ar X iv : a st ro - p h / 05 11 54 1 v 1 1 7 N ov 2 00 5 Discovery of excess O i absorption towards the z = 6 . 42 QSO

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 NO I > 10 13.7 cm in the redshift intervals where O i λ1302 falls redward of the Lyα forest. Four of these lie towards 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 towards SDSS J1030+0524 (zQSO = 6.30), whose spectrum shows complete absorption in Lyα and Lyβ over ∆z ∼ 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 (2σ), 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 which is insensitive to ionization corrections. The ionic comoving mass densities along the zQSO > 6.2 sightlines, including only the detected systems, are ΩO I = (7.0 ± 0.6) × 10, ΩSi II = (9.6 ± 0.9) × 10, and ΩC II = (1.5± 0.2)× 10 . Subject headings: cosmology: observations — cosmology: early universe — intergalactic medium— quasars: absorption lines — quasars: individual (SDSS J22250014, SDSS J1204-0021 SDSS J0915+4244, SDSS J0231-0728, SDSS J0836+0054, SDSS J0002+2550, SDSS J1623+3112, SDSS J1030+0524, SDSS J1148+5251) The observations were made at the W.M. Keck Observatory which is operated as a scientific partnership between the California Institute of Technology and the University of California; it was made possible by the generous support of the W.M. Keck Foundation. Palomar Observatory, California Institute of Technology, Pasadena, CA 91125, USA; [email protected], [email protected] Carnegie Observatories, 813 Santa Barbara Street, Pasadena, CA 91101, USA; [email protected] MIT Center for Space Research, 77 Massachusetts Ave. #37-664B, Cambridge, MA 02139, USA; [email protected]