The Physics of Cold Gas in Galaxy Low Excitation Regions
After decades of investigations, the interstellar medium (ISM) still appears as extraordinarily complex. Therefore, as a sound strategy, it is appropriate to return considering the simplest situations. The low excitation regions in or around galaxies, such as the galaxy outer disks, the inter-arm regions, the well shielded starless cores in molecular clouds, or the high velocity clouds (HVC’), appear as ideally suited to test our understanding of gas in simple conditions. A priori, regions far from star formation regions should be much less perturbed by their associated energetic effects, like supernovae explosions and subsequent cosmic rays, or protostellar jets. Another good reason to be interested by low excitation regions is that almost all the matter that shines today in galaxies is thought to have spend some time in a very cold form around 10 K. Therefore the processes that occur during this cold phase are important since they determine the fate of almost everything we can observe.
But precisely because the lack of excitation prevents various radiative emissions to occur, cold gas is difficult to observe. Most matter is then neutral and at the lowest energy levels. The restricted choice of observables that we have, mainly emissions in the spectral domain from the far-infrared (FIR), sub-mm to cm radio waves (CO and HI), or absorptions from bright sources also in shorter wavelengths, have deeply challenged our effective physical understanding of cold gas. Namely, what has been collected up to now are features that strongly contradict the bold prediction of physics about a non-self-gravitating cold gas in vacuum: such an unconfined gas has a positive energy and therefore should expand indefinitively until it meets a larger scale confiner, such as a galaxy potential. Some basic questions remain wide open about cold gas:
KeywordsStar Formation Molecular Cloud Star Formation Region Local Thermal Equilibrium Extensive Thermodynamic
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