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ABSTRACT We report the detection of the far-infrared (FIR) fine-structure line of singly ionized nitrogen, [N ii] 205 $\mu$m , within the peak epoch of galaxy assembly, from a strongly lensed galaxy, hereafter ‘The Red Radio Ring’; the RRR, at z = 2.55. We combine new observations of the ground-state and mid-J transitions of CO (Jup = 1, 5, 8), and the FIR spectral energy distribution (SED), to explore the multiphase interstellar medium (ISM) properties of the RRR. All line profiles suggest that the H ii regions, traced by [N ii] 205 $\mu$m , and the (diffuse and dense) molecular gas, traced by CO, are cospatial when averaged over kpc-sized regions. Using its mid-IR-to-millimetre (mm) SED, we derive a non-negligible dust attenuation of the [N ii] 205 $\mu$m line emission. Assuming a uniform dust screen approximation results a mean molecular gas column density >1024 cm−2, with a molecular gas-to-dust mass ratio of 100. It is clear that dust attenuation corrections should be accounted for when studying FIR fine-structure lines in such systems. The attenuation corrected ratio of $L_{\rm N\,{\small II}205} / L_{\rm IR(8\!-\!1000\, \mu m)} = 2.7 \times 10^{-4}$ is consistent with the dispersion of local and z > 4 SFGs. We find that the lower limit, [N ii] 205 $\mu$m -based star formation rate (SFR) is less than the IR-derived SFR by a factor of 4. Finally, the dust SED, CO line SED, and $L_{\rm N\,{\small II}205}$ line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM. 

We studied the molecular gas properties of AzTEC/C159, a star-forming disk galaxy at $z=4.567$. We secured $^{12}$CO molecular line detections for the $J=2\to1$ and $J=5\to4$ transitions using the Karl G. Jansky VLA and the NOEMA interferometer. The broad (FWHM$\sim750\,{\rm km\,s}^{-1}$) and tentative double-peaked profiles of both $^{12}$CO lines are consistent with an extended molecular gas reservoir, which is distributed in a rotating disk as previously revealed from [CII] 158$\mu$m line observations. Based on the $^{12}$CO(2$\to$1) emission line we derived $L'_{\rm{CO}}=(3.4\pm0.6)\times10^{10}{\rm \,K\,km\,s}^{-1}{\rm \,pc}^{2}$, that yields a molecular gas mass of $M_{\rm H_2 }(\alpha_{\rm CO}/4.3)=(1.5\pm0.3)\times 10^{11}{\rm M}_\odot$ and unveils a gas-rich system with $\mu_{\rm gas}(\alpha_{\rm CO}/4.3)\equiv M_{\rm H_2}/M_\star=3.3\pm0.7$. The extreme star formation efficiency (SFE) of AzTEC/C159, parametrized by the ratio $L_{\rm{IR}}/L'_{\rm{CO}}=(216\pm80)\, {\rm L}_{\odot}{\rm \,(K\,km\,s}^{-1}{\rm \,pc}^{2})^{-1}$, is comparable to merger-driven starbursts such as local ultra-luminous infrared galaxies (ULIRGs) and SMGs. Likewise, the $^{12}$CO(5$\to$4)/CO(2$\to$1) line brightness temperature ratio of $r_{52}= 0.55\pm 0.15$ is consistent with high excitation conditions, similar to that observed in SMGs. We constrained the value for the $L'_{\text{CO}}-{\rm H}_2$ mass conversion factor in AzTEC/C159, i.e. $\alpha_{\text{CO}}=3.9^{+2.7}_{-1.3}{\rm \,M}_{\odot}{\rm \,K}^{-1}{\rm \,km}^{-1}{\rm \,s\,pc}^{-2}$, that is consistent with a self-gravitating molecular gas distribution as observed in local star-forming disk galaxies. Cold gas streams from cosmological filaments might be fueling a gravitationally unstable gas-rich disk in AzTEC/C159, which breaks into giant clumps forming stars as efficiently as in merger-driven systems and generate high gas excitation. 

We present an analysis of the dust attenuation of star-forming galaxies at z = 2.5–4.0 through the relationship between the UV spectral slope (β), stellar mass (M*), and the infrared excess (IRX = LIR/LUV) based on far-infrared continuum observations from the Atacama Large Millimeter/sub-millimeter Array (ALMA). Our study exploits the full ALMA archive over the COSMOS field processed by the A3COSMOS team, which includes an unprecedented sample of ∼1500 galaxies at z ∼ 3 as primary or secondary targets in ALMA band 6 or 7 observations with a median continuum sensitivity of 126 $\rm {\mu Jy\, beam}^{-1}$ (1σ). The detection rate is highly mass dependent, decreasing drastically below log (M*/M⊙) = 10.5. The detected galaxies show that the IRX–β relationship of massive (log M*/M⊙ > 10) main-sequence galaxies at z = 2.5–4.0 is consistent with that of local galaxies, while starbursts are generally offset by $\sim 0.5\, {\rm dex}$ to larger IRX values. At the low-mass end, we derive upper limits on the infrared luminosities through stacking of the ALMA data. The combined IRX–M* relation at $\rm {log\, ({\it M}_{\ast }/\mathrm{M}_{\odot })\gt 9}$ exhibits a significantly steeper slope than reported in previous studies at similar redshifts, implying little dust obscuration at log M*/M⊙ < 10. However, our results are consistent with earlier measurements at z ∼ 5.5, indicating a potential redshift evolution between z ∼ 2 and z ∼ 6. Deeper observations targeting low-mass galaxies will be required to confirm this finding.