Yo dawg, herd you like criticism

An excellent criticism of the criticism of that galaxy without any (or "lacking" dark matter). I'm a big fan of the faded error bars in the figure. I'm not persuaded that they didn't claim the galaxy has no dark matter at all - this is an inevitable conclusion from the wording and they should have been much more overt if they really wanted to say "not much dark matter". Still, the essential point is that it has, at best, hardly any dark matter compared to other galaxies, which is well-made. I remain more skeptical of the author's other big claim about a galaxy with an anomalously high dark matter content, but this one seems reasonable. Not exactly rock solid, but interesting enough in my opinion to warrant the Nature paper.

On the critiques that the velocity dispersion has been measured incorrectly :

The distribution of the 10 objects is non-Gaussian with high significance, and this is borne out by the fact that different estimators for the observed dispersion give wildly different answers: the normalized median absolute deviation gives 4.7 km/s, the biweight estimator gives 8.4 km/s, and the rms is 14.3 km/s. Collins and Martin do not take this into account, but apply a maximum likelihood estimator blindly to all 10 objects. This seems a logical approach, #thedataareboss (to speak with Nicolas) and all that, but it means that their confidence limits are largely driven by a single object. This is problematic in itself, but the more fundamental issue is that the distribution of the 9 other objects rules out dispersions as high as 20 km/s. There is just no chance (that is, less than 1 part in 10,000 or so) to measure a median absolute deviation as low as 4.7, or a biweight dispersion as low as 8.4, if the true intrinsic dispersion is 20 km/s.

On the importance of the External Field Effect in the MOND analysis :

Now, I did actually consider the EFE - I had seen Stacy's nice 2016 paper about the "feeble giant" Crater II, and did an order of magnitude estimate while working on this section in the Nature paper. NGC1052-DF2 is about 1000x more massive (in stars) than Crater II, and based on the approximations in that paper the EFE should be negligible for DF2. It seemed so far off that it did not seem worth mentioning in the paper - I also was not sure how to do the actual calculation (rather than a scaled estimate). After publication of the Nature paper Stacy (and also Pavel Kroupa, independently I believe) did the (non-trivial) calculation, and concluded that it is actually important, lowering the expected dispersion to about 14 km/s. This is still higher than our upper limit, but Michelle Collins and Nicolas Martin, for two, would argue that this is getting to be pretty close. (I'm still not quite sure why the simple estimate I did was so far off, but I feel better for the fact that Stacy also initially thought the EFE would be negligible. In an email, he wrote "indeed, my first thought was that the EFE was unlikely to be important, for the reasons you give.")

In any case, getting back to the question whether MOND is able to fit other galaxies than DF2, the closer MOND gets to explaining NGC1052-DF2, the harder it is to explain Dragonfly 44. This galaxy, as discussed in a 2016 paper, is even larger than DF2, has a similar stellar mass, and a velocity dispersion of 47 +- 7 km/s. It lives in the Coma cluster - I imagine the EFE is important there, also, but even if it isn't, the dispersion is significantly higher than even the isolated MOND prediction (about 22 km/s for Dragonfly 44). The key issue for alternatives to dark matter is the difference between NGC1052-DF2 and Dragonfly 44, as highlighted in Fig. 3. If a theory is able to fit one object, it will have a hard time fitting the other. This is what we should have said more clearly in the paper - my misunderstanding was that I thought MOND could fit Dragonfly 44. (There are always "outs" of course: for Dragonfly 44 those might include measurement errors, non-equilibrium of the galaxy, and perhaps some form of cluster-specific unseen matter).

Via Winchell Chung and for the attention of Abhijeet Borkar.


https://www.pietervandokkum.com/ngc1052-df2