More criticism of that galaxy without dark matter

More criticism of that galaxy without dark matter again. Unfortunately, while some other critiques are legitimate, this one is just plain silly.

The first problem we see is that, beside the positional proximity in the sky, there is no other evidence that the globular clusters used to derive the velocity dispersion are physically bound to the galaxy.

Umm... you mean other than the fact they're at an almost identical velocity to the galaxy ? Because that's pretty compelling evidence. I'd say it's virtually certain that they're associated.

Neither is there any evidence that the system is virialized and isotropic.

Other than the fact that it appears to be spherical.

What if, because of anisotropy, the line of sight velocity dispersion is not representative of the true value... it could perfectly be the case that the globular cluster system studied by van Dokkum and collaborators forms a flattened disk with substantial rotational support and is being observed close to face-on, as the image of the galaxy itself might suggest.

It could, but disc galaxies rarely if ever look that smooth. Possible though.

And what if the system is not virialized? Of course, if not virialized, the velocity dispersion of the clusters is not representative of the mass of the galaxy and the claim is invalidated.

Sure, but then why are the velocities of the clusters so damn similar to the galaxy ? That makes no sense.

Thus, according to van Dokkum and collaborators, this galaxy is unique in at least three different aspects: it is the only galaxy know not to contain dark matter, it is the only galaxy know to have such an extremely large number of globular clusters, and it is the only galaxy hosting a population of globular clusters not obeying to the universal luminosity function. All this supported only by their own claim that the selected clusters are physically related to the galaxy.

Oh lordy. This is trying to say that the galaxy is too weird to exist, so therefore it doesn't. Well, it does. And the globular clusters are clearly associated with it. You can't dismiss weird objects just because you don't like them. Outliers do occasionally overthrow theories, but more often they just point to incompleteness : implicit assumptions that were previously made that don't really say much about the fundamental mechanism proposed by the theory.

Any scientist knows that doing statistic with such a small number of measurements is dangerous, and not sufficient to base an exceptional claim on. Nevertheless, the authors went on to calculate the dispersion and instead of doing the strait forward calculation, they used a biweight approach that basically kicked out one cluster (as any one can see in fig 3b of their Nature paper). They basically neglected the velocity of cluster 98, precisely the value that disagrees with their claim!

Right, but these criticisms have already been made and the author's have responded (reasonably well, in my opinion).

'Tis true that the small numbers are a concern. But the other globular clusters are so close to the systemic velocity of the galaxy, it's damn hard to believe that one outlier is anything other than an outlier. There's also a whiff of too many explanations here : a determination to show that because there are so many other possibilities, it's surely more likely that one of them must be correct and therefore downgrade the probability of the original claim. This is a lot like climate denier strategies to say that global warming is either a hoax, natural but not significant, natural but beneficial, etc... anything besides the claim that warming is real, artificial and harmful.

Of course the problem is that having lots of crappy alternatives is no match so a single really good option. Here, the authors first imply the globular clusters aren't associated with the galaxy, then that they are associated but the system isn't stable, or is stable but measured from the wrong angle, or is a stable sphere after all but the statistics are wrong. This is not a sensible critique, especially because none of them are discussed in any detail. Without some estimate of the probabilities of the alternatives - even just a very hand-wavy one - then this amounts to little more than making stuff up. It's been long enough since the original to do some crude estimates of the different options. The original paper may not be a watertight piece of statistical analysis, but it's a damn sight better than not doing any at all.

Having said all the above, we cannot avoid to ask ourselves how could such a result have been published at all, wondering how a similar paper would have been received if the conclusion were the other way around. This points to the responsibility of journals that at present adopt standards orders of magnitude lower to publish results favouring the dark matter hypothesis compared to the ones required to papers claiming the opposite. Our sad conclusion is that science cannot progress this way.

Utter nonsense. The critics didn't even bother to check that these criticisms have already been raised. I was going to say that this kind of approach belongs on a blog, but van Dokkum's blog is far better than this !

Much more interesting, I think, is that the velocity dispersion of this object is so low it's amazing it's (apparently) spherical at all - there hasn't been time for the stars to have more than a couple of orbits. No-one seems to want to mention that.

https://arxiv.org/abs/1805.04817