"Yes," say I. "But don't forget, other, similar candidates have also emerged, and it's much more difficult to explain them by dodgy distance measurements. So this new paper may still be relevant even if it's about the wrong galaxies."
How weird are these objects ? It depends on which model of cosmology you adopt. The vast majority of galaxies appear to be rotating so quickly that they ought to fly apart. The standard model says, "okay, that's weird, there must be some undetected enormous mass holding the galaxies together." Modified gravity theories say, "nope, there must be a problem with our current theory of gravity."
It's perfectly possible in standard cosmology for dark matter and stars to remain separate, albeit unusual. But in modified gravity cosmologies, if you have two galaxies which are basically identical (same mass and same mass distribution), then they should always show the same rotation speed since gravity should always work in same way. For one of them to be rotating much more slowly - as in these objects - is seriously weird. A significant caveat is that some modified gravity theories depend strongly on the large-scale distribution of material, so it's not really as simple as that - galaxies close to other galaxies can be expected to show the low rotation speeds.
Even though it's possible (in the standard model) to separate dark matter and stars, this is still really weird for objects as large as these. This has prompted claims that actually the distance measurement of these galaxies in incorrect : if they're far away then they'd be weird and lack dark matter, whereas if they were closer they'd be perfectly normal and dark matter dominated. I've become persuaded that the latter explanation is probably correct.
(Or, if you prefer, if they're close by then their dynamics are typical of galaxies regardless of which model you adopt, whereas if they're far away then their dynamics are much more unusual.)
The problem with this, though, is that if the galaxies are nearby then they must be moving with strong peculiar motions along our line of sight. That is, they deviate from the overall large-scale flow (the Hubble expansion of the Universe). This is common enough in massive groups or clusters, where the mass of other galaxies can accelerate them, but it's less obvious how this could happen for more isolated objects. So does that in itself challenge the standard model ?
According to this paper the answer is "yes", but I would tone this down to a cautious "maybe". They search the latest state-of-the-art simulations for similar galaxies and assess how frequently such objects are found. This is a common strategy, but as always, the devil's in the statistics : just what is a fair comparison in this case, and what does probability really mean ?
So far as I can tell, they assess probability by looking at the fraction of galaxies with similar parameters (size, velocity, and both together) of galaxies in a very generous mass range (40 million to 4 billion solar masses). On the one hand, this high range gives a huge scope to find similar objects, but on the other hand, if such objects are only common over a small mass range, then this could be misleading as to how common they really are. They explore this over a range of distances, correcting - I think - for the respective change in properties that implies. That's more reasonable, and a nice way to show which sorts of objects are really more common.
They find that if the parameters are restricted to just the structural properties, then such galaxies are indeed most common at the lower distance estimates. But if their peculiar velocities are used instead, then they're much more common at the higher distances. Overall, the latter dominates, so such objects are actually more compatible with the standard model if they're far away. In other words, normal objects moving weirdly are harder to explain than weird objects moving normally. Galaxies without dark matter are less contradictory to the standard model than normal galaxies moving against the general Hubble flow.
But then again, these galaxies have a weird population of globular clusters too. Taking that into account as well, the result switches back : galaxies with similar structures, velocities and globular clusters all together are more common in the simulations at low distances. So we're back to ordinary, dark matter dominated galaxies moving weirdly.
How improbable are such objects ? That's where it gets tricky, not least because the results are strongly dependent on which simulation is used. In the best case, such galaxies account for about 1 in 10,000 of the overall population, while in the worst case they make up more like one in a million.
Given that we're talking about a total galaxy population in the visible universe measured in the billions at the very least, are these numbers worrying ? Maybe. As far as I can tell, they examined only one fixed region in the simulations, extending to their maximum allowed distance of 20 Mpc. This means that if such objects are more common elsewhere then they won't be detected.
That's probably a small effect though. What's more problematic is that they don't discuss the formation scenarios of the rare matching objects that they do find. They could, I suppose, be formed purely by happenstance, but if there's a physical mechanism at work then they could be very much more common in certain conditions. So the low probabilities may not mean anything : if the real objects do indeed have similar environments, then this would be a powerful vindication of the models, not a refutation. Worse of all, because their are two (real) galaxies, they multiply the probabilities together to say that it's fantastically unlikely that we'd really detect a system like this. But you can only multiply probabilities if they're independent, which they have not demonstrated either for the simulation or observational reality.
So overall I'm not at all convinced these galaxies are going to be a challenge for the standard model. It's true that the peculiar velocities are worthy of further investigation, but those who challenge the standard model have this odd habit of saying, "the model can't explain this" without actually checking if this is true. Which is a shame, since they've already identified similar objects in the models, so it would be relatively straightforward to investigate them.
The model-dependent result is also important. These simulations are very fancy, but that doesn't mean they're flawless. Indeed, their staggering complexity is a weakness. It's entirely possible and credible that their input physics will need substantial improvements before we can even use them to make sensible comparisons to reality, without needing to change the basic dark matter paradigm. I don't think this result is wrong so much as I think that it's simply premature.
The ultra-diffuse dwarf galaxies NGC 1052-DF2 and 1052-DF4 are in conflict with standard cosmology
Recently van Dokkum et al. (2018b) reported that the galaxy NGC 1052-DF2 (DF2) lacks dark matter if located at $20$ Mpc from Earth. In contrast, DF2 is a dark-matter-dominated dwarf galaxy with a normal globular cluster population if it has a much shorter distance near $10$ Mpc.
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