Un-missing matter

When you find that the missing matter of a galaxy is itself missing, and then you realise that actually it isn't missing missing matter, it's just the regular missing matter, you're in a right proper linguistic pickle.

Readers are advised to consult this previous post for a full summary. In brief, there's this galaxy which doesn't seem to have the usual dark matter (unseen and hence missing by direct observations) needed to hold it together. Based on the mass of its visible stars and gas, its rotation speed is so slow that it's apparently gravitationally bound without needing any of this additional matter at all. And this just doesn't fit the general picture of galaxy evolution : yes, some systems like this can be produced in interactions, but this object is nowhere near anything else. And a generally overlooked detail is that the slow rotation speed means the galaxy wouldn't have had much time to settle into its apparent stability.

My major worry last time was that the rotation speed of the galaxy might have been underestimated. This is dependent on estimating how inclined the object is along our line of sight : if it's directly edge on, we'd be able to measure its full rotation directly, whereas if it's face on, we wouldn't be able to measure rotation at all; at intermediate inclinations we can apply a correction factor. The observations indicate a low inclination angle, which makes this correction already uncertain, but the very ragged edge of the gas disc used for estimating the angle makes this process extra difficult.

To be fair, the authors of the observational studies have had many good arguments in favour of the higher inclination angle they say is preferred by the data. Indeed, it wasn't until the last paper that I would even have questioned this. And they do admit that the inclination angle is the biggest uncertainty.

Today's paper is by a different group coming at the problem with numerical simulations. They cite a lot of private communication with the original team, so it's nice to see that this dispute is being addressed cooperatively.

You might remember my own extensive efforts to model a galaxy without dark matter that met with assorted dismal failures. Only when I added a nice big dark matter halo, to ensure stable circular orbits, did everything magically click into place. This is nothing very new, as the author's of the latest paper explain - in fact, disc stability was even one of the motivations for dark matter in the first place.

What they do is much as you might expect. They model the galaxy as according to the original observations and see what happens. Which, as I found myself, is that the object tears itself apart in short order. Even if they use a very peculiar dark matter halo the original team say the observations do permit, it just doesn't work - it's a bit better, but not much. And then, lo and behold, if they allow a lower inclination angle and hence higher velocities and a big dark halo, everything works out nicely.

This is an awfully tempting explanation. It's just possible that there is some combination of parameters that would be compatible with the observations where a dark halo is not required, but this needs a lot of fine tuning and feels contrived. In contrast, accepting the inclination angle is wrong and there actually is a dark halo after all is a hell of a lot simpler and more satisfying. There is one other option that the object is not in fact stable, and is in the process of disintegrating, but this would seem to be incredibly unlikely in the case of an isolated object.

Not that we should discount these alternatives altogether, mind you. Remember one of those other claimed galaxies without dark matter, which had a prolonged controversy over its distance. A lower value would have made it a very satisfactorily normal galaxy after all, but in fact it was the more puzzling higher value that won out. So galaxies without dark matter are still, for the time being, very much a thing. But in this particular case, my money would now be quite firmly on the "inclination angle measurement" problem. And given that those other cases may well be tidal objects, this particular controversy just might be wrapping itself up.