Probing the ionization state of the universe at z > 6

We present high signal-to-noise ratio Keck ESI spectra of the two quasars known to have Gunn-Peterson absorption troughs, SDSS J1030+0524 (z = 6.28) and SDSS J1148+5251 (z = 6.37). The Lyalpha and Lybeta troughs for SDSS J1030+0524 are very black and show no evidence of any emission over a redshift interval of similar to0.2, starting at z = 6. On the other hand, SDSS J1148+5251 shows a number of emission peaks in the Lybeta Gunn-Peterson trough along with a single weak peak in the Lyalpha trough. The Lyalpha emission has corresponding Lyalpha emission, suggesting that it is indeed a region of lower optical depth in the intergalactic medium at z = 6.08. The stronger Lybeta peaks in the spectrum of SDSS J1148+5251 could also conceivably be the result of "leaks'' in the intergalactic medium (IGM), but we suggest instead that they are Lyalpha emission from an intervening galaxy at z = 4.9. This hypothesis gains credence from a strong complex of C IV absorption at the same redshift and from the detection of continuum emission in the Lyalpha trough at the expected brightness. If this proposal is correct, the quasar light has probably been magnified through gravitational lensing by the intervening galaxy. The Stromgren sphere observed in the absorption spectrum of SDSS J1148+5251 is significantly smaller than expected on the basis of 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 the troughs of SDSS J1148+5251 are only lower limits [albeit still quite strong, with tau(Lyalpha) > 16 inferred from the Lybeta trough]. The Lybeta absorption trough of SDSS J1030+0524 gives the single best measurement of the IGM transmission at z > 6, with an inferred optical depth of tau(Lyalpha) > 22.