Lights in the darkness

At long last I rouse myself from a prolonged refusal to read any papers with this one about molecular gas in the Leo Ring.

The Ring, you might remember, is a giant atomic hydrogen (HI) structure over 200 kpc in diameter whose origins remain unclear. It's of particular interest to me for two reasons. First, we have AGES data of the Ring which found a set of small, discrete clouds nearby that may or may not shed light on its origins. Second, we might be able to get time with the APEX telescope to look for molecular gas here, so today's paper was some much-needed background reading on my part.

Actually many of the authors involved in this paper were also involved in this one in 2021, which found several small patches of star formation happening in the densest parts of the Ring. Here they follow-up those patches using the IRAM 30 m radio telescope to look for molecular gas. Star formation seems to be reasonably well-behaved in typical galaxies, but nobody is quite sure if this is because of the fundamental physics of gas collapse or only because the conditions within galaxies tend to be similar. Looking at how it works in the Ring would be a great way to test this, because here there's little or no rotation and the gas density is much lower, yet the chemical composition is similar to ordinary galaxies.

I have to say I found parts of this paper a bit of a slog and some of the narrative could have been clearer. As far as I can tell, they did 11 different pointings in and around two Halpha (ionised gas) regions, but their choice of nomenclature is horrendous. They also stack the individual observations to give one equivalent to 72 hours of observing time, but don't seem to say how long each individual observation took. 

The basic result is that they don't detect anything. They have some hints of marginal detections, but with commendable honesty, they amply stress just how tentative these are. For example some of the "detections" have mismatched velocities at different frequencies, and the stack doesn't reveal a detection. Additionally, given their velocity widths the lines are brighter than the usual scaling relations suggest. All things considered, it's quite probable that they're all spurious.

Fortunately this doesn't matter too much. Their sensitivity is high enough that this becomes a genuinely interesting non-detection exercise; based on the observed star formation rates, typical clouds should have been massive enough to be detectable. So why aren't they ?

There are several possibilities. It could be that the physics of star formation is indeed different in this environment. It might be so efficient that much lower mass clouds are able to form stars, and so rapid that the onset of star formation quickly suppresses any further collapse (e.g. by stellar winds and supernovae, which would disperse the gas on local scales though would have no affect at all on the much larger scales of the Ring). Interestingly they say this could explain Ultra Diffuse Galaxies, though I suppose we'll have to wait for metallicity measurements of those objects to see if they're comparable with the patchy star formation in the Ring.

There are other options. It could be that the star-forming gas here is dominated by atomic rather than molecular gas, an idea that's been floating around for a while. Alternatively it could be that the CO they observe here isn't such a good tracer of molecular hydrogen as in normal galaxies (so-called "CO dark" molecular gas), an idea that's extremely popular with theorists. Or perhaps there is molecular gas but it's a little further away from the star-forming regions than usual, just beyond where the telescope was pointed.

Bottom line ? Something is different about star formation in the Ring than in galaxies. That's progress. I also have to give them high praise for describing their sensitivity estimates in considerable detail, and while of course detections always give you more to work with, they seem to have thoroughly exploited what they've got. But to really say what's going on here, I think needs even deeper observations at different wavelengths. Constraints can only get you so far, at some point, you need to see something.