It's taken a while, but Mordehai Milgrom, MOND's creator, has responded on arXiv, and he's very cross about it. He says the problem is all due to the way they measured the rotation speed. Rotation varies as a function of galacto-centric distance, so there are a number of different values one could use : the peak, the flat part, or the extrapolated value to infinity. These last two are normally the same, and this is what MOND predicts. But the previous authors used the peak value, which can be substantially different. He also notes that this sort of deviation has been seen many times before, and is well-understood to be due to using the wrong sort of rotation velocity. It's the flat rotation section, he says, which minimises the scatter in the TFR, not some other parameter.
He may well be right. But at first blush, it's difficult to believe the difference could be that strong in this case. The example rotation curve the previous authors show extends to a not inconsiderable 40 kpc, and it's still rising. It's difficult to believe that the flat part could be substantially lower than the rising part, and goodness only knows how far out we'd have to go to find it if it's still rising at 40 kpc.
Milgrom also raises a fair point in that the sample of the Ogle results is small, so what looks like a deviating relationship to them looks like a handful of a few outliers to him. That is, some of their sample lie near the standard TFR but a few are offset, so it might just be creating the illusion of a true change of slope. On the other hand, almost all of their sample are found to be rotating more quickly than the standard slope, so I think it can still be convincingly argued that there really is a change of slope here. At the very least, both interpretations are consistent with the data. And the rotation curve Ogle shows is a nice one, so outlier or not, it's far from obviously wrong. Just because a galaxy is unusual doesn't mean its data can or should be discarded; that it's an outlier from the general trend does not necessarily grant it immunity from causing problems for MOND.
So just how different can the peak and flat velocities really be ? Milgrom is right to point out that it would have been far better for Ogle et al. to show all their rotation curves and not just one galaxy from their sample. For comparison, he references this paper*, which clearly shows a very different set of curves : generally, they rise steeply in the very centre, go a bit crazy for a short radius (a few kpc), and then quickly stabilise at levels a bit below their peak. The one Ogle et al. show is one is nothing like those. True, they're measuring the curve in the innermost regions (given the scale length of the disc**), but it hardly seems likely that a smoothly-rising curve over 40 kpc (!) is likely to plateau at a much lower value, given that it never even peaks at all in the measured radius.
* He also references this one, which plots the overall relationship between the different velocity measures. But I found the relevant figure hard to interpret, as it appears to show an almost perfectly linear relationship between maximum and flat velocities, with only miniscule deviations from a 1-1 correlation. Perhaps I'm reading it wrong.
** EDIT : Milgrom says that Ogle et al. measure the rotation at less than one scale length of the disc. This appears to be simply wrong : Ogle measure the rotation out to a radius of 40 kpc, whereas they say the scale length is 22 kpc. I also wonder how accurate this scale length is, as it seems exceptionally large while the galaxy doesn't look all that unusual.
Milgrom further points out an earlier paper of massive galaxies, in which the fastest rotator in that sample has a maximum speed twice as high as its flat value. But that peak, and also in the case of the few others which vaguely resemble it, is found in the innermost few kpc or so, and the curve is already almost flat by 20 kpc, never mind 40. And that particular galaxy is almost face-on, which makes rotation hard to measure (though I'm not going to dig so deeply as to check how accurate the values are). The next two fastest rotators in the sample have ratios that only differ by 20%.
So when Milgrom says that he expects the even faster rotators in Ogle to show even higher ratios, I think he's making a completely unjustified extrapolation. I don't see any evidence at all that faster rotators have different max/flat speed ratios; the various curves presented don't really resemble each other very much. Most galaxies show the curve going a bit wild in the innermost few kpc, not a few tens of kpc. His argument would be a lot more convincing if he gave an example of a galaxy with a peak velocity at several tens of kpc which then substantially declined, but as far as I know, no such galaxies are known to exist. So Ogle's data looks just fine to me, and it's Milgrom's comparison sample that doesn't stand up.
Fast-rotating galaxies do not depart from the MOND mass-asymptotic-speed relation
Ogle et al. have fallaciously argued recently that fast-rotating disc galaxies break with the predictions of MOND: the 6 fastest rotators of the 23 galaxies in their sample appear to have higher rotational speeds than is consistent with the MOND relation between the baryonic mass of a galaxy, $M$, and its `rotational speed', $V$.
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