The subthermal excitation of the C 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 3P1 → 3P0, and 3P2 → 3P1 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) modeled by the exact solutions of the corresponding three-level system, without any special assumptions about the kinematic state of the concomitant H2 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 ∼25 K reported recently by Valentino et al. (2020), which is uncorrelated to the actual gas temperatures. The non-LTE CI line excitation in the interstellar medium 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 R(ci)21/10 = F(n,Tk) 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 large velocity gradient approximation). Finally we speculate that the combination of low ratios R(ci)21/10≲1 and high Tdust 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