Hubble Advanced Camera for Surveys Observations of the Z = 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-band image of the weak emission peak seen at λ = 7205 Å in the Gunn-Peterson Lyβ absorption trough of the highest redshift quasar, SDSS J1148+5251. The emission looks perfectly point-like; 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 Lyβ trough is leaking through two unusually transparent, overlapping windows in the IGM absorption, one in the Ly β forest at z ∼ 6 and one in the Lyα forest at z ∼ 5. If there are significant optical depth variations on velocity scales small compared with our spectral resolution (∼ 150 km/s), the Lyα trough becomes more transparent for a given Lyβ optical depth. 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 Lyβ trough is not only a more sensitive measure of the neutral fraction than is Lyα, it also provides a less biased estimator of the neutral hydrogen fraction than does the Lyα transmission. Subject headings: comology: observations — early universe — galaxies: high-redshift — intergalactic medium — quasars: absorption lines — quasars: individual (SDSS J1148+5251)