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We examine a sample of 106 galaxies for which the total luminosities of thetwo fine structure lines $^{3}P_1$$\rightarrow $$^{3}P_{0}$, and$^{3}P_2$$\rightarrow $$^{3}P_{1}$ of neutral atomic carbon (C) are available,and find their average excitation conditions to be strongly subthermal. This isdeduced from the CI(2-1)/(1-0) ratios ($R^{(ci)}_{21/10}$) modelled by theexact solutions of the corresponding 3-level system, without any specialassumptions about the kinematic state of the concomitant $\rm H_2$ gas (andthus the corresponding line formation mechanism). This non-LTE excitation ofthe CI lines can induce the curious clustering of (CI,LTE)-derived gastemperatures near $\sim $25 K reported recently by Valentino et al. (2020),which is uncorellated to the actual gas temperatures. The non-LTE CI lineexcitation in the ISM of galaxies deprives us from a simple method forestimating molecular gas temperatures, and adds uncertainty in CI-basedmolecular gas mass estimates especially when the J=2-1 line is used. Howeverthe $\rm R^{(ci)}_{21/10}$=$\rm F(n, T_{k})$ ratio is now more valuable forjoint CO/CI SLED and dust SED models of galaxies, and independent of theassumptions used in the CO radiative transfer models (e.g. the LVGapproximation). Finally we speculate that the combination of low ratios $\rmR^{(ci)}_{21/10} \leq 1$ and high $\rm T_{dust}$ values found in some extremestarbursts indicates massive low-density molecular wind and/or circumgalacticgas reservoirs. If verified by imaging observations this can be a usefulindicator of the presence of such reservoirs in~galaxies.

The subthermal excitation of the C{\sc i} lines in the molecular gas reservoirs of galaxies: its significance and potential utility