2022A&A...659A.154I

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 K_AB > 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, z_CO = 5.2383 ± 0.0005. Energy-coupled spectral energy distribution modelling from optical to radio wavelengths indicates an infrared luminosity L_IR=8.3_–1.4^+1.5x 10¹²L_☉, a star formation rate SFR=630_–380⁺²⁶⁰M_☉/yr, a dust mass M_d=4.4_–0.3^+0.4x10⁸M_☉, a stellar mass M_stellar=3.5_–1.4^+3.6x10¹¹M_☉, and a dust temperature T_d=37.4_–1.8^+2.3K. The CO luminosity allows us to estimate a gas mass M_gas=3.1±0.3x10¹⁰M_☉, suggesting a gas-to-dust mass ratio of around 70, fairly typical for z∼2 SMGs. ASXDF1100.053.1 has ALMA continuum size R_e=1.0_–0.1^+0.2kpc, so its surface infrared luminosity density Σ_IR is 1.2_–0.2^+0.1x10¹²L_☉/kpc². 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 M_gas/M_stellar=0.09, SFR/SFR_MS(R_SB)=0.6, and a gas-depletion time τ_dep of ≃50Myr, modulo assumptions about the stellar initial mass function in such objects. ASXDF1100.053.1 has extreme values of M_gas/M_stellar, R_SB, and τ_dep 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.