UV CONTINUUM SPECTROSCOPY OF A 6L_* z = 5.5 STARBURST GALAXY

摘要

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 ～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 μJy at 1300 A) 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 ≤ 1.4 x 10~(-18) ergs cm~(-2) s~(-1), 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_(0.7)~(-2) solar mass yr~(-1) (142 h_(0.7)~(-2) solar mass 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 (delensed) in the 3.6 and 4.5 μm channels, respectively, implying a mass of (1-6) x 10~(10) solar mass from population synthesis models. This galaxy is brighter than any confirmed z ～6 i-dropout to date in the z_(850) band, and in the 3.6 and 4.5 μm channels, and is the most massive starbursting galaxy known at z > 5.