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 Ly \alpha and Ly \beta troughs for SDSS J1030 + 0524 are very black and show no evidence for any emission over a redshift interval of \sim 0.2 starting at z = 6 .
On the other hand , SDSS J1148 + 5251 shows a number of emission peaks in the Ly \beta Gunn-Peterson trough along with a single weak peak in the Ly \alpha trough .
The Ly \alpha emission has corresponding Ly \beta emission , suggesting that it is indeed a region of lower optical depth in the intergalactic medium at z = 6.08 .

The stronger Ly \beta peaks in the spectrum of SDSS J1148 + 5251 could conceivably also be the result of “ leaks ” in the IGM , but we suggest that they are instead Ly \alpha 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 Ly \alpha 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 Strömgren sphere observed in the absorption spectrum of SDSS J1148 + 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 the troughs of SDSS J1148 + 5251 are only lower limits ( albeit still quite strong , with \tau ( \hbox { Ly$ \alpha$ } ) > 16 inferred from the Ly \beta trough . )
The Ly \beta absorption trough of SDSS J1030 + 0524 gives the single best measurement of the IGM transmission at z > 6 , with an inferred optical depth \tau ( \hbox { Ly$ \alpha$ } ) > 22 .