Phase plots of T _gas vs. the number density of the three C⁺ colliding partners for the t = 170 Myr snapshot (see also Figure 5). Here L _CII is weighted with the abundance of each colliding partner. From left to right, each panel shows the number densities of atomic hydrogen (H I), molecular hydrogen (H₂), and electrons (e). The black solid lines correspond to the critical density of each partner vs. T _gas (Goldsmith et al. 2012). We find that collisional de-excitation is negligible.