Phys.org's Mystery gas discovered near center of Milky Way links to Teodoro et al 2020 in Nature Cold gas in the Milky Way’s nuclear wind who's abstract is shown below.

Question: My reading of the Phys.org summary and the abstract tells me that the gas is known to be fairly cold and molecular, and yet it is not known what the gas is! Is it so obviously hydrogen as H2 that nobody says this explicitly? If not, how does one determine a gas is molecular without knowing the chemical nature of the gas? It seems that if one resolves vibrational/rotational features in optical, IR or radio spectra one could simultaneously identify the species, but perhaps I'm naive. (Okay, I'm almost certainly naive!)


The abstract from Nature:

The centre of the Milky Way hosts several high-energy processes that have strongly affected the inner regions of our Galaxy. Activity from the super-massive black hole at the Galactic Centre, which is coincident with the radio source Sagittarius A*, and stellar feedback from the inner molecular ring1 expel matter and energy from the disk in the form of a galactic wind. Multiphase gas has been observed within this outflow, including hot highly ionized (temperatures of about 106 kelvin), warm ionized (104 to 105 kelvin) and cool atomic (103 to 104 kelvin) gas. However, so far there has been no evidence of the cold dense molecular phase (10 to 100 kelvin). Here we report observations of molecular gas outflowing from the centre of our Galaxy. This cold material is associated with atomic hydrogen clouds travelling in the nuclear wind. The morphology and the kinematics of the molecular gas, resolved on a scale of about one parsec, indicate that these clouds are mixing with the warmer medium and are possibly being disrupted. The data also suggest that the mass of the molecular gas outflow is not negligible and could affect the rate of star formation in the central regions of the Galaxy. The presence of this cold, dense and high-velocity gas is puzzling, because neither Sagittarius A* at its current level of activity nor star formation in the inner Galaxy seems to be a viable source for this material.

  • $\begingroup$ They know is molecular because of the T. Why it is not spelled out as H2 I don't know. The same is true for the other phases. $\endgroup$ – Alchimista Aug 20 at 9:57

They know there's molecular gas because they observed emission from the CO molecule in two of the previously identified (atomic-hydrogen-emitting) gas clouds. From the paper:

... we targeted two objects (hereafter, MW-C1 and MW-C2), highlighted by red boxes in Fig. 1, in the 12CO(2 → 1) emission line at 230.538 GHz with the 12-m Atacama Pathfinder Experiment (APEX) telescope.... We mapped both clouds in 12CO(2 → 1) emission over a 15′ × 15′ field centred on the peak of the H I emission,... These data revealed molecular gas outflowing from the centre of our Galaxy.

CO emission is always assumed to come from molecular gas clouds which are, by mass, mostly H$_2$, though the conversion factor needed to turn a CO measurement into an H$_2$ measurement is a matter of debate (this is discussed in the paper).

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  • $\begingroup$ Okay thanks! Armed with both a better understanding of the assumptions and now a copy of the actual paper, I'll dive in. $\endgroup$ – uhoh Aug 22 at 12:07

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