摘要：We have obtained deep 1 and 3 mm spectral-line scans towards a candidate z & 5 ALMA-identified AzTEC submillimetre galaxy (SMG) in the Subaru/XMM-Newton Deep Field (or UKIDSS UDS), ASXDF1100.053.1, using the NOrthern Extended Millimeter Array (NOEMA), aiming to obtain its spectroscopic redshift. ASXDF1100.053.1 is an unlensed optically dark millimetre-bright SMG with S 1100 µm = 3.5 mJy and KAB > 25.7 (2σ), which was expected to lie at z = 5–7 based on its radio–submillimetre photometric redshift. Our NOEMA spectral scan detected line emission due to 12CO(J = 5–4) and (J = 6–5), providing a robust spectroscopic redshift, zCO = 5.2383 ± 0.0005. Energy-coupled spectral energy distribution modelling from optical to radio wavelengths indicates an infrared luminosity LIR = 8.3 +1.5 −1.4 × 1012 L , a star formation rate SFR = 630+260 −380 M yr−1 , a dust mass Md = 4.4 +0.4 −0.3 × 108 M , a stellar mass Mstellar = 3.5 +3.6 −1.4 × 1011 M , and a dust temperature Td = 37.4 +2.3 −1.8 K. The CO luminosity allows us to estimate a gas mass Mgas = 3.1 ± 0.3 × 1010 M , suggesting a gas-to-dust mass ratio of around 70, fairly typical for z ∼ 2 SMGs. ASXDF1100.053.1 has ALMA continuum size Re = 1.0 +0.2 −0.1 kpc, so its surface infrared luminosity density ΣIR is 1.2 +0.1 −0.2 × 1012 L kpc−2 . These physical properties indicate that ASXDF1100.053.1 is a massive dusty star-forming galaxy with an unusually compact starburst. It lies close to the star-forming main sequence at z ∼ 5, with low Mgas/Mstellar = 0.09, SFR/SFRMS(RSB) = 0.6, and a gas-depletion time τdep of ≈50 Myr, modulo assumptions about the stellar initial mass function in such objects. ASXDF1100.053.1 has extreme values of Mgas/Mstellar, RSB, and τdep compared to SMGs at z ∼ 2–4, and those of ASXDF1100.053.1 are the smallest among SMGs at z > 5. ASXDF1100.053.1 is likely a late-stage dusty starburst prior to passivisation. The number of z = 5.1–5.3 unlensed SMGs now suggests a number density dN/dz = 30.4 ± 19.0 deg−2 , barely consistent with the latest cosmological simulations.
