Missing Matter Still Even More Missing, Study Finds

Last year a galaxy that seemed to have no dark matter was doing the rounds because that's freakin' weird. Virtually all galaxies appear to be heavily mass dominated by dark matter : it's arguably the best way to define a galaxy as opposed to a giant star cluster. The whole mainstream basis of galaxy formation and evolutionary theory depends on dark matter as an integral feature. While there are some cases of dwarf galaxies formed by tidal encounters that don't have much (or any) dark matter, these tend to be still embedded in the debris associated with their formation. As far as I know there are no good cases of a dark matter free galaxy just sitting there minding its own business.

If such an object were to be found, it would raise awkward questions for the standard theories of galaxy evolution but make life even more difficult for the main alternative : modified gravity. Tidal encounters between galaxies can strip away dark matter, so it's at least possible to reduce the dark matter content in standard theories (but remove it completely ? I doubt it). For modified gravity, on the other hand, any two star systems of the same size and shape ought to have the same dynamics : gravity should work the same everywhere, more or less. A nice control test where one can compare similar objects is not so easy to find as you might think, but such systems have been found - and the results don't look good for modified gravity.

Then there is this galaxy, NGC1052-DF2. This is an ultra-diffuse galaxy, meaning it's very large but with few stars per unit area. That makes it difficult to measure how fast its stars are moving, which is what you need to work out its total mass. So previously astronomers used its globular clusters, which are much brighter and easier to measure (though there are only a few of them). They found a velocity dispersion of 8-10 km/s - so low that it's consistent with the galaxy having no dark matter at all.

This made a lot of people very upset. Claims were made that the distance measurements were wrong and that would mean the galaxy was perfectly normal, but then an independent team came along and said nope, the distance measurements are correct, this galaxy really is weird.

Still, having only 10 globular clusters has always raised concerns that the estimate of the velocity resolution is reliant on small number statistics. Other teams have questioned the rigour of the claim for such a low dispersion, though in my opinion the original van Dokkum claim always looked stronger. Now, two teams have used extremely powerful instruments to measure the velocity dispersion of the stars directly. Note that both papers are still under review.

The first paper was by an independent group and came out just before Christmas :
http://adsabs.harvard.edu/abs/2018arXiv181207345E

I read this, but didn't bother writing about it because I found it rather badly-worded : I could not easily extract the main point about just how massive the galaxy is supposed to be. Fortunately the new paper (by the original team) is much more clearly written and comments on the Emsellem work.

The bottom line is that this galaxy does indeed seem to have very little or possibly no dark matter whatsoever. This is in conflict with the Emsellem claim in two ways : first, Emsellem claimed that the velocity dispersion could be much higher (13-27 km/s), whereas this paper says it's 8 km/s (just as the original globular cluster measurements indicated); second, Emsellem claimed the galaxy is rotating (albeit slowly) whereas this paper finds no evidence of that. The authors comment directly on the disagreement , noting that they aren't able to explain it. The only hint is that this latest study has a much greater velocity resolution than the Emesllem paper so it should be more accurate. And their fitted velocity dispersion profiles do seem to match the data extremely well.

As for how well this galaxy does or doesn't fit with modified gravity, as usual there's the complication of the external field effect. In modified dynamics, the presence of nearby galaxy can change the velocity dispersion in a very different way to conventional theories. Based on this, the earlier prediction was that the dispersion should be 13 km/s. This is not consistent with the new results, and only just about consistent with the Emsellem range which is actually more favourable to the galaxy having some dark matter than the modified gravity prediction.

I would expect a great deal of back-and-forth on this issue. My money's on the original van Dokkum team. Though a very strange result, it does seem to stand up to scrutiny so far. Watch this space.

https://arxiv.org/abs/1901.03711