A Redshift [CLC][ITAL]z[/ITAL][/CLC] = 6.56 Galaxy behind the Cluster Abell 370

We report the discovery of a redshift z=6.56 galaxy lying behind the clusterAbell 370. The object HCM 6A was found in a narrowband imaging survey using a118 Angstrom bandpass filter centered at 9152 Ang in the LRIS camera on the 10m Keck II Telescope. Candidate Lyman alpha emitters were identified by theequivalent width of the emission and the absence of lower wavelength flux inultradeep broadband images. HCM6A is the first galaxy to be confirmed atredshift z>6, and has W_lambda(observed)=190 Angstroms, flux = 2.7 x 10^-17 ergcm^-2 s^-1. Spectra obtained with LRIS confirm the emission line and thecontinuum break across the line, and show an asymmetric line profile with steepfall-off on the blue side. Deep Subaru near-infrared CISCO images in J, H andK' which extend the sampled continuum to longer wavelengths give a consistentestimate of the continuum flux density in these line-free regions of 2.6+/-0.7x 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 theline as a redshifted Lyman alpha feature. From the detailed lensing model ofthis cluster, we estimate a lensing amplification of 4.5 for this galaxy, whichis located slightly over an arcminute from the center of the cluster, for anunlensed flux of 6.5 x 10^-18 erg cm^-2 s^-1. The presence of such a galaxysuggests that the reionizing epoch is beyond z=6.6.Comment: 5 pages, 4 PostScript figures, uses emulateapj5, to appear in the April 1, 2002 issue of the Astrophysical Journal Letter