[C ii] /FIR luminosity ratio as a function of FIR luminosity, with our new estimate of L_[CII]/L_FIR in SDSS J1148+5251, obtained separately for the extended and compact components, as explained in Sect. 4.2. For comparison, the plot also includes previous ISO observations of local star-forming galaxies (asterisks, Malhotra et al. 2001) and of local ULIRGs (diamonds, Luhman et al. 2003) along with their median and scatter (dashed lines), recent Herschel/PACS observations of local LIRGs (small open circles, Díaz-Santos et al. 2013), the CSO-ZEUS survey of star forming and active galaxies at redshift 1 <z< 2 (triangles, Stacey et al. 2010), observations of sub-millimetre galaxies (SMGs) at 2 <z< 5 (open circles, Ivison et al. 2010; De Breuck et al. 2014; Valtchanov et al. 2011; Cox et al. 2011; Swinbank et al. 2012; Carniani et al. 2013), sub-millimetre galaxies (SMGs) at z> 5 (filled circles, Riechers et al. 2013, 2014; Neri et al. 2014; Rawle et al. 2014), quasars at 2 <z< 5 (open stars, Maiolino et al. 2009; Wagg et al. 2010; Carniani et al. 2013), and z> 5 quasars (filled stars, Venemans et al. 2012; Wang et al. 2013; Willott et al. 2013). The dotted horizontal line indicates the Milky Way value (L_[CII]/L_FIR ~ 3 × 10^-3, Carilli & Walter 2013). All measurements refer to the FIR luminosity as integrated over 42.5–122.5 μm. Where necessary, we have converted L_{FIR(40 − 500 μm)} and L_{TIR(8 − 1000 μm)} into L_{FIR(42.5 − 122.5 μm)} by using the conversion factors provided by Carilli & Walter (2013), i.e. L_{FIR(40−500 μm)} ≃ 1.2 L_{FIR(42.5−122.5 μm)} and L_{TIR(8−1000 μm)} ≃ 1.3 L_{FIR(40−500 μm)}.