Line luminosity ratio of [CII] 158 μm and [OI] 145 μm against [OIII] 88 μm and [OI] 145 μm. The low-z samples are drawn from Fernández-Ontiveros et al. (2016), Brauher et al. (2008), Herrera-Camus et al. (2018), Cormier et al. (2015) and Spinoglio et al. (2015). We note that relative to [OI] 145 μm, SPT 0418−47 (red star) has an [OIII] 88 μm luminosity that is rather consistent with normal (black, brown and blue) rather than dwarf galaxies (green), but a weaker [CII] 158 μm. Arrows indicate which physical processes dominate these line ratios (see text).