Abstract
Detectable flux is visible in the Lyα and Lyβ 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 Lyγ trough of SDSS J1148+5251 and show that this interpretation is untenable: the spectrum does not show the continuum break in a z = 4.94 galaxy expected from absorption by the intervening Lyα 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 Lyα and Lyβ troughs are consistent with pure transmission. From the Lyγ trough, we place an upper bound on the effective Lyα optical depth at z ~ 6.2 of τeff < 15.4 (2 σ). This implies a highly ionized intergalactic medium along this line of sight and significant cosmic variance in the transition toward complete Gunn-Peterson absorption.
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