The "Red Radio Ring": Ionised and Molecular Gas in a Starburst/Active Galactic Nucleus at z 2.55
Abstract
We report the detection of the far-infrared (FIR) fine-structure line of singly ionised nitrogen, , 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 , 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 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 NII205 / L IR(8-1000μ m) = 2.7 × 10-4 is consistent with the dispersion of local and z > 4 SFGs. We find that the lower-limit, -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 NII205 line-to-IR luminosity ratio of the RRR is consistent with a starburst-powered ISM.