Does the MDAR challenge MOND ?

Investigating the radial acceleration relation in early-type galaxies using the Jeans analysis

The first author lives in the office across the corridor from me, but I didn't know he was working on this. I put this one under the 'My' Astronomy Articles collection only (as usual) to offer my own commentary.

The MDAR, also called the RAR, MDR and various others, is a relation between the predicted and observed acceleration of material in a galaxy. The prediction is based on only the observed material - assuming that all the material we see is all that's there, and that Newtonian gravity is correct. But since many other observations indicate the presence of additional dark matter, we don't expect this to agree with the actual measured MDAR. Naively, we might expect there to be not much of any relation at all, since dark matter is so much more dominant than normal matter according to those other observations.

Strangely, the actual MDAR is a very tight relation with almost no scatter ! It seems that you can use the baryons to predict exactly how much dark matter should be there, which doesn't make a lot of sense according to the standard model... at least, not obviously. In fact that naive interpretation turns out to be wrong. There are a whole bunch of selection effects at work as to which dark matter halos actually host galaxies, and standard models have had no difficulty whatsoever in reproducing the observed MDAR (I have a detailed write-up here : http://astrorhysy.blogspot.cz/2017/08/this-isnt-law-youre-looking-for.html).

(I completely disagree with the authors of this study that "The tightness of the RAR for the LTGs [spirals] remains unexplained with the DM hypothesis.")

Still, the MDAR was actually predicted decades ago by MOdified Newtonian Dynamics, which does away with the need for dark matter, so that's interesting. But there were also observations indicating that the MDAR breaks down in certain dwarf galaxies, potentially a challenge for MOND : acceleration is acceleration, it shouldn't work differently in different places (this is a gross oversimplification since MOND is bloody complicated, but it gets the point across).

In this Proceedings article (i.e. not a peer-reviewed paper), the authors show that the MDAR doesn't work for elliptical galaxies either. And they're not just poxy little dwarf galaxies where the data might not be very reliable, they're stonking great objects where the measurement should be safe enough (inasmuch as anything ever is in astronomy). Interestingly, the deviation looks a lot like the deviation that was already known for dwarf galaxies, which the standard model explains very well.

It gets even more fun. Two thirds of ellipticals don't follow the standard MDAR. The ones that do appear to be much more similar to disc galaxies, dominated by rotation (I'd guess these are actually lenticulars, not ellipticals). So disc galaxies follow the MDAR, but galaxies dominated by random motions don't. That could be a really serious challenge for MOND, as the authors note that such deviant galaxies would "need copious amounts of dark matter in their outer regions even in the MOND approach". What exactly they mean by "copious", they don't say. MOND does require a little bit of dark matter in clusters, but only by a factor ~2-3 compared to tens for the standard model. The authors note :

It is possible to reconcile MOND with our results by supposing additional invisible matter in the galaxies. MOND is already known to require some DM in galaxy clusters. The most discussed candidates are sterile neutrinos (Angus 2007) and compact baryonic objects (Milgrom 2008). We note that the DM required in our galaxies might be connected with the yet undetected gas which is predicted to flow into galaxies to explain various observations (Sancisi et al. 2008).

Michal is very pro-MOND for some reason, though to his credit he acknowledges the possibility that these results disprove it in the abstract. Since this Proceedings reports on a conference which took place last month, it misses the very recent result of the discovery of much of that undetected gas, which is now known not to reside in galaxies but in much larger filaments :
https://plus.google.com/u/0/+RhysTaylorRhysy/posts/FZXbAWmoHgY

It would be interesting to see what the standard model predicts for elliptical galaxies. Thus far, AFAIK no-one's tried to simulate that because the MDAR was measured for spiral galaxies. But it feels like MOND is becoming ever-more desperate : with the caveat that "copious" is undefined here, if it requires as much dark matter as the standard model then I really don't see the point of it at all any more. On the other hand, MOND is also still lacking a good numerical simulation to show what it really predicts.
https://arxiv.org/abs/1711.06335