We have obtained a high S/N ( 22.3 hr integration ) UV continuum VLT FORS2 spectrum of an extremely bright ( z _ { 850 } = 24.3 ) z = 5.515 \pm 0.003 starforming galaxy ( BD38 ) in the field of the z = 1.24 cluster RDCS 1252.9-2927 .
From HST Advanced Camera for Surveys imaging this object was selected as a potential z \sim 6 Lyman break galaxy ( LBG ) based on its red i _ { 775 } - z _ { 850 } = 1.5 color .
This object shows substantial continuum ( 0.41 \pm 0.02 \mu Jy at \lambda 1300 ) and low-ionization interstellar absorption features typical of LBGs at lower redshift ( z \sim 3 ) ; this is the highest redshift LBG confirmed via metal absorption spectral features .
The equivalent widths of the absorption features are similar to z \sim 3 strong Ly \alpha absorbers .
No noticeable Ly \alpha emission was detected ( F \leq 1.4 \times 10 ^ { -18 } ergs cm ^ { -2 } s ^ { -1 } , 3 \sigma ) .
This object is at most amplified 0.3 mag from gravitational lensing by the foreground cluster .
The half-light radius of this object is 1.6 kpc ( 0 \farcs 25 ) and the star formation rate derived from the rest-frame UV luminosity is SFR _ { UV } = 38 h ^ { -2 } _ { 0.7 } M _ { \sun } { yr ^ { -1 } } ( 142 h ^ { -2 } _ { 0.7 } M _ { \sun } { yr ^ { -1 } } corrected for dust extinction ) .
In terms of recent determinations of the z \sim 6 UV luminosity function , this object appears to be 6 L _ { * } .
The Spitzer IRAC fluxes for this object are 23.3 and 23.2 AB mag ( corrected for 0.3 mag of cluster lensing ) in the 3.6 \mu and 4.5 \mu channels , respectively , implying a mass of 1-6 \times 10 ^ { 10 } M _ { \sun } from population synthesis models .
This galaxy is brighter than any confirmed z \sim 6 i -dropout to date in the z _ { 850 } band , and both the 3.6 \mu and 4.5 \mu channels , and is the most massive starbursting galaxy known at z > 5 .