UV continuum spectroscopy of a 6L* z=5.5 starburst galaxy

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 +/- 0.003 star- forming 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 similar to 6 Lyman break galaxy ( LBG) based on its red i(775) -z(850) = 1.5 color. This object shows substantial continuum ( 0.41 +/- 0.02 mu Jy at 1300 angstrom) and low- ionization interstellar absorption features typical of LBGs at lower redshift A ( z similar to 3); this is the highest redshift LBG confirmed via metal- absorption spectral features. The equivalent widths of the absorption features are similar to z similar to 3 strong Ly alpha absorbers. No noticeable Ly alpha emission was detected (F <= 1.4 x 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 delensed half- light radius of this object is 1.6 kpc ( 0 .25), and the star formation rate derived from the rest- frame UV luminosity is SFRUV = 38h(0.7)(-2) M-circle dot yr (-1) corrected for dust extinction). In terms of recent determinations of the z similar to 6 UV luminosity function, this object appears to be 6L(*). The Spitzer IRAC fluxes for this object are 23.3 and 23.2 AB mag ( delensed) in the 3.6 and 4.5 mu m channels, respectively, implying a mass of ( 1 - 6) x 10(10) M-circle dot from population synthesis models. This galaxy is brighter than any confirmed z similar to 6 i- dropout to date in the band, and in the 3.6 and 4.5 mu m channels, and is the most z (850) massive starbursting galaxy known at z > 5.