Comparison of the CO line ratios observed for 3 positions of the circum-nuclear disc with RADEX simulations. As discussed in the text, we ran RADEX simulations assuming a large velocity gradient with a kinetic temperature of 15 K and N_CO/ΔV = 1.9 × 10¹⁶ cm⁻² km⁻¹ s. The bullets indicate the results of these simulations for different volume density (n). We superimposed the R₂₁ line ratios computed in Table 5, and R₃₁ ratio derived from the CO(3-2) James Clerk Maxwell Telescope observations of Li et al. (2019). For the positions 15 and 28, there is a good velocity agreement, while Li et al. (2019) have found a different CO(3-2) velocity for the position 36, when compared to our measurements. This is why there are 2 points corresponding to this position. The measurement uncertainties are shown for each point. Interestingly, most R₂₁ points are incompatible with a critical density and the R₃₂ points at 2σ.