The worryingly astute reader may remember that I briefly mentioned this paper back in this post last year. I didn't give it its own post then because of various problems : confusing language, poor figure labels, lack of any statement about which journal it was in, and lack of citation of a similar analysis from the same data set. Much of that is now cleared up - it's accepted in Nature Astronomy and had a press release late last year. Some figures still aren't labelled as clearly as they should and "inclination angels" wins the Typo Of The Day award, but mostly it seems in much better shape than it was. I still think they should have cited the earlier analysis, even though this is admittedly a different sample.
Previously we've seen a slew of claims that some Ultra Diffuse Galaxies (large, fluffy galaxies that are very spread out and usually quite faint) have weird dynamics. Whereas most galaxies are rotating so quickly that they should fly apart without dark matter, some UDGs are rotating exactly as their baryonic mass (that is, ordinary gas and stars) predicts. It seems that they don't need any dark matter at all, or at least, much less than other galaxies of comparable baryonic mass. One early worry was that there might be a problem correcting the inclination angles (not angels), meaning that the rotation speeds could have been underestimated, but recent data seems to make that less likely.
The galaxies in this sample are quite different. The deviant UDGs are extremely gas-rich, optically faint objects, whereas the ones in this sample are neither*. There gas content looks entirely normal compared to typical galaxies, so their star formation is - presumably - ticking over just fine. Nor are they especially faint. Indeed, their self-imposed brightness cutoff stands out quite clearly in their data, tentatively suggesting that there could be eve more massive galaxies hidden in the sample. So in essence these appear to be relatively normal galaxies chock-full of gas that's forming stars at a normal rate, but don't have nearly as much dark matter as normal galaxies (if they have any at all). And most of them aren't in groups or clusters, so interactions are probably not responsible.
* Strictly speaking we can't say if these galaxies are UDGs or not without more precise measurements of the radial stellar profile, but looking at some of the images, it's clear that they're pretty normal-looking objects.
This is getting very strange indeed. If you don't have much dark matter, feedback from stars and supernovae should make it much easier to blast apart any luckless galaxies that happened to form like that. But this hasn't happened in these cases. Why not ? The two options they suggest are either that there is an inclination angle estimation error here (if not in the other cases), so their true velocity width is actually much higher and they have dark matter after all, or that the feedback helps "flatten out" the dark matter potential during formation. So they'd still have dark matter, it would just be distributed in an unusual way.
The second option may or may not work but it would take detailed numerical studies to properly investigate (so far as I know, no such objects have shown up in existing simulations). The first option doesn't look likely either. Although they'd like to have resolved gas observations to measure the gas disc angle directly, looking at the optical images it seems very unlikely that there could be a big error in the estimated angle. They say that perhaps the presence of strong bars may have screwed things up, but again, inspecting the optical images, that doesn't seem likely either. Look, here's a few :
Those are as normal a set of galaxies that you could ever hope to meet. Except, apparently, they're just very slow.
Unfortunately they only plot the optical Tully-Fisher relation (optical brightness as a function of rotation speed). Baryonic mass would have been better, but since they're brighter than expected, there's no way including the gas mass can solve the problem.
Finally, they note that if you plot the distribution of the ratio between dynamic and baryonic matter, you get a nice Gaussian but with a tail at the low end where these galaxies live. They say this indicates they're a truly separate population rather than outliers. I'm not sure I'm convinced by that - maybe the normal population just has a non-Gaussian distribution. And in terms of their deviation from the Tully-Fisher, they look like a continuation of the general scatter, not outliers. But it's an interesting thing to plot even so.
So what's going on here ? Damned if I know. Apparently some otherwise normal galaxies just don't have any dark matter. How many more objects like this could there be ? Well, their main sample was 324 objects, of which 19 are weirdos, so 6%. That used the now-obsolete 40% ALFALFA catalogue. From the 100% catalogue size, that's about 1,850 objects - possibly more due to their brightness limit. As for such deviants affect MOND, which predicts a low scatter in the TFR, I've no idea, though I assume it wouldn't be good news given that most of these objects are quite isolated.
Spare a thought for poor van Dokkum, who had to face a barrage of skepticism over the claimed distance to his "original" galaxy without dark matter. That may be a concern for some of the objects here, but not all of them - they're just too far away for there distance to be drastically over-estimated. So regardless of whether van Dokkum's original claim stands up, it really does seem as though there's a population of very strange objects out there. Fun times !
EDIT : A very short rebuttal paper claims that it is all due to inclination angle after all, essentially saying that the galaxies could simply be not circular - i.e. they might be close to face-on, just not-disc shaped (as is quite usual for dwarf galaxies). I'm not convinced by this. No dwarf I ever plotted in the TFR showed any deviation like this, and I didn't correct for their shape either. Nor are these particularly low-mass dwarfs, and again, there seems to be cut-off in mass that's purely an arbitrary choice, hinting at more massive objects. And looking at the images, it just doesn't feel right - if this was the explanation, we ought to have heard previous similar claims, and to my knowledge we never have. Finally, the claim that the sample is biased because of their large axial ratios doesn't make any sense to me, since the authors specify they deliberately chose the sample in this way (since this is a necessary criteria for edge-on galaxies). Still, I could be persuaded with a longer paper : show me some comparable objects and simulations showing that such objects would be stable.
EDIT 2 : And because these are big, bright objects, whether they are rotating or not would be relatively easy to test. High resolution HI observations or spectroscopy with an IFU ought to give a definitive answer with little or no wriggle room.
Further evidence for a population of dark-matter-deficient dwarf galaxies
In the standard cosmological model, dark matter drives the structure formation of galaxies and constructs potential wells within which galaxies may form. The baryon fraction in dark halos can reach the Universal value (15.7%) in massive clusters and decreases rapidly as the mass of the system decreases 1,2 .
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