Tantalisingly tidal

Here's a paper on that ever-popular topic : those two funky galaxies which apparently don't have any dark matter.

It seems that the great distance controversy has at last been settled; to my knowledge no challenges were raised to the latest measurements on that score, despite an abundance of dispute on similar previous claims. This means the galaxies are indeed very far away, so their small velocity dispersions do indeed imply a lack of dark matter. While if they were closer they would just be perfectly normal galaxies, which Occam's Razor could imply is the more likely scenario, the data really seems to rule this out.

Recall also that galaxies without dark matter do not, paradoxically, challenge the dark matter paradigm itself. If dark matter exists, then it can in principle exist independently of ordinary stars and gas. By contrast, if it's just an illusion and it's actually our theory of gravity that's at fault, then it shouldn't be possible for similar objects to have dissimilar velocity dispersions : similar objects, in similar environments, should always be kinematically similar. What they might potentially challenge instead is the nuances of how galaxies assemble, though conceivably they may just have had their dark matter removed in ordinary tidal encounters.

Searching for the effects of tidal encounters could give important clues to how these objects were formed. One such previous study clearly shows there are tidal streams of stars in the group in which our two little galaxies are likely located, but there's no sign of such features associated with the oddball galaxies themselves. Another claimed to have found characteristic S-shaped tails associated with one of them, but I was very skeptical about this because it looked marginal at best.

Today's paper uses the Dragonfly telescope to conduct a new search for tidal features associated with the weird galaxies. They give a very nice but probably unnecessarily thorough introduction, given that they don't actually find any tidal tails per se. Ideally, if they found nice classical extended tails, that would settle the argument pretty unambiguously, but alas such features aren't seen yet.

However, what they do find is evidence of tidal distortion. Looking at their images I was a bit skeptical - the galaxies look pretty normal, and the outermost measurements look like they're almost within the noise. But their measured surface brightness profiles (which show how the stellar density changes with radius) are rather more convincing, showing a clear, coherent change of gradient in both density and ellipticity over several data points. It certainly doesn't appear that they're really probing much into the noise, although of course even more sensitive measurements would always be better.

(The only real caveat I might raise to this is that there's no discussion of alternative explanations, especially how long some mild ellipticity might persist in a galaxy consisting purely of stars as these do. But that's probably not very important.)

Using this, they can estimate the radius within each galaxy at which the change of profile occurs. And from theories of tidal distortions, they can then estimate how close they must be to the larger galaxies present within the group. The neat finding is that if the galaxies do actually have dark matter, then for them to be this distorted, they'd have to be even closer to the bigger galaxies than is possible according to the observations. That is a pretty strong result that these galaxies really do lack dark matter. You can always say a galaxy might be at a different distance along the line of sight, but the distance across the sky is a very much harder limit. They also find no evidence of those S-shaped tails I was wary about, though this doesn't really change anything.

What does this mean ? To be honest, not that much as far as understanding their origins goes - though I got a bit bored towards the end because it is a rather long paper. We can now, I think, quite definitively say that these objects are strange - we're just going to have to wait for even deeper data to say more about what the bloody things actually are. Weird ? Yes. Challenge to theory ? Not so sure, for now.