NOEMA confirmation of an optically dark ALMA-AzTEC submillimetre galaxy at $z=5.24$. A late-stage starburst prior to quenching

Abstract

We have obtained 1 and 3 mm spectral scans of ASXDF1100.053.1 using NOEMA. ASXDF1100.053.1 is an unlensed optically dark millimetre-bright SMG with $K_{\mathrm{AB}}>25.7$ ($2\sigma$), which was expected to lie at $z=$5-7 based on its radio-submm photo-$z$. Our data detected line emission due to ${}^{12}$CO($J=$5-4) and ($J=$6-5), providing a $z_{\mathrm{CO}}=5.2383\pm 0.0005$. Energy-coupled SED modelling indicates properties of $L_{\mathrm{IR}}=8.3_{-1.4}^{+1.5}\times 10^{12}$ L${}_{\odot }$, SFR $=630_{-380}^{+260}$ M${}_{\odot }$ yr${}^{-1}$, $M_{\mathrm{dust}}=4.4_{-0.3}^{+0.4}\times 10^8$ M${}_{\odot }$, $M_{\mathrm{stellar}}=3.5_{-1.4}^{+3.6}\times 10^{11}$ M${}_{\odot }$, and $T_{\mathrm{dust}}=37.4_{-1.8}^{+2.3}$ K. The CO luminosity allows us to estimate a gas mass $M_{\mathrm{gas}}=3.1\pm 0.3\times 10^{10}$ M${}_{\odot }$, suggesting a gas-to-dust mass ratio of around 70, fairly typical for $z\sim 2$ SMGs. ASXDF1100.053.1 has $R_{\mathrm{e},\mathrm{mm}}=1.0_{-0.1}^{+0.2}$ kpc, so its surface $L_{\mathrm{IR}}$ density ${\Sigma }_{\mathrm{IR}}$ is $1.2_{-0.2}^{+0.1}\times 10^{12}$ L${}_{\odot }$ kpc${}^{-2}$. These properties indicate that ASXDF1100.053.1 is a massive dusty star-forming (SF) galaxy with an unusually compact starburst. It lies close to the SF main sequence at $z\sim 5$, with low $M_{\mathrm{gas}}$/$M_{\mathrm{stellar}}=0.09$, SFR/SFR${}_{\mathrm{MS}}(R_{\mathrm{SB}})=0.6$, and a gas-depletion time ${\tau }_{\mathrm{dep}}$ of $\approx 50$ Myr, modulo assumptions about the stellar initial mass function in such objects. ASXDF1100.053.1 has extreme values of $M_{\mathrm{gas}}/M_{\mathrm{stellar}}$, $R_{\mathrm{SB}}$, and ${\tau }_{\mathrm{dep}}$ compared to SMGs at $z\sim$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 $dN/dz=30.4\pm 19.0$ deg${}^{-2}$, barely consistent with the latest cosmological simulations.

9 pages, 5 figures, 2 tables, accepted for publication in A&A

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