While both objects have been mired in controversy, the dark matter deficient galaxy (NGC 1052-DF2) has stood up well to repeated scrutiny. Or, to be very strictly accurate, its measured dynamics are quite definitely different to other galaxies of comparable size and stellar mass. The stars in the Milky Way are rotating around the centre of the disc at ~200 km/s, so fast that without an enormous amount of dark matter to bind them together they ought to quickly be flung out into intergalactic space. Those of the galaxy without dark matter are sluggish, lazily milling about in random directions at a mere 10 km/s, like a swarm of partially anaesthetised bees. There's no need for this object to have any dark matter at all - indeed, if it did, the stars ought to be moving an awful lot faster. It's a simple relation : Mass => acceleration => speed.
On the other hand, the stars in Dragonfly 44, which seems to have too much dark matter, were previously estimated to be moving at about 47 km/s. You might think that implies it should have some dark matter, but quite a bit less than the Milky Way and certainly not an unusually high amount as was claimed. And you'd be right. Well, sort of.
When it comes to estimating total mass you need size as well as velocity dispersion, and "ultra diffuse galaxies" like these are very difficult to measure in their outermost regions (actually, exactly how far they really extend is a bit of an under-discussed issue in my opinion). But the main point here is that you should try and make a fair comparison : i.e. see how much dark matter they have at a fixed,well-defined point, rather than extrapolating to much larger distances to get the grand total. Dragonfly 44 might indeed have a huge total mass, but this relies on making an extrapolation to distances far beyond the stars we can actually see and measure. As I pointed out when it was discovered, the authors were very clear about that in their paper, but neglected this in the press release. Ho hum.
Anyway, if you do restrict it to the range where you have direct measurements, Dragonfly 44 does seem to have more dark matter than the Milky Way at least. Or perhaps not. This paper revisits the galaxy with new observations, and they find that this might not be the case. Their previous analysis looks to have overestimated the velocity dispersion, and the new value drops to about 33 km/s. That seems to be consistent with the original data : it was an mistake in the analysis that was responsible for the over-estimate.
It now looks far less likely that this galaxy has an unusual amount of dark matter, though it does depend on what comparisons you use. If you compare it based on its stellar density then it looks a bit unusual, but not much. If you use a simpler fixed radius then it looks completely normal : it's still heavily dominated by dark matter, but not to an unusual degree compared to other galaxies with similar stellar properties. So it probably doesn't have a total dark matter content as great as the Milky Way, as was previously claimed. It's more likely to be just another dwarf, albeit a fairly large one.
But wait ! This doesn't mean the galaxy isn't interesting. The most popular interpretation of UDGs is that they are "huge dwarfs" - that is, they have total masses the same as dwarf galaxies but are rotating much more quickly and hence are more extended. In that view, UDGs are just part of the normal galaxy population - they just have rather extreme properties. The alternative is that UDGs are massive galaxies that somehow fail to form stars. That now looks less likely for Dragonfly 44... but it shows no signs of rotating - its stellar motions are completely random. Which means the fast-spinning dwarf theory doesn't seem to work either ! We might be back to square one as far as their formation goes.
And the same analysis showing that Dragonfly 44 has a normal amount of dark matter for its size also reinforces the fact that NGC 1052-DF2 apparently has no dark matter at all. What's especially strange is that these objects are similar in terms of stellar mass and size. So the central problem remains : why do some galaxies apparently have very different amounts of dark matter and/or dynamical properties ?
It also potentially poses a problem for alternatives to dark matter that replace it with modified gravity. Since the two galaxies have very similar distributions of stars, they ought to show almost identical motions, but they clearly don't. Although it's not well understood, for the dark matter paradigm it's at least possible in principle to separate stars and dark matter (this can even be demonstrated for tidal dwarf galaxies). So we can at least conceive of galaxies with similar structural parameters but different dynamics in the dark matter paradigm, even if we don't fully understand them. But you can't do the same trick if you don't have any dark matter. If dynamics are due to baryonic mass distribution, then all objects with the same baryonic mass distribution ought to have the same dynamics. And they don't.
Checkmate, modified gravity ? Not quite. The major caveat is where the galaxy lives. NGC 1052-DF2 is in a small group, whereas Dragonfly 44 is in a big cluster. While Dragonfly 44 looks to be stable and in equilibrium (it doesn't show any signs of disturbance), in some modified gravity theories the presence of external masses can change dynamics considerably, as has been suggested is the case for NGC 1052-DF2. So the different environments may pose a problem for comparing dark matter and modified gravity predictions, at least if we want to make a direct comparison of these two particular galaxies. That said, a compelling case has been made for two dwarf galaxies in the same environment that do appear to favour the dark matter paradigm.
While these new observations, then, do make the exciting "failed giant galaxies" scenario less likely, they doesn't mean that UDGs are now boring, or that the results are just too uncertain and unclear to tell us anything interesting. Indeed, they show that the leading theory of their formation is still highly problematic. And they also open the door a little further to using UDGs as a way to test different theories of gravity. I'd say they remain as exciting and mysterious as ever.
Spatially-resolved stellar kinematics of the ultra diffuse galaxy Dragonfly 44. I. Observations, kinematics, and cold dark matter halo fits
We present spatially-resolved stellar kinematics of the well-studied ultra diffuse galaxy (UDG) Dragonfly 44, as determined from 25.3 hrs of observations with the Keck Cosmic Web Imager. The luminosity-weighted dispersion within the half-light radius is $σ_{1/2}=33^{+3}_{-3}$ km/s.
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