UV Continuum Spectroscopy of a 6 L * z = 5.5 Starburst Galaxy

Abstract

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 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 × 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 (025), and the star formation rate derived from the rest-frame UV luminosity is SFRUV = 38 h M☉ yr-1 (142 h M☉ 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) × 1010 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.