One of my all-time favourite dark galaxy candidates was discovered by FAST back in 2023. An isolated gas cloud with no obvious optical counterpart, it also had an apparently flat rotation curve. This is the quintessential dark galaxy candidate : small, to be sure, but it would be simply unrealistic to hope for more.
Two small concerns stopped it just short of the Platonic ideal that would convert skeptics into believers. First, its flat rotation curve was marginal, because it wasn't well-resolved by FAST. The 500m telescope is enormous, but to see structure properly, you need something like the multiple-kilometre baselines of the VLA. Second, the optical data of the area wasn't especially deep, and it was always possible that a faint optical counterpart might be lurking there.
Earlier this year came two papers in quick succession, both targeting this same object. The first used deep optical imaging to search for faint stellar emission, while the second used the VLA to better resolve the HI detection itself. Both came to the same conclusion. This isn't a dark galaxy, it's just an unusually faint object.
The first paper is straightforward enough. They pointed a relatively small 1.4m telescope at the gas cloud for a whopping 27 hours to get really deep data, and lo and behold, a galaxy appears ! A small, faint one, to be sure, but it's very clear. It's a little bit offset from the position determined by FAST, but this is entirely consistent with FAST's big beam. And then, just for good measure, they observed this for three hours on a much larger 6m telescope to get the optical redshift... and bingo, it matches that of the HI cloud. So this is definitely the optical counterpart, no ifs or buts.
The odd thing is that the stellar mass of this object is much higher than the upper limit FAST determined could evade detection. Based on the discovery paper, I said that this would almost certainly turn out to be something really interesting, but the stellar mass of the counterpart turns out to be almost exactly ten times greater than what FAST predicted could be there. This makes it unusually faint, but by no means exceptionally so. The authors don't care to venture a guess as to why the upper limit from FAST was so much more optimistic.
The second paper arrives at the same conclusion by a different method. The better resolution of the VLA means they can more accurately locate the centre of the gas cloud. They don't have the same deep optical data as the first team, but by carefully stacking PanSTARRS data, they can do well enough. And lo, the same galaxy emerges from the noise. They estimate the stellar mass to be about half that of the first team, but factor two variations in stellar masses are quite normal at the best of times – and with the noisier optical data this group have to work with, this isn't surprising at all. It's still much more massive than the original upper limit.
The VLA data doesn't show the same flat rotation curve as the FAST data did, but it does clearly show that the gas has ordered motions. That's actually the more relevant factor, demonstrating that it is indeed rotating and so likely stable. So again, it's definitely a galaxy, and a pretty normal one at that.
To be fair it is notably faint. The second team are open to a bit more speculation than the first, suggesting that this might be something similar to the notorious Ultra Diffuse Galaxies but a bit smaller. And it's true that while being a bona fide dark galaxy would have been way cooler, it's still important to study objects like this : they still raise the question of what keep star formation suppressed, albeit not quite so dramatically as originally suggested.
Nor does this rule out the notion of "dark galaxies" more generally. What I think we can rule out, pretty definitively, is the idea of massive dark galaxies. There may very well be absolutely no truly dark galaxies as massive as our own Milky Way (although for sure there may be some very faint ones), or even a fraction of our mass. Okay, sure, in all the unimaginable vastness of the cosmos, there might be one or two hiding somewhere. But as a population, this is something I think we can now safely rule out.
What we can't and shouldn't rule out is the idea of dark minihalos. The reason dark galaxies originally gained traction was to explain the "missing satellite" problem of the Milky Way, where the very smallest dark matter halos never accumulate enough gas to form stars. This would explain the huge discrepancy between theory and observations – and these kinds of dark galaxies are still very much permitted by the data. Indeed, it was only a few theoretical models that allowed for massive dark galaxies at all, so dark satellites remain as plausible now as they did 30 years ago.
Finding those, however, is turning out to be bloody difficult. The challenge, I think, lies mainly on the theoretical end, in proving whether we really expect any of them to be detectable with radio telescopes at all – and if not, in establishing some other way of verifying their existence. At the moment they feel a little bit unfalsifiable : not to a fatal extent by any means, but to a degree where we should definitely be going "hmm" and doing a good bit of head-scratching. Nothing wrong with that... it wouldn't be research if you knew what you were doing.
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