[C ii] /FIR luminosity surface density ratio as a function of the projected distance from the quasar optical position. The black curve represents our model of Σ_[CII]/ Σ_FIR(r) for the extended source, obtained from the analysis of the uv data as explained in Sect. 4.2. The grey shaded region indicates the statistical uncertainty on the Σ_[CII]/ Σ_FIR predicted by the model based on the uv analysis. The blue triangles show the values of L_[CII]/L_FIR calculated at different radii by integrating the extended [C ii] and FIR emission over concentric annuli. For comparison, the [C ii] /FIR luminosity ratio of the compact (unresolved) component is also shown, plotted at a representative upper limit radius of FWHM_beam/ 2, where FWHM_beam refers to the beam-size of the FIR continuum observations (Fig. 11).