The last few years have seen a lot of hoo-hah about galaxies apparently lacking dark matter, but this may at last be drawing to a rather boring conclusion.
First, we had a whole kerfuffle about two dwarf galaxies, NGC 1052-DF2 and DF4. These seemed to have such a low velocity dispersion that they wouldn't require any additional matter apart from their stars to hold them together. There was some dispute about the magnitude of the velocity dispersion (in my opinion not a very credible one) which went away quite quickly because yes, their velocities really were very low. A much more protracted and complex dispute arose about their distance, which was only settled quite recently and found that yes, they are far away (and lacking dark matter) after all.
Those first discoveries were pure starballs. Then, we've also had the saga of galaxies with gas, for which we can much more easily examine the rotation velocity. They were also so far away that there was no prospect of a distance controversy. But just the other day, despite many arguments in favour of these being a population of weird objects, simulations showed that such objects can't be stable. It's far more likely that there's been a slight measurement error in their inclination angle. I wouldn't personally call that one settled just yet, but it's definitely going in that direction.
There was also bad news for those of us hoping the original objects would turn out to be something truly weird : simulations found a plausible way of producing objects like this through unusual but entirely plausible encounters, specifically requiring direct collisions between galaxies, which are relatively rare. Which is great news for the boring old standard model of cosmology.
Today's paper feels like another nail in the coffin for anyone hoping these objects would turn out to be truly strange. Not the final nail by any means, and it's still just about possible they could burst back out like an enraged vampire, but it's definitely making it harder for any would-be children of the night to go on a killing spree.
It's a nice little paper but I do have to wonder why on earth it made it into Nature. Noting the simulations showing that collisions are a viable explanation, they go out and have a look at the context of DF2 and DF4 on a larger scale. They show they're in a neat line of maybe 11 objects in total, stretching over 2 Mpc. This is nicely consistent with the collisional origin hypothesis.
And honestly... that's it. Yes, it's a nice discovery, but they don't go on - as I assumed they would - to do numerical simulations to back this up. They don't have any new observations of these additional galaxies to see if any of them have anomalously low velocity dispersions. They just report the basic discovery along with their (undeniably) very interesting hypothesis. I mean, I'm glad they reported this, but it feels much more worthy of a letter to MNRAS than to Nature.
As I said though, this is not the final nail. More observations of those new galaxies are essential to determine their distances and velocities, otherwise this is just a chance alignment. More simulations to examine exactly what we expect in a collision similar in quantitative detail (major kudos to the authors for proposing this, which largely offsets publishing it in the wrong journal) would also help, a lot. But it's awfully tempting to declare the mystery... well solved is too strong a word, but shall we say, addressed then ? Yeah, that'll do. Mystery addressed !
There is also perhaps some prospect of a weird afterlife for these oddballs. If they do form through good old-fashioned collisions, they're like teeny-weeny versions of the famous Bullet Cluster. This has caused a lot of controversy because Standard Model enthusiasts have pointed and laughed at MOND believers by saying, "Hahah ! Look, we found a case where dark matter can be separated from normal matter, and you can't do that just by modifying gravity !". To which MOND devotees responded by saying, "Yes it can, you don't understand our theory at all, and look the collision velocity you need is just too dang high." But then the Standard Modellers retorted with, "Yeah, but you don't have a unique relativistic solution, and anyway, the collision velocity is way lower than we though, so pfffffp !".
Or something like that anyway. I tend to heavily favour the Standard Model, but if we had more cases to compare, especially on such dramatically smaller scales like these galaxies, this might give us a much more rigorous comparison. The thing about MOND though it's that it gives such similar results to the dark matter paradigm that one wonders what the point of it is. So, we'll see.
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