2005ApJ...630L.137D

We have obtained a high S/N (22.3 hr integration) UV continuum VLT FORS2 spectrum of an extremely bright (z₈₅₀=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 ∼ 6 Lyman break galaxy (LBG) based on its red i₇₇₅-z₈₅₀=1.5 color. This object shows substantial continuum (0.41±0.02 µJy at 1300 Å) 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 Lyα absorbers. No noticeable Lyα emission was detected (F≤1.4x10^–18 ergs/cm2/s, 3 σ). 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 SFR_UV=38 h^–2_0.7M_☉/yr (142 h^–2_0.7M_☉/yr 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 (delensed) in the 3.6 and 4.5 µm channels, respectively, implying a mass of (1-6) x10¹⁰ M_☉from population synthesis models. This galaxy is brighter than any confirmed z ∼ 6 i-dropout to date in the z₈₅₀ band, and in the 3.6 and 4.5 µm channels, and is the most massive starbursting galaxy known at z > 5.