CO excitation (CO(5−4)/CO(2−1) in the left panel and CO(12−11)/CO(5−4) in the right panel) versus [CII]/[NII] luminosity ratio (the ratio reported for ALESS73.1 is computed using the updated [CII] flux of De Breuck et al. 2014). Red star: SPT2132-58. Salmon diamonds: other high-z sources from the literature (see text). Blue circles: HERCULES local sample (Rosenberg et al. 2015). [NII] luminosities are from Kamenetzky et al. (2015). The left and right arrows indicate that this source does not have available measurement of its CO ratio. The range of CO(5−4)/CO(2−1) ratio for various samples is indicated in the bottom of the left panel (Daddi et al. 2015). The ranges of ratio expected for HII and PDR/XDR/shock dominated regions from Decarli et al. (2014) are indicated on the left. The metallicity estimated from [CII]/[NII], following the Nagao et al. (2012) method, is indicated on the right for log (n) = 1.5 (navy) and log (n) = 3.0 (blue). We assume log(U_HII) = −3.5 for both.