Yo Dawg, We Herd You Like Missing Matter...

...So We Lost Some Of Your Missing Matter So You Can Miss Matter While You're Missing Matter And Then We Found It Again So You Can Miss Your Missing Missing Matter

So much of astronomy seems to consist of finding lost stuff. First, there didn't seem to be enough matter in galaxies to hold them together. This "missing matter" was deemed dark matter, and then a bunch of other lines of evidence also suggested it should be there. Like a slightly racist uncle, dark matter is somewhat reluctantly but nevertheless accepted by most astronomers as unavoidable. Realistically it's gotta be there, but we'd all be a heck of a lot happier if our uncle would stop making racist jokes we knew what the damn thing was.

Then along came a galaxy which seemed not to have any dark matter at all. Wait, our missing matter is itself missing ? I mean, come on....

An interesting implication of this, though, would be that the dark matter theory completely allows for such objects to exist. It says that there's absolutely no reason in principle that you can't just have a bunch of stars buzzing around each other, held in a cloud by their own gravitational field. There'd be some practical difficulties as to whether such objects could form in the first place but no physical reason preventing them from existing. In contrast, theories which reject dark matter and modify gravity instead would have serious problems. If dark matter is just an illusion caused by gravity making stars move differently than expected, then this ought to be the same in all galaxy-scale objects, with a few caveats of detail that aren't terribly interesting. So this galaxy of pure stars ought to be bad news for modified gravity theories, as well as really weird for the standard model.

"But wait !" cry the authors of this latest paper. "Maybe this is all just because we've got the distance to this object wrong. If that's the case, then the mass estimate would be way off."

There's been lots of previous criticism of the claims about this galaxy. Most of this I haven't found at all convincing, but this one is different. The authors of the original study explicitly stated that if the distance was wrong, the conclusions would be have to be altered. And this new study is a very careful, precise look at the distance estimates. They have five independent estimates of distance, and they all place it significantly lower than the original authors - four of them in very good agreement with each other, with the fifth just a bit higher. They show that the original team used extrapolations which, while not crazy, were not well-justified. Distance relations are complex things relying on a basic understanding of the stellar properties of the galaxy, and it's those assumptions which were at fault.

They also show that at this lower distance, the galaxy would be a pretty unremarkable and normal sort of object. Instead of having lots of different anomalies, it wouldn't have any. At most it would have a lower dark matter content than expected based on its stellar mass, but this would be rather uncertain, and it would definitely be a dark matter dominated object. That would make it far easier to explain in the standard model.

One of the other key features of the galaxy was that if the lower distance was correct, it would have to have a strongly deviant systemtic velocity (not to be confused with the velocity dispersion of the individual stars) from the general Hubble flow. To me, that originally looked like a convincing argument that it probably was at the higher distance, but here the authors show that actually lots of galaxies in this region have equally strongly peculiar velocities anyway. Although it's not in a galaxy cluster, which such weirdness if the norm, it might be at the intersection of several filaments (which are much larger, more diffuse, and harder to discern) which would also cause a strong peculiar velocity.

There was also a claim (not mentioned here) that the low velocity dispersion of the stars at high distances from the galaxy's center means there hasn't been enough time in the history of the Universe for them to do more than one or two orbits. That would make the neatly spheroidal shape of the object very strange. However, this claim is weakened by the lower distance and also, I suspect, by an incorrect estimate of the angular distance of the furthest stars.

I find the arguments in this paper very convincing and thorough. Which is something of a disappointment because this would have been a very strange object, which are always fun. A very interesting galaxy just got a lot less interesting. Boooo !
http://adsabs.harvard.edu/abs/2018arXiv180610141T