We report the discovery of a redshift z = 6.56 galaxy lying behind the cluster Abell 370 .
The object HCM 6A was found in a narrowband imaging survey using a 118 Å bandpass filter centered at 9152 Å in the LRIS camera on the 10 m Keck II Telescope .
Candidate Ly \alpha emitters were identified by the equivalent width of the emission and the absence of lower wavelength flux in ultradeep broadband images .
HCM 6A is the first galaxy to be confirmed at redshift z > 6 , and has W _ { \lambda } ( observed ) =190 Å , flux = 2.7 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } .
Spectra obtained with LRIS confirm the emission line and the continuum break across the line , and show an asymmetric line profile with steep fall-off on the blue side .
Deep Subaru near-infrared CISCO images in J , H and K ^ { \prime } which extend the sampled continuum to longer wavelengths give a consistent estimate of the continuum flux density in these line-free regions of 2.6 \pm 0.7 \times 10 ^ { -30 } erg cm ^ { -2 } s ^ { -1 } Hz ^ { -1 } .
The line width and strength , asymmetric profile , and very deep spectral break are only consistent with the interpretation of the line as a redshifted Ly \alpha feature .
From the detailed lensing model of this cluster , we estimate a lensing amplification of 4.5 for this galaxy , which is located slightly over an arcminute from the center of the cluster , for an unlensed flux of 6.5 \times 10 ^ { -18 } erg cm ^ { -2 } s ^ { -1 } .
The presence of such a galaxy suggests that the reionizing epoch is beyond z = 6.6 .