How universal is the Gunn-Peterson trough at z∼6?:: A closer look at the quasar SDSS J1148+5251

Detectable flux is visible in the Lyalpha and Lybeta troughs of the highest redshift (z = 6.42) quasar found to date, SDSS J1148 + 5251. This has previously been interpreted as continuum contamination from an interloper galaxy at z = 4.94. We examine the Lygamma trough of SDSS J1148 + 5251 and show that this interpretation is untenable: the spectrum does not show the continuum break in a galaxy expected from absorption by the intervening z = 4.94 Lyalpha forest. Therefore, flux must be leaking through at least one of the troughs from the quasar itself. Contrary to previous claims, the flux ratios in the Lyalpha and Lybeta troughs are consistent with pure transmission. From the Lygamma trough, we place an upper bound on the effective Lyalpha optical depth at z similar to 6.2 of tau(eff) < 15.4 (2 sigma). This implies a highly ionized intergalactic medium along this line of sight and significant cosmic variance in the transition toward complete Gunn- Peterson absorption.