Hubble Space TelescopeAdvanced Camera for Surveys Observations of thez= 6.42 Quasar SDSS J1148+5251: A Leak in the Gunn-Peterson Trough

The Hubble Advanced Camera for Surveys has been used to obtain a narrow-bandimage of the weak emission peak seen at lambda=7205 A in the Gunn-Peterson Lybeta absorption trough of the highest redshift quasar, SDSS J1148+5251. Theemission looks perfectly point-like; there is no evidence for the interveninggalaxy that we previously suggested might be contaminating the quasar spectrum.We derive a more accurate astrometric position for the quasar in the twofilters and see no indication of gravitational lensing. We conclude that thelight in the Ly beta trough is leaking through two unusually transparent,overlapping windows in the IGM absorption, one in the Ly beta forest at z ~ 6and one in the Ly alpha forest at z ~ 5. If there are significant optical depth variations on velocity scales smallcompared with our spectral resolution (~150 km/s), the Ly alpha trough becomesmore transparent for a given Ly beta optical depth. Such variations can onlystrengthen our conclusion that the fraction of neutral hydrogen in the IGMincreases dramatically at z>6. We argue that the transmission in the Ly betatrough is not only a more sensitive measure of the neutral fraction than is Lyalpha, it also provides a less biased estimator of the neutral hydrogenfraction than does the Ly alpha transmission.Comment: Submitted to the Astronomical Journa