Two's Company : A Second Galaxy Without Dark Matter

A short letter submitted to ApJ by the same team who brought you the first galaxy without dark matter. This second discovery is very similar to the first : it's in the same group of galaxies, at the same distance, has a low surface brightness, is very extended, and has a similar smooth and boring-looking morphology. Its all-important velocity dispersion is even lower than the first, at a mere 6 km/s compared to 8-10 km/s. Which is again consistent with its dynamics being completely dominated by its stellar mass, with little or no additional dark matter needed.

This is very strange indeed. During tidal encounters between galaxies, it's possible for the gravity to tear off enough material to form a brand new (low mass) galaxy without its own dark matter component - that's been known for ages. But such a process ought to be messy. It shouldn't be able to form big, smooth objects that move very slowly. There ought to be debris all over the place : stellar streams and other weird-looking structures. It just shouldn't be able to make anything that looks this damn boring. Well, not quite true : after a good long while things should settle down and most of the crazier stuff ought to disperse, but to form very smooth things like these galaxies should take a very, very long time indeed because their motions are so low. And galaxies produced by this mechanism are chemically different to other galaxies, whereas this one isn't.

Could this just be a normal (though faint) galaxy observed close to face-on where we wouldn't be able to detect any rotation ? It was possible with one object, but that becomes highly unlikely with two - where are all the faint edge-on galaxies, eh ? Similarly, while tidal encounters can act to strip away large amounts of dark matter from ordinary galaxies, it seems incredibly unlikely that we'd find two such objects in a group without the expected tidal debris.

The other weird feature of these objects is that they have a large number of globular clusters given their low stellar mass. What the connection might be with the lack of dark matter is anyone's guess, but it does make it even more likely that they're part of a distinct population rather than being weird but rare exotica. Having a second object changes the picture considerably, but we need many more objects to have any kind of statistical view.

What I'm a bit surprised at is that no-one is talking much about the other (rather large !) population of objects which are known to show unusually low velocity dispersion from their gas measurements : ultra-diffuse galaxies. I e-mailed van Dokkum about that because it seems like something that should be mentioned more in the literature somewhere, but after a couple of weeks I didn't get a response so I guess I never will.

https://arxiv.org/abs/1901.05973