2016A&A...586L...7B

We present ALMA detections of the [NII] 205µm and CO(12-11) emission lines, and the tentative detection of [CI] ³P₁-³P₀ for the strongly lensed (µ=5.7±0.5) dusty, star-forming galaxy SPT-S J213242-5802.9 (hereafter SPT2132-58) at z=4.77. The [NII] and CO(12-11) lines are detected at 11.5 and 8.5σ levels, respectively, by our band 6 observations. The [CI] line is detected at 3.2σ after a reanalysis of existing band 3 data. The [CI] luminosity implies a gas mass of (3.8±1.2)x10¹⁰M_☉, and, consequently, a very short depletion timescale of 34±13Myr and a CO luminosity to gas mass conversion factor α_CO of 1.0±0.3M_☉/(K.km/spc²). SPT2132-58 is an extreme starburst with an intrinsic star formation rate of 1100±200M_☉/yr. We find a [CII]/[NII] ratio of 26±6, which is the highest ratio reported at z>4. This suggests that SPT2132-58 hosts an evolved interstellar medium (0.5Z_☉<Z<1.5Z_☉), which may be dominated by photodissociation regions. The CO(2-1) and CO(5-4) transitions have lower CO to far-infrared ratios than local and high-redshift samples, while CO(12-11) is similar to these samples, suggesting the presence of an additional very excited component or an active galactic nucleus.