UV Continuum Spectroscopy of a 6L* 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 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 μJy at λ1300) 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, 3σ). The 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-20.7 Msun yr-1 (142 h-20.7 Msun 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 3.6μ and 4.5μ channels, respectively, implying a mass of 1-6 * 1010 Msun from population synthesis models. This galaxy is brighter than any confirmed z ~ 6 i-dropout to date in the z850 band, and both the 3.6μ and 4.5μ channels, and is the most massive starbursting galaxy known at z > 5. -- Abstract Abridged
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