lie toward SDSS J1148+5251 (z_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 Lyα and Lyβ. 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_QSO=6.30), whose spectrum shows complete absorption in Lyα and Lyβ over Δz∼0.2. Assuming no ionization corrections, we measure mean abundance ratios <[O/Si]≥-0.04±0.06, <[C/O]≥-0.31±0.09, and <[C/Si]≥-0.34±0.07 (2 σ), 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_QSO>6.2 sight lines, including only the detected systems, are Ω_OI_=(7.0±0.6)x10^–8, Ω_SiII_=(9.6±0.9)x10^–9, and Ω_CII_=(1.5±0.2)x10^–8.