Very long-term readers may remember my anguished efforts (almost a decade ago) to build a stable disc galaxy. Sweet summer child that I was, I began by trying to set up the simulations to just have gas or stars, but no dark matter. I thought – understandably enough – that adding more components would just make things more complicated, so best to start simple. I was planning to gradually ramp up the complexity so I could get a feel for how simulations worked, eventually ending up with a realistic galaxy that would sit there quietly rotating and not hurting anyone.
That wasn't what I got. Instead of a nice happy galaxy I got a series of exploding rings instead. Had that been a real galaxy, millions of civilisations would have been flung off into the void.
It turns out that dark matter really is frightfully necessary when it comes to keeping galaxies stable. Dark matter is a galaxy's emotional support particle, preventing it from literally flying apart whenever it has a mild gravitational crisis. Stable discs are easy when you have enough dark mass to hold them together.
(Of course, this is only true in standard Newtonian gravity. Muck about with this and you can make things work without dark any matter at all, but I'm not going there today.)
You don't always need dark matter to keep things together though. Plenty of systems manage just fine without it, like planetary systems and star clusters. But it's come as a big surprise to find that there are in fact quite large numbers of galaxies which have little or no dark matter, a result which is now reasonably (and I stress that this is an ongoing controversy) confirmed. We always knew there'd be a few such oddballs, if only from galaxies formed from the debris of other galaxies as they interact. But nobody thought there'd be large numbers of them existing in isolation. So what's going on ?
Enter today's paper. This is one in a short series which to be quite honest I'd completely forgotten about, partially because the authors forgot to give the galaxy a catchy nickname. Seriously, they could learn a lot from those guys who decided to name their galaxy Hedgehog for no particular reason. I'm only half-joking here : memorable names matter !
But anyway, this was an example of a UDG with lots of gas that appeared to have no dark matter at all. I wasn't fully convinced by their estimated inclination angle though, for which even a small error can change the estimated rotation speed and thus the inferred dark matter content substantially. A independent follow-up paper by another team ran numerical simulations and found that such an object would quickly tear itself to bits, whereas if if was just a regular galaxy with a very modest inclination angle error then everything would be fine. And there have been many other such studies of different individual objects, all of them mired in similar controversies.
Since then, however, I've become much more keen on the idea that actually, a lot of these UDGs really do have a deficit or even total lack of dark matter after all. The main reason being this paper, which is highly under-cited in my view. Now it's entirely plausible that any one object might have its inclination angle measured inaccurately*. But they showed that the inclination-corrected rotation velocity of the population as a whole shows no evidence of any bias in inclination. Low inclinations, high inclinations, all can give fast or slow rotating galaxies, consistent with random errors. That some show a very significant lower than expected rotation therefore seems very much more likely to a a real effect and not the result of any systematic bias.
*Though all of these terms like "bias", "errors" and "inaccuracies" are, by the way, somewhat misleading. It's not that the authors did a bad job, it's that the data itself does not permit greater precision. That is, it allows for a range of inclination angles, some of which lead to more interesting results than others. The actual measurements performed are perfectly fine.
What about that original galaxy though ? AGC 11405 might itself still have had a measurement problem. Here the original authors return to redress the balance.
It seems that in the interim I missed one of their other observational papers which changes the estimates of exactly how much dark matter the galaxy should have; probably this is lost somewhere in my extensive reading list. The earlier simulation paper found that the object could be stable only (if at all) with a rather contrived, carefully fine-tuned configuration of dark matter, and there wasn't any reason to expect such a halo to form naturally. Couple that with the findings that it could easily be a normal galaxy if the inclination angle was just a bit off, and that made the idea of this particular object seem implausible, even if a population of other such objects did exist.
But that interim paper changes things. Whereas previously they used the gas of the object to estimate the inclination angle, now they got sufficiently sensitive optical data to measure it from the stars, and that confirms their original finding independently. They also improved their measurements of the kinematics from the gas, finding that it's rotating a bit more quickly than their original estimates, meaning it has a little bit more scope for dark matter. More significantly, the same correction found that the random motions are considerably higher than they first estimated.
What this means is that the dark matter halo can be a bit more massive than they first thought, and the disc of the galaxy doesn't have to be so thin. A thick disc with more random motions isn't so hard to keep stable because it's fine if things wander around a bit. So they do their own simulations to account for this, with the bulk of the paper given to describing (in considerable detail) the technicalities of how this was done.
They find that an object with these new parameters can indeed be stable. Rather satisfyingly, they also run simulations using the earlier parameters, as the other team already did independently. And they confirm that with that setup, the galaxy wouldn't be stable at all. So the modelling is likely sound, it's just that it depends quite strongly on the exact parameters of the galaxy. They confirm this still further with analytic formulae for estimating stability, showing that the new measurements of the rotation and dispersion are, once again, predicted to be stable.
But if the galaxy actually does have a hefty dark matter halo after all, doesn't that mean it's just like every other galaxy and therefore not interesting ? No. As far as I can tell, the amount of dark matter is still significantly less than expected, but also its concentration (essentially its density) is far lower : a 10 sigma outlier ! So yes, it's still really, really weird, with the implied distribution of dark matter still apparently very contrived and unnatural.
So how could such a galaxy form ? That's the fun part. It's important to remember that just because dark matter doesn't interact with normal matter except through gravity, this is not at all the same as saying it doesn't interact at all ! So some processes you'd think couldn't possible affect dark matter... probably can*. Like star formation, for instance. Young, massive stars tend to have strong winds and also like to explode, which can move huge amounts of gas around very rapidly. It's been suggested, quite plausibly, that this is what's responsible for destroying the central dark matter spikes which are predicted in simulations but don't seem to be the case in reality. The mass of the gas being removed wouldn't necessarily be enough to drag much dark matter along with it, but it could give it a sufficient yank to disrupt the central spike.
* And it's also worth remembering that just because dark matter dominates overall, this isn't at all true locally. This means that movement of the normal baryonic matter can't always be neglected.
The problem for this explanation here is that the star formation density must be extremely low to get objects this faint. So whether there were ever enough explosively windy stars to have a significant effect isn't clear. Quantifying this would be difficult, especially because dwarf galaxies are much more dominated by their dark matter than normal galaxies – yes, they'd be more susceptible to the effects of massive stars because they're less massive overall, but the effect on the dark matter might not necessarily be so pronounced.
The authors here favour a more exotic and exciting interpretation : self interacting dark matter. The most common suggestion is self-annihilating dark matter that's its own anti-particle, which would naturally lead to those density spikes disappearing. There could be other forms of interaction that might also "thermalize" the spike... but of course, this is very speculative. It's an intriguing and important bit of speculation, to be sure : that we can use galaxies to infer knowledge of the properties of dark matter beyond its mere existence is a tantalising prospect ! But to properly answer this would take many more studies. It could well be correct, but I think right now we just don't have enough details of star formation to rule anything out. Continuing to establish the existence of this whole unuspected population of dark matter-deficient galaxies is enough, for now, to be its own reward.
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