An Extremely Luminous Galaxy at [CLC][ITAL]z[/ITAL][/CLC] = 5.74

We report the discovery of an extremely luminous galaxy lying at a redshiftof z=5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22field using a 105 Angstrom bandpass filter centered at 8185 Angstroms duringthe course of the Hawaii narrowband survey using LRIS on the 10 m Keck IITelescope, and was identified by the equivalent width of the emissionW_lambda(observed)=175 Angstroms, flux = 1.7 x 10^{-17} erg cm^{-2} s^{-1}).Comparison with broadband colors shows the presence of an extremely strongbreak (> 4.2 at the 2 sigma level) between the Z band above the line, where theAB magnitude is 25.5, and the R band below, where the object is no longervisible at a 2 sigma upper limit of 27.1 (AB mags). These properties are onlyconsistent with this object's being a high-z Ly alpha emitter. A 10,800 sspectrum obtained with LRIS yields a redshift of 5.74. The object is similar inits continuum shape, line properties, and observed equivalent width to thez=5.60 galaxy, HDF 4-473.0, as recently described by Weymann et al. (1998), butis 2-3 times more luminous in the line and in the red continuum. For H_0 = 65km s^{-1} Mpc^{-1} and q_0 = (0.02, 0.5) we would require star formation ratesof around (40, 7) solar masses per year to produce the UV continuum in theabsence of extinction.Comment: 5 pages, 4 figures, Latex with emulateapj style file; to appear in the Astrophysical Journal (Letters