Probing the Ionization State of the Universe atz>6

We present high signal-to-noise ratio Keck ESI spectra of the two quasarsknown to have Gunn-Peterson absorption troughs, SDSS J1030+0524 (z=6.28) andSDSS J1148+5251 (z=6.37). The Ly alpha and Ly beta troughs for SDSS J1030+0524are very black and show no evidence for any emission over a redshift intervalof ~0.2 starting at z=6. On the other hand, SDSS J1148+5251 shows a number ofemission peaks in the Ly beta Gunn-Peterson trough along with a single weakpeak in the Ly alpha trough. The Ly alpha emission has corresponding Ly betaemission, suggesting that it is indeed a region of lower optical depth in theintergalactic medium at z=6.08. The stronger Ly beta peaks in the spectrum of SDSS J1148+5251 couldconceivably also be the result of "leaks" in the IGM, but we suggest that theyare instead Ly alpha emission from an intervening galaxy at z=4.9. Thishypothesis gains credence from a strong complex of C IV absorption at the sameredshift and from the detection of continuum emission in the Ly alpha trough atthe expected brightness. If this proposal is correct, the quasar light hasprobably been magnified through gravitational lensing by the interveninggalaxy. The Stromgren sphere observed in the absorption spectrum of SDSSJ1148+5251 is significantly smaller than expected based on its brightness,which is consistent with the hypothesis that the quasar is lensed. If our argument for lensing is correct, the optical depths derived from thetroughs of SDSS J1148+5251 are only lower limits (albeit still quite strong,with tau(LyA)>16 inferred from the Ly beta trough.) The Ly beta absorptiontrough of SDSS J1030+0524 gives the single best measurement of the IGMtransmission at z>6, with an inferred optical depth tau(LyA)>22.Comment: To appear in July 2003 AJ, 34 pages, 11 figures; minor changes/typos fixe