An Extremely Luminous Galaxy at z = 5.74

We report the discovery of an extremely luminous galaxy lying at a redshift of z = 5.74, SSA22-HCM1. The object was found in narrowband imaging of the SSA22 field using a 105 Å bandpass filter centered at 8185 Å during the course of the Hawaii narrowband survey using the Low-Resolution Imaging Spectrograph (LRIS) on the 10 m Keck II telescope, and it was identified by the equivalent width of the emission [W_λ(observed) = 175 Å, flux = 1.7 × 10^-17 ergs cm^-2 s^-1]. Comparison with broadband colors shows the presence of an extremely strong break (>4.2 at the 2 σ level) between the Z band above the line, where the AB magnitude is 25.5, and the R band below, where the object is no longer visible at a 2 σ upper limit of 27.1 (AB magnitudes). These properties are only consistent with this object's being a high-z Lyα emitter. An 18,000 s spectrum obtained with LRIS yields a redshift of 5.74. The object is similar in its continuum shape, line properties, and observed equivalent width to the z = 5.60 galaxy HDF 4-473.0, as recently described by Weymann et al., but is 2-3 times more luminous in the line and in the red continuum. For H₀ = 65 km s^-1 Mpc^-1 and q₀ = (0.02, 0.5), we would require star formation rates of around (40, 7) M_☉ yr^-1 to produce the UV continuum in the absence of extinction.