Hubble Space Telescope Advanced Camera for Surveys observations of the z=6.42 quasar SDSS J1148+5251:: A leak in the Gunn-Peterson trough

The Hubble Space Telescope Advanced Camera for Surveys has been used to obtain a narrowband image of the weak emission peak seen at lambda = 7205 angstrom in the Gunn- Peterson Ly beta absorption trough of the highest redshift quasar, SDSS J1148+5251. The emission looks perfectly pointlike; there is no evidence for the intervening galaxy that we previously suggested might be contaminating the quasar spectrum. We derive a more accurate astrometric position for the quasar in the two filters and see no indication of gravitational lensing. We conclude that the light in the Ly beta trough is leaking through two unusually transparent, overlapping windows in the intergalactic medium (IGM) absorption, one in the Ly beta forest at z similar to 6 and one in the Ly alpha forest at z similar to 5. If there are significant optical depth variations on velocity scales small compared with our spectral resolution (similar to 150 km s(-1)), the Ly alpha trough has the potential to become more transparent for a given Ly alpha optical depth because the Ly alpha transmission is extremely sensitive to the most transparent sight lines in the IGM while Ly alpha is sensitive to sight lines with a broader range of optical depths. Such variations can only strengthen our conclusion that the fraction of neutral hydrogen in the IGM increases dramatically at z > 6. We argue that the transmission in the Ly alpha trough is not only a more sensitive measure of the neutral fraction than is Ly alpha but that it also provides a less biased estimator of the neutral hydrogen fraction than does the Ly alpha transmission.