Following on from those couple of papers on possible dark galaxies, comes... another paper on dark galaxies !
This one is a completely different sort of beast. While identifying optically dark galaxies is normally done by looking for their gas instead of their stars, here they use good old-fashioned optical telescopes instead. Even weirder, having found something which is optically faint but not dark, they then go on to infer its dark matter content without measuring its dynamics at all !
If this all sounds very strange, that's because it is. It's by no means crazy, but it must be said that some of the claims here should be taken with a very large pinch of salt.
Let's go right back to basics. A good working definition of a galaxy is a system of gas and/or stars bound together by dark matter. True, there are some notable exceptions like so-called tidal dwarf galaxies, but it's questionable whether we shouldn't drop the "galaxy" for those objects altogether (maybe replace it with "system" or something instead). Clearly they're physically very different from most galaxies, which are heavily mass-dominated by their dark matter.
A dark galaxy, then, is just a dark matter halo with maybe some gas but definitely no stars. Or is it ? For sure, if it really has literally zero stars, then such an object would definitely count as a dark galaxy. But what if it had just one star and billions of solar masses worth of dark matter ? Would it really be worth getting hung up on that point ? Presumably the physics involved in its formation would be basically the same as a truly dark object.
Generally speaking, most people would allow an object to qualify as a dark galaxy even if it had some small mass in stars. At present there's no strict definition, however, and so few candidate objects are known that setting a quantitative limit wouldn't really help. Right now, we don't know nearly enough about the physics of the formation of such objects, and indeed the jury's still out on when any of them exist at all.
(Some people prefer the term "almost dark", which annoys me intensely. I prefer to call them dim when they have some detectable stars, but it hasn't caught on).
Anyway, you can see how this explains using an optical telescope to search for dark galaxies. But actually, here they go a step further. Rather than looking for the ordinary stellar emission from galaxies, which are normally in diffuse discs, they look only for the light emitted by the compact, relatively bright globular clusters. Most galaxies have these dense starballs which orbit around in their halos quite separately from their main stellar disc. What these authors are looking for are cases where they find groups of globular clusters without an accompanying disc : essentially, star clusters orbiting all by themselves in their dark matter halos.
This is an interesting grey area in terms of calling something a dark galaxy, but I'd be inclined to say such objects would qualify. The physics at work in forming dense globular clusters and the diffuse stellar disc is quite different, so at the very least, these would certainly be extremely interesting.
Here they present the imaginatively named "Candidate Dark Galaxy 2". Really ? Yes, really. That's the name they're going with. Bravo, team.
(Actually, snarkcasm aside, this is a wee bit insulting, considering that there have been many candidate dark galaxies over the years, but I'll let that pass).
It turns out they had a previous candidate (you can guess the name) which is even more extreme than this one. CDG-1* consists of four globular clusters in close proximity to each other with no detectable diffuse emission between them at all. I won't attempt to discuss the complicated statistical methods they use to identify globular clusters without parent galaxies; at the words "trans-dimensional Markov chain" my eyes glazed over anyway. I can safely mention a few points though : 1) They don't have spectroscopic measurements of the globular clusters so they can't robustly estimate their distances*; 2) Their initial catalogues of globular cluster candidates are surely incomplete, but 3) Since they do careful inspection of the candidate cluster groups they do find, we can be confident that the associations they identify are real.
* I honestly can't remember if I heard about this at the time or not. I may have missed it or just forgotten about it.
* Spectroscopy gives you velocity, which is a very powerful constraint on (though not quite a direct measure of) distance.
CDG-2 initially consisted of three globular clusters, but here, using new data from Hubble and Euclid, they identify a fourth. While they still don't have spectroscopy, the new data confirms that the candidates are all unresolved. That means they cannot possibly be close objects, and in fact their colours and other parameters are consistent with their being in the Perseus galaxy cluster* at 75 Mpc distance. So it seems very unlikely that they're either significantly closer or further away. And while their might be a few free-floating globular clusters in Perseus (ripped off their parent galaxies by tidal encounters and the like), it's not very likely that they'd happen to be so close together.
* This can sometimes get very confusing. A globular cluster is a cluster of stars that orbits around a parent galaxy; a giant galaxy might host, say, several dozen such objects. A galaxy cluster is a whole bunch of galaxies, each with their own population of globular clusters, all swarming around together.
The killer argument that this is highly likely to be an actual galaxy, though, is that here they detect diffuse stellar mission between the globular clusters. The thing just looks like a galaxy, albeit an extremely faint one. The chance of a tidal encounter creating something like this isn't worth considering.
Ahh, but is it a dark galaxy ? That's where things get a lot more speculative. While we can be pretty sure about the distance of the object and their physical association, only spectroscopic measurements would really give a good handle on the total mass. Measuring how fast things are moving lets you infer how much mass you need to hold them together. Without this, they rely on scaling relations, extrapolating based on the globular clusters to infer a massive amount of dark matter : probably there are a few million solar masses of stars present in total, but it could easily have a hundred billion solar masses of dark matter based on the scaling relations.
These are, however, truly enormous extrapolations. Given that Ultra Diffuse Galaxies are now known which have significantly lower dark matter contents than typical galaxies, but these too have globular clusters, I'd be wary about digging any deeper into this one until they get some spectroscopy.
Even so, it's clearly a very interesting object indeed. Arguably even more interesting, however, is CDG-1, which still has no diffuse emission detected at all. Even if the extreme dark matter content turns out to be a wrong estimate, if either of them have any at all, they're still super weird objects. Hopefully when they find CDG-3 I won't be caught quite so unawares.
