  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 starforming
galaxy (BD38) in the field of the z = 1.24 cluster RDCS 1252.9-2927. This
object shows substantial continuum (0.41 +/- 0.02 \muJy at \lambda1300) and
low-ionization interstellar absorption features typical of LBGs at lower
redshift (z ~ 3); this is the highest redshift LBG confirmed via metal
absorption spectral features. The equivalent widths of the absorption features
are similar to z ~ 3 strong Ly\alpha absorbers. No noticeable Ly\alpha emission
was detected (F <= 1.4 * 10^-18 ergs cm^-2 s^-1, 3\sigma). The half-light
radius of this object is 1.6 kpc (0\farcs25) 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 ~ 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
(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 * 10^10 M_sun from population synthesis
models. This galaxy is brighter than any confirmed z ~ 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. -- Abstract Abridged
