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 LRIS on the 10 m Keck II Telescope , and was identified by the equivalent width of the emission ( W _ { \lambda } ( observed ) =175 Å , flux = 1.7 \times 10 ^ { -17 } erg cm ^ { -2 } s ^ { -1 } ) .
Comparison with broadband colors shows the presence of an extremely strong break ( > 4.2 at the 2 \sigma 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 \sigma upper limit of 27.1 ( AB mags ) .
These properties are only consistent with this object ’ s being a high- z Ly \alpha emitter .
A 10 800 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 .
( 1998 ) , but is 2–3 times more luminous in the line and in the red continuum .
For H _ { 0 } = 65 { km } { s } ^ { -1 } { Mpc } ^ { -1 } and q _ { 0 } = ( 0.02 , 0.5 ) we would require star formation rates of around ( 40 , 7 ) M _ { \odot } yr ^ { -1 } to produce the UV continuum in the absence of extinction .