2019MNRAS.488.1489H

We report the detection of the far-infrared (FIR) fine-structure line of singly ionized nitrogen, [N II] 205 µm , 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 (J_up = 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 µm , 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 µm 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_ N II205_/ L_ IR(8-1000 µm)_ = 2.7 ×10^–4 is consistent with the dispersion of local and z > 4 SFGs. We find that the lower limit, [N II] 205 µm -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_ N II205_ line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM.