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 Ly-alpha and Ly-beta troughs of the highestredshift ($z=6.42$) quasar found to date, SDSS J1148+5251. This has previouslybeen interpreted as continuum contamination from an interloper galaxy atz=4.94. We examine the Ly-gamma trough of SDSS J1148+5251 and show that thisinterpretation is untenable: the spectrum does not show the continuum break ina z=4.94 galaxy expected from absorption by the intervening Ly-alpha forest.Therefore, flux must be leaking through at least one of the troughs from thequasar itself. Contrary to previous claims, the flux ratios in the Ly-alpha andLy-beta troughs are consistent with pure transmission. From the Ly-gammatrough, we place an upper bound on the effective Ly-alpha optical depth atz~6.2 of tau < 14.3 (2 sigma). This implies a highly ionized IGM along thisline of sight and significant cosmic variance in the transition toward completeGunn-Peterson absorption. Detailed study of the observed transmission featureswill shed light on this era.Comment: Submitted to ApJ Letter