Ratio of the [OIII] 88 μm and [CII] 158 μm line luminosities as a function of (intrinsic) far-IR luminosity. The comparison low-z samples are the Herschel Dwarf Galaxies (green circles; Cormier et al. 2015), nearby Seyferts (orange diamonds; Spinoglio et al. 2015), and the GOALS sample (grey squares, assuming a 20% uncertainty in L_FIR; Sanders et al. 2003; Díaz-Santos et al. 2017). The high-z sources marked in blue are SDP.81 (Valtchanov et al. 2011), the Eyelash (George et al. 2013), the Cloverleaf (Uzgil et al. 2016), SXDF-NB1006-2 (Inoue et al. 2016), SPT 0311−58 (Marrone et al. 2018), CFHQS J2100 (Walter et al. 2018), COSMOS-AzTEC1 (Tadaki et al. 2019), B14-65666 (Hashimoto et al. 2019), and J1342+0928 (Novak et al. 2019).