NOEMA confirmation of an optically dark ALMA–AzTEC submillimetre galaxy at z = 5.24

A late-stage starburst prior to quenching

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
² Department of Microelectronics, Delft University of Technology, PO Box 5031, 2600 GA Delft, The Netherlands
³ Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
e-mail: soh.ikarashi@durham.ac.uk
⁴ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
⁵ Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
⁶ Research Center for the Eary Universe, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan

Received: 27 April 2021
Accepted: 2 October 2021

Abstract

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^+1.5 × 10¹² L_⊙, a star formation rate SFR  = 630₋₃₈₀⁺²⁶⁰ M_⊙ yr⁻¹, a dust mass M_d = 4.4_−0.3^+0.4 × 10⁸ M_⊙, a stellar mass M_stellar = 3.5_−1.4^+3.6 × 10¹¹ M_⊙, and a dust temperature T_d = 37.4_−1.8^+2.3 K. The CO luminosity allows us to estimate a gas mass M_gas = 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 R_e = 1.0_−0.1^+0.2 kpc, so its surface infrared luminosity density Σ_IR is 1.2_−0.2^+0.1 × 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 M_gas/M_stellar = 0.09, SFR/SFR_MS(R_SB) = 0.6, and a gas-depletion time τ_dep of ≈50 Myr, 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.