The subthermal excitation of the C{\sc i} lines in the molecular gas
  reservoirs of galaxies: its significance and potential utility
  We examine a sample of 106 galaxies for which the total luminosities of the
two 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 is
deduced from the CI(2-1)/(1-0) ratios ($R^{(ci)}_{21/10}$) modelled by the
exact solutions of the corresponding 3-level system, without any special
assumptions about the kinematic state of the concomitant $\rm H_2$ gas (and
thus the corresponding line formation mechanism). This non-LTE excitation of
the CI lines can induce the curious clustering of (CI,LTE)-derived gas
temperatures near $\sim $25 K reported recently by Valentino et al. (2020),
which is uncorellated to the actual gas temperatures. The non-LTE CI line
excitation in the ISM of galaxies deprives us from a simple method for
estimating molecular gas temperatures, and adds uncertainty in CI-based
molecular gas mass estimates especially when the J=2-1 line is used. However
the $\rm R^{(ci)}_{21/10}$=$\rm F(n, T_{k})$ ratio is now more valuable for
joint CO/CI SLED and dust SED models of galaxies, and independent of the
assumptions used in the CO radiative transfer models (e.g. the LVG
approximation). Finally we speculate that the combination of low ratios $\rm
R^{(ci)}_{21/10} \leq 1$ and high $\rm T_{dust}$ values found in some extreme
starbursts indicates massive low-density molecular wind and/or circumgalactic
gas reservoirs. If verified by imaging observations this can be a useful
indicator of the presence of such reservoirs in~galaxies.
