A dark galaxy by any other name

Back in February I got very excited by the discovery of a really good, solid, dark galaxy candidate courtesy of China's FAST telescope. I haven't heard anything more about that since then so I remain excited about it.

Today's paper also comes via the FAST telescope and concerns another dark galaxy candidate. However this isn't by the same authors. It's actually based on this paper from June (which I've not read) by the FAST team, but the authors are unconnected. Basically what they're doing is saying, "hey, the FAST paper said that this might be a dark galaxy, and here's our more detailed analysis of the same data supporting that". There isn't any new data in this paper, just pure analysis. In fact I'm not sure if they even have access to the FAST data (which I would have thought was private) or just scraped it from the published images... 

Look, dark galaxies are the thing that gets me most excited about astronomy, but I have to say the more I think about it, the less I'm a fan of this particular paper. It's not that there's necessarily anything wrong with it, it's just... odd, as a piece of work.

It's not just the lack of new data and their unclear access to the original data* that irk me about this paper. They also don't cite me, even though my work would be directly relevant here. And they insist on calling "dark galaxies" RELHICs ("RE-ionisation-Limited HI Clouds").... urrgh, that's an ugly term. Most irritating of all, they use the term "isocontour". The hell is that ? Are normal contours not plotting things at the same (iso) value anyway ?

* If they had it, surely they could have reprocessed it in a few other ways and at least reported what they did, even if it didn't show anything new ? Not to do this is very strange.  

To be fair, RELHIC might be better as a term were it reserved for a very specific kind of dark galaxy : low HI mass, low rotation speed, small dark matter halos, as in this putative case. I'm not sure we really need another term for this besides the already-common "minihalo", but still.

Anyway what they do is to show that this particular object is consistent with being a RELHIC/minihalo/dark galaxy, in which the HI is in hydrostatic equilibrium. It would need a dark matter halo to be stable because the HI mass is undoubtedly far too low to keep it self-bound. And it can't be much further away or closer than the best-guess distance estimates, because that would make it an altogether stranger object.

And that's pretty much it. Even this depends quite a lot on the shape of the HI contours, and I'm a bit concerned that this circular shape - the distinguishing feature of a non-rotating system in equilibrium - is only due to the low resolution of FAST (though as they say, higher resolution would be better). All this is extrapolating quite a lot from very marginal data - I don't doubt the detection is real, only the confidence about the conclusions. 

Nor do they consider other explanations either. True, it's quite well-separated from the nearest big spiral, M94, but 70 kpc is not that far. And as we and others have shown, objects with line widths this narrow (a mere 20 km/s, only twice the width of the line itself) can be stable on very long timescales even if they're completely unbound, because by definition they're dispersing slowly. 

In short, the paper presents a perfectly valid summary of what the object could be, but it doesn't offer anything new and doesn't consider alternatives. I'd have been far happier about it if they'd at least applied this analysis to other objects and/or used at least some other data sets here. On the other hand, at least dark galaxies are getting more attention again, and that at least is something to be welcomed.

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.