Distribution of the derived τ(JF₁ F) = (123 − 012) opacities as function of the estimated total N₂ H⁺ column density in the ISF. The plot represents the kernel density estimates of >25 000 individual components extracted from our ALMA observations. The last contour encloses 95% of the components fitted in our spectra. For comparison, we overplot the different RADEX predictions for an isothermal gas at kinetic temperatures of 10 K (red), 20 K (green), and 30 K (blue) at densities n(H₂) between 10⁵ cm⁻³ (dashed lines) and 10⁸ cm⁻³ (solid lines), all calculated assuming a constant linewidth of Δ V = 0.7 km s⁻¹.