The subthermal excitation of the C{\sc i} lines in the molecular gas reservoirs of galaxies: its significance and potential utility
The subthermal excitation of the C{\sc i} lines in the molecular gas reservoirs of galaxies: its significance and potential utility
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星系分子气库中C{sc i}线的次热激发：其意义和潜在用途
DOI：
10.1093/mnras/stab3194
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
S. Maddox
中科院分区：
文献类型：
--
作者：
P. Papadopoulos;L. Dunne;S. Maddox
我们研究了106个星系的样本，其中中性原子碳(C)的两条精细结构线$^{3}P_1$$\rightarrow $$^{3}P_{0}$和$^{3}P_2$$\rightarrow $$^{3}P_{1}$的总光度是可用的，发现它们的平均激发条件是强亚热的。这是从CI(2-1)/(1-0)比率（$R^{(ci)}_{21/10}$）推导出来的，该比率由相应的3级系统的精确解建模，而没有对伴随的$\rm H_2$气体的运动状态（以及相应的线形成机制）进行任何特殊假设。这种CI线的非LTE激发可以诱导（CI，LTE）衍生的气体温度在$\sim $ 25 K附近的奇怪聚类，这是Valentino等人（2020）最近报道的，这与实际的气体温度无关。星系ISM中的非lte CI线激发使我们失去了一种简单的估计分子气体温度的方法，并且增加了基于CI的分子气体质量估计的不确定性，特别是当使用J=2-1线时。然而，$\rm R^{(ci)}_{21/10}$ = $\rm F(n, T_{k})$比值现在对于CO/CI SLED和星系尘埃SED联合模型更有价值，并且独立于CO辐射传输模型（例如LVG近似）中使用的假设。最后，我们推测，在一些极端恒星爆发中发现的低比例$\rm R^{(ci)}_{21/10} \leq 1$和高$\rm T_{dust}$值的组合表明了大质量的低密度分子风和/或星系周围的气藏。如果通过成像观测得到证实，这可能是星系中存在此类储层的有用指示。
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.