Yet another Dark Matter versus MOND paper

Well... partially. Mainly this paper is about whether the standard dark matter model can explain some of the recently discovered "ultra faint" dwarf galaxies in the Local Group. Cold Dark Matter (CDM) models predict that really small dark matter halos, which have low rotational speeds, shouldn't be able to form any stars at all. They shouldn't have enough mass to pull in enough gas to form stars... but that's just what these new galaxies seem to have done.

In this paper, the authors use new simulations with much higher resolution and more complex physics than previous efforts. They show that similar objects can form through tidal stripping (i.e. gravitational effects), which can remove large fractions of the dark matter and stars but, interestingly, doesn't change the size of the galaxy. So apparently weird new objects (like the "giant" Crater II) may make sense after all.

They also show more quantitatively that the main objections to this idea aren't as fatal as might be thought. First, tidal stripping is thought to be very disruptive, with stars and gas and dark matter flying all over the place. And it is... but not for very long. The damage is done only when the dwarf galaxy flies closest to its bigger, scarier neighbour. The rest of the time the tidal effects don't really do a lot, and afterwards the galaxy can settle down into a nice round shape, like those that are observed. In essence it's like that bit in Jaws where the decapitated head suddenly appears - everyone jumps, but you don't go running off down the street screaming your head off for ten minutes afterwards. Usually.

The second objection is that these tidally-disrupted galaxies should only be found near their larger neighbours. But because the scary moment of disruption is short-lived, the galaxy can continue on its merry way back out to the hinterlands. So it's no surprise that the observed galaxies are found quite far away from the Milky Way.

All well and good, but now we reach the controversial stuff....

First, there were several recent papers claiming the discovery of a "new law of nature", where the gravity from dark matter appeared to be closely correlated with the gravity from normal matter. Which seemed a bit odd, but then there were several other papers which said, "yeah, but we see this is dark matter simulations anyway, shut your ugly face, it's not very interesting." Or words to that effect.

This paper shows that the scatter in this "mass discrepancy acceleration relation" is much higher for faint galaxies than was previously reported. They also claim that their simulations explain this. I think this is true, but the agreement is not that great. It's OK, but not particularly impressive. More interesting is simply the claim about the size of the scatter, which I find fairly convincing. Our resident tame MOND expert says that the original papers already discussed this, but I don't think this is true. However, they do mention that they only use the highest-quality data available. A better objection might be whether these new galaxies are rotating at all or if the stars are just moving on random orbits - this might not change the conclusions, but it should be discussed.

The second controversial point was the claim that they have discovered something which poses a "possibly insurmountable challenge" to Modified Newtonian Dynamics, the main alternative to dark matter. Strong words. Strong, angry words - which have been used before against MOND... but with admirable if cockroach-like tenacity the bloody thing keeps coming back.

MOND has this funny thing called the "external field effect". If there's another galaxy nearby, acceleration in a smaller galaxy reverts back to standard Newtonian behaviour. When you account for this, it seems that the velocities in the faint galaxies are in strong disagreement with MOND's predictions (they're also in strong disagreement even if you ignore it).

The tame MOND expert agrees with the method used, but disputes the conclusion. He's probably right that the conclusion is too strong, although personally I find claims of "a new law of nature" to be far worse on that front. His objection is that the fraction of binary stars in a galaxy can change the overall measured velocity dispersion, or it could be that even those the galaxies aren't in the process of being tidally disrupted right now, they're still out of equilibrium. It's possible, but I'm not convinced this can really explain the strong systematic offset seen in the data. It needs to be quantified though.

The paper concludes by worrying that this model requires some pretty dramatic tidal disruption to explain the galaxies - they've have to have lost 99% of their original mass (whether this is common in their simulations is not clear). But with measurements of the 3D velocities of the galaxies it would be possible to trace their orbits back in time and work out whether they really have experienced close encounters with the giant galaxies of the Local Group. So, as usual, watch this space.
https://arxiv.org/abs/1707.03898