We report the detection of the far-infrared ( FIR ) fine-structure line of singly ionised 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 ( J _ { up } = 1,5,8 ) , and the FIR spectral energy distribution ( SED ) , to explore the multi-phase interstellar medium ( ISM ) properties of the RRR .
All line profiles suggest that the HII regions , traced by [ N ii ] 205 \mu m , and the ( diffuse and dense ) molecular gas , traced by the CO , are co-spatial 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 > 10 ^ { 24 } 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 _ { NII 205 } / L _ { 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 four .
Finally , the dust SED , CO line SED and L _ { NII 205 } line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM .