The last time I tried to count the number of times objects had been claimed to be the first dark galaxy candidates, I stopped at ten because I got bored. Today's paper adds another one to the list.
To be fair, not all dark galaxy claims are equal. Some would say a galaxy only counts as dark if it really has no stars at all, others that it just needs to be sufficiently dominated by its gas and/or dark matter. Others would insist that it has to have certain dynamics or only be found in the nearby universe*. Most would probably demand it had a primordial origin rather than just being stripped out of a galaxy, but not everyone would agree. So that list of ten could very plausibly be extended or contracted considerably.
* Pretty much everyone agrees that galaxies started off as dark, so we accept that dark galaxies did exist at one point. The controversy is over whether any still remain dark today.
This paper concerns a very particular type of dark galaxy they annoyingly dub a RELHIC. Why annoying ? Because we also have radio relics, which are completely different beasts : they're incomparably larger and more diffuse, and have little or no direct relation to individual galaxies. These objects, on the other hand, are Reionisation Limited HI Clouds, a term coined by one of the authors that I'd urge them to stop using.
But no matter. What they report on is a very interesting object that in some ways is the sort of dark galaxy candidate everyone wants to find. One of the major reasons to suppose such objects exist at all is that they would solve the long-standing missing satellite problem, whereby simulations produce far more galaxies than are actually observed. The idea is that while the physics of gravity is pretty simple, the physics of star formation is anything but. So maybe these objects do exist, it's just that they've never formed stars. This is now the widely-accepted explanation for missing satellites – indeed, arguably there isn't such a problem at all any more, as (in some models) the only such "dark galaxies" are now so small and so lacking in stars that we wouldn't detect them.
One of the complexities of the physics behind these is that of the Epoch of Reionisation. The first stars as thought to have been super-powerful monsters powerful enough to ionise most of the gas in the early Universe, heating it to the point where it would be driven out of the smallest galaxies completely. Models show that below a critical mass threshold, galaxies would lose all of their gas and never form any stars at all. It's not quite a sharp cut-off, with some near or slightly above the threshold able to form some stars before reionisation brought the process to a permanent halt, but it's close.
Such objects are thought to be extremely difficult to find. Their HI masses should be only a few million times the mass of the sun, about a hundred times less than a typical dwarf galaxy. And their line widths might be only 20 km/s or less, barely wider than the HI line itself . In principle these objects could be almost numberless, just bloody hard to spot. In contrast, most dark galaxy candidates that hit the headlines are much bigger, and usually by the author's own admissions fairly exceptional – massive dark hulks that are relatively easy to find despite being so rare as to indicate little or nothing about how most galaxies form.
Here the authors present a candidate discovered with China's mighty FAST telescope. In contrast to this awful RELHIC term, I can't fault them for the name of this particular object : Cloud 9. Yes, really. Apparently this was first reported in 2023 but I seem to have missed that paper when it came out.
Here they report on deep Hubble images and confirm that it's really, really dark, with no more than a few thousand solar masses of stars against it's million or so of gas. That, together with its line width of just 12 km/s (!) and small size (1.4 kpc radius), with an estimated halo mass that's extraordinarily close to the mass threshold, make it a compelling RELHIC candidate. It's certainly one of the darkest objects ever found, which is always a pretty cool thing to find.
But just how good, exactly ? My verdict would be... yeah, this one's pretty interesting. Is it definite ? By no means. But it's a good candidate, and absolutely needed to be published.
Glancing at the discovery paper, it seems that Cloud 9 is a little over 100 kpc from M94 itself, with HI clouds closer to the galaxy that are clearly some form of debris. 100 kpc is quite far, but only a few times the size of a large galaxy, and certainly there are many extended streams known which are much larger than this. So this cloud could be a leftover far-flung bit of debris as well, but like Cloud 6 in Leo, it doesn't really fit the general pattern of the other clouds.
A perhaps more serious difficulty would be that estimating the total mass of the feature must be extraordinarily difficult. HI tends to be found at ~10,000 K, corresponding to a line width of 10 km/s. A width of 12 km/s tells you pretty much nothing beyond the thermal state of the gas, so inferring its dynamics from this is... well, my worry is that you simply can't get a meaningful estimate when things get this narrow. Even if there was no dark matter here at all, the line width wouldn't get much narrower because this is about as narrow as the line can get. It's not a matter of better data in this case : nothing will help, at least not very much.
Another issue is the question of how long the cloud could survive, and conversely, how long it's been in existence. Currently its gas density is well below the threshold for star formation. Assuming it began life so small that the density would reach the threshold (so as to have always been dark), and given its expansion velocity and small size, it would have taken perhaps a hundred million years to reach its present size (without dark matter, as expected if it's just debris). In a another hundred megayears or so it'll double its and likely be undetectable. So if it's debris, we're detecting it at an unusual point in its existence, but sadly this doesn't constrain things too much.
I think this is a case where what's needed is a good set of simulations, especially given the timing constraints from the size of the cloud. What kind of interactions could affect M94 that would produce debris like this ? How often are such things formed, and are those simulations compatible with all the other data of the system ? What happens to existing minihalo RELHICS like this one in a system like this, where there's clear evidence that M94 experienced a merger – can they survive in such a place ?
This one's going to take a lot of work to answer. It would be easy to dismiss this as just another bit of HI fluff... and it might be. But it's so close to what we expect minihalos to be like that the workload might be worth it. And perhaps, just maybe, some other clouds already lurking in the data aren't the boring bits of debris we all thought they were.
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