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

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Abstract
We present a search for O I in the spectra of nine 4.9 ≤ zQSO ≤ 6.4 QSOs taken with Keck HIRES. We detect six systems with N > 1013.7 cm-2 in the redshift intervals where O I λ1302 falls redward of the Lyα forest. Four of these lie toward SDSS J1148+5251 (zQSO = 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 (zQSO = 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 zQSO > 6.2 sight lines, including only the detected systems, are Ω = (7.0 ± 0.6) × 10-8, Ω = (9.6 ± 0.9) × 10-9, and Ω = (1.5 ± 0.2) × 10-8.
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