Of all the candidate dark galaxies that have come and gone over the years, this new one quite likely deserves to jump straight to the top of the tree.
Apparently there's a dedicated filler project at FAST to search for dark galaxies, though the authors don't say very much about it. It's found "some" candidates, but this one is so good that I guess the others just aren't worth mentioning yet.
What makes it so compelling ? It's isolated. Like, very, very isolated. They don't actually emphasise this much, but a cursory NED search (and examination of WWT images) shows there's bugger all nearby. The object is about 29 Mpc away and there's no other galaxies at all within a 500 kpc search radius, at least with known optical redshift measurements. And that's about as isolated as it's ever going get. And the optical images, as a sanity check, don't show any obvious galaxies anywhere nearby. That immediately makes the popular "debris" explanation incredibly hard to sustain in this case.
The object itself is a hydrogen cloud of about 80 million times the mass of the Sun, a few times larger than the ones I've spent an inordinate amount of time investigating in Virgo. Its spectrum appears to have a hint of a double-horn profile that's characteristic of rotating discs, which is backed up by the position-velocity diagram which looks quite a lot like the standard flat rotation curves which are associated with normal, dark matter-dominated galaxies. Optically, it appears to be completely dark : not just dim, but nothing there at all, whereas typical galaxies with this much hydrogen ought to be readily detectable at optical wavelengths.
This means it's ticking all the boxes : it's detected in radio but not other wavelengths, it's isolated, it's got a flat rotation curve. Numerically its estimated dynamical mass is a few tens of times greater than its hydrogen mass so it would have to be strongly dominated by dark matter. And estimates of the gas density put it well below the threshold above which star formation tends to occur.
In terms of caveats I'm struggling to find any. It can't just be hidden behind a nearby dust cloud because the reddening in this region is negligible. It fits the baryonic Tully-Fisher relation between mass and velocity width for much brighter galaxies. In all honesty it's pretty darn close to the Platonic ideal of a dark galaxy candidate.
Is there anything at all to spoil the excitement ? Not really. I think it's virtually certain that whatever this turns out to be, it'll be something interesting. My only slight concern is that the double-horn shape at detection of a flat rotation curve are both marginal, but that it shows both makes this more convincing. And this marginal nature isn't unexpected for a small galaxy like this one anyway : for low mass galaxies, the gas tends to only just reach the flat parts of the curve.
To me the isolation is the crucial feature here. Higher resolution gas observations ought to be able to pretty decisively determine if it's really rotating or not, and it should be easy enough to get some really deep optical data to make sure there's no low surface brightness component hiding nearby. Since the object is so small and lonely, there's no need for massive levels of surrounding data for this : just point the telescopes and go.
The only thing I can think of that would change the picture is if there were more hydrogen clouds nearby, which would point towards some much larger scale feature. But that is hugely unlikely, because if the gas came from galaxies then at least few ought to be known about, and if it's a large-scale gas feature then there's no way anyone wouldn't report this. I'm scraping the bottom of the barrel pretty hard in an effort to find cautionary notes to end on here.
In terms of excitement levels I... have to rate this one 10/10. I have to. To be blunt, in terms of research directly relevant to me, it's potentially the most exciting thing I've heard about in my whole career. There have been dark galaxy candidates aplenty over the years but none of them come close to matching this one : every single one has particular extenuating circumstances whereas this one has none of that.
The only thing that's keeping me together is the pressing need to debug code and a very very strong habituation to avoid getting excited until everyone else does, in case there's some rudimentary error in the whole thing that I've missed, or the whole thing turns out to be bunk for some reason. That's been known to happen to similar discoveries before, so fingers crosses that this is one that's genuine.
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