In this paper Joe Silk chimes in with his own explanation. It's not a catastrophically crazy idea, but I have to say the presentation of the idea is terrible. It lacks any kind of narrative flow, with facts being disconnected from each other and seemingly plucked out of the air at random, with the same subject being oddly split across multiple short paragraphs. Worse, he concentrates on the least interesting aspects of the system, which is very frustrating And there are typos too, just for good measure. Honestly, I find it difficult to believe this paper would have been accepted in its current form if its author wasn't already an established leader in the field.
The two objects are galaxies of comparable size to the Milky Way but with far fewer stars. The most remarkable feature is that whereas the stars and gas in the Milky Way are rotating around the centre at about 220 km/s, the stars in these systems are buzzing around randomly at a leisurely 10 km/s or so. That pretty much rules out any sort of significant extra mass component : there's enough mass in the visible stars alone to account for the motions. If there was extra mass, the stars would necessarily be moving faster. The only way to avoid this would be if the distance had been estimated incorrectly, but independent measurements have more or less confirmed the original estimate. So they really do seem to be devoid of dark matter.
There are two possibilities. Either these objects have been like this since their formation, or they started off as more normal galaxies but lost their dark matter somehow. Now we know some galaxies (or galaxy-like objects at any rate) can be produced by close encounters, where tidal forces strip away large amounts of stars and gas that can become gravitationally self-bound. But it doesn't look awfully likely that that explanation is correct is this case. And it's hard to see how objects like this could have formed directly : their stars are moving so slowly that there just hasn't enough time for them to become the smooth spheroidal objects we see. They'd also be extremely vulnerable to any interactions with their more massive neighbours.
Silk's idea is that they formed like a mini version of the Bullet Cluster. Instead of colliding two clusters of galaxies, Silk postulates that two dwarf galaxies had a (fairly) high-speed collision. But it's not at all clear to me how the hell this is supposed to work, because he goes off on one about the relatively minor details of its unusual globular star cluster system and chemical composition, and - for some reason - some random speculations about intermediate mass and primordial black holes. Oh, and he refers to, "the simulations" in a way that's horribly unclear as to what he's referring, i.e. other people's existing simulations or his own. I just wanted to shake the man and shout, "TALK ABOUT THE KINEMATICS ! THEY'RE THE INTERESTING FEATURE ! DAMMIT MAN, NO-ONE CARES ABOUT THE METALLICITY !".
What would I intuitively expect to happen in a dwarf galaxy high-speed, head-on collision ? I guess the gas would go splat* and form some weird starry structure in the middle. I also know from first-hand experience that the displacement of the gas, if sufficiently massive, would cause a strong distortion of the pre-existing stars. There would, for sure, be some very nice fireworks. What I cannot see is how this is supposed to remove the dark matter and leave behind at least one (possibly two) nice spheroidal swarms of lazy stars. This badly needs a simulation or at the very least an illustration as to what the heck is supposed to happen, because currently it feels more like, "randomly making stuff up" than an actual paper.
* Or squelch. Or splwergh. Or schwpoof. Perhaps flwhooop.
Ultra-diffuse galaxies without dark matter
I develop a high velocity galaxy collision model to explain a rare but puzzling phenomenon, namely the apparent existence of ultra-diffuse galaxies with little dark matter.
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