UV Continuum Spectroscopy of a 6L_* z = 5.5 Starburst Galaxy1,

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 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<=1.4×10^-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 (0.25"), and the star formation rate derived from the rest-frame UV luminosity is SFR_UV=38 h^-2_0.7 M_solar yr^-1 (142 h^-2_0.7 M_solar 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_solar 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.

Based on observations collected at the European Southern Observatory, Paranal, Chile (LP166.A-0701 and 169.A-0458).

Based in part on observations made with the Spitzer Space Telescope, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under NASA contract 1407.