[C II]-FIR deficit. Left panel: [C II]/FIR luminosity ratio as a function of FIR surface brightness. The red open diamonds with error bars connected by the solid red line represent measurements for J2310+1855. The comparison samples are spatially resolved z ∼ 6 quasars (green triangles connected by dashed lines; Wang et al. 2019) and z ∼ 3 SMGs (purple triangles connected by dashed lines; Rybak et al. 2019), other z > 6 quasars (yellow stars; Wang et al. 2013; Venemans et al. 2016, 2017; Decarli et al. 2018), other high-z SMGs (pink triangles; Riechers et al. 2013; Neri et al. 2014; Gullberg et al. 2018; Rybak et al. 2019), the GOALS sample of local luminous infrared galaxies (gray circles; Díaz-Santos et al. 2013, 2017), and LBGs (green triangles; Capak et al. 2015; Jones et al. 2017; Hashimoto et al. 2019a). Right panel: Comparison between the trends of the surface brightnesses of [C II] (green circles with error bars) and the scaled FIR (black circles with error bars) luminosities, as a function of dust temperature. The green and black lines are the power-law fits to the Σ_[C II] and Σ_FIR as a function of T_dust, respectively.