Fraction of [CII]_158 μm arising from the PDR as a function of dust temperature, color-coded by the logarithm of the total IR luminosity. The QSO PJ231−20 and its companion galaxy are marked by stars. For the companion we report the cases with an assumed dust temperature of 47 K and 35 K. These points are connected by a dashed line. The diamond marks the 3σ upper limit on quasar ULAS J1342+0928 at z = 7.54 (Novak et al. 2019), while the square is the upper limit on SDSS J2310+1855 at z = 6.00 (Li et al. 2020a,b). The latter limits are computed using the [NII]_122 μm/[NII]_205 μm line luminosity ratio to estimate the electron density and then the fraction of [CII]_158 μm from the ionized gas from Oberst et al. (2006). T_dust and L_TIR for SDSS J2310+1855 are derived via a dust SED fitting by Shao et al. (2019), and for ULAS J1342+0928, L_TIR is estimated in Novak et al. (2019) by assuming T_dust = 47 K (here slightly shifted for better visualization). Circles represent the LIRG sample presented in Díaz-Santos et al. (2017). The solid black line shows the best-fit model , where S₆₃ and S₁₅₈ are the continuum flux density at 63 μm and 158 μm, respectively. The gray shaded area is the 1σ confidence interval. The approximate relation between the flux density ratio and T_dust is provided by Díaz-Santos et al. (2017), T_dust = 20.24 + 14.54(S₆₃/S₁₅₈)−3.75(S₆₃/S₁₅₈)² + 0.46(S₆₃/S₁₅₈)³.