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A Redshift z = 6.56 Galaxy Behind the Cluster Abell 370

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 Angstrom bandpass filter centered at 9152 Ang in the LRIS camera on the 10 m Keck II Telescope. Candidate Lyman alpha emitters were identified by the equivalent width of the emission and the absence of lower wavelength flux in ultradeep broadband images. HCM6A is the first galaxy to be confirmed at redshift z>6, and has W_lambda(observed)=190 Angstroms, flux = 2.7 x 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' 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+/-0.7 x 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 Lyman 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 x 10^-18 erg cm^-2 s^-1. The presence of such a galaxy suggests that the reionizing epoch is beyond z=6.6.