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

TL;DR

This study shows that the neutral atomic carbon lines in galaxy molecular gas are often subthermally excited, affecting temperature estimates and offering new insights into galaxy gas reservoirs, especially in starbursts.

Contribution

It demonstrates the subthermal excitation of CI lines in galaxies and explores its implications for molecular gas diagnostics and galaxy evolution studies.

Findings

CI lines are strongly subthermally excited in galaxy reservoirs.

Non-LTE excitation causes clustering of derived gas temperatures near 25 K.

Low R^{(ci)}_{21/10} ratios combined with high dust temperatures may indicate massive low-density gas reservoirs.

Abstract

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.