Back from the grave ?

I'd thought that the controversy over NGC 1052-DF2 and DF4 was at least partly settled by now, but this paper would have you believe otherwise. 

To recap, these are two small, extremely faint galaxies that appear to have no dark matter. This claim caused a prolonged furore when it was announced, most contentious of which was the distance dilemma. If they were at 20 Mpc distance, as originally claimed, then they'd indeed lack dark matter and be quite strange in other aspects too, like how many globular clusters they have. This is weird because galaxy formation models don't predict objects like this. If on the other hand they were at about 13 Mpc, then they'd be pretty normal and uninteresting.

After a lengthy ping-pong of claims and counter-claims over the distance, this seemed to have been settled decisively with a wholly-independent measurement placing DF2 at 22 Mpc and DF4 at 20 Mpc. Very clearly then, they have no or negligible amounts of dark matter.

So, exciting stuff ? New physics ? Alas, not so fast. Simulations showed that such objects could be produced by a rare but perfectly normal process. It turns out that the direct collision of a dwarf galaxy with a larger object can almost completely remove its dark matter, a process made easier because dark matter particles should orbit radially around the centre of their parent object. That is, if a particle happens to be deep inside the galaxy at some point, then eventually it will wander to the outskirts where it can be more easily removed. This is quite different to the stars and gas, which remain much more tightly confined to the inner regions at all times, making them much harder to disturb.

All well and good. A very few oddball galaxies having experienced a weird but entirely conventional process : fun, but not revolutionary for our understanding of physics or even just plain old galaxy evolution. Except, of course, that we now know of much greater numbers of galaxies which seem to at least be significantly deficient in dark matter, if not lacking it entirely. And many of those are isolated, where encounters where other galaxies wouldn't be a likely explanation.

Anyway, today's paper goes back to the original DF2 and DF4. They use really, seriously deep optical images to search for the signatures of tidal encounters which are expected to be present if the galaxies really had lost their dark matter thanks to interactions with other galaxies. They note that simulations show that these features should be surprisingly long-lasting, on timescales of several gigayears. So even if it happened in the distant past, something should still be visible.

Most of the paper is a very technical description of exactly how they reduced their data. I skipped over this; it's likely only of interest to anyone planning to actually do it for themselves. The bottom line is that they say they confirm earlier evidence that DF4 does have signatures of tidal encounters, whereas DF2 shows no indication of any disturbance.

First, DF4. As I describe in the last link, I was very skeptical about this claim, which looks like a marginal variation in the image to me, and not at all like the S-shaped feature they say they've found. The new, deeper images, which show much larger and stranger features... well, I don't know what to make of them. I'll take the authors at their word and assume they've processed the data correctly; certainly they understand this much better than I do. What I think would have been really valuable would have been to actually reduce the sensitivity by using shorter segments of their exposures, to get to the same sensitivity as the previous images. If these had then also shown the same claimed feature as that earlier paper, that would have least been a bit more convincing that they were real.

But instead (understandably) they go straight for the jugular of getting the highest sensitivity possible. To me the resulting features in the image look like a rather haphazard variation in the data that's not at all indicative of the tidal features expected. The extremely small size of the image is a major limitation here, though, showing little beyond the target galaxy itself. So it's difficult to tell if these apparently-random variations are really part of some larger, more coherent structures that would be far more persuasive for the claim of detecting tidal features.

DF2, however, is a case where you can prove a negative. The image shows a classic case of a non-detection. What was supposed to be there, according to theory, undeniably isn't there, and the authors don't dispute this.

Does this mean the tidal hypothesis is done, at least for DF2 ? They say yes, and venture to suggest that maybe there is a distance tension after all, given that some of the major galaxies in the group have been found to be a bit closer (17 Mpc). This feels like a weak argument to me. By all means dispute the distance, but I think that you need to do so based on direct measurements, not those of galaxies which are nearby in projection. Close sky-alignments of galaxies which are at radically different distances happen all the time. 17 Mpc is anyway not 13 Mpc.

A slightly better argument is that if DF2 were associated with NGC 1042 instead (which is presumably closer), their projected separation would be enough that no tidal features would be expected. But again this is circumstantial, and not really a direct argument in favour of a different distance. It also ignores all those other dark matter deficient galaxies, making the whole thing feel just a tad... petty.

In yet another post about these objects, I mentioned that observations showing a neat line of galaxies on larger scales was consistent with a tidal origin, describing this as another nail in the coffin for anyone hoping these would turn out to be seriously weird, physics-challenging objects. BUT :

Not the final nail by any means, and it's still just about possible they could burst back out like an enraged vampire, but it's definitely making it harder for any would-be children of the night to go on a killing spree.

So do I have to say that now the vampire is slowly rising from the tomb after all, ready to drain the blood of mainstream physics ? Nah. Even leaving aside the explanation that they're actually just closer than first thought, exotic physics doesn't seem necessary to me. As the authors of today's paper themselves note, the same data has been interpreted differently by different teams even when they don't dispute the numerical values. The clear lesson from this storm in a teacup is just how much the details matter. What at first seems like a Eureka moment can, on repeat analysis, turn out to be erroneous or incomplete, which is why having multiple analyses, multiple lines of attack, varying perspectives and teams digging into the dirt, really matters. Discovery in this case is a slow and unglamorous process of slowly chipping away at best, and more often a case of two-steps-forward-one-step-sideways-oops-I've-tripped-and-broken-my-ankle.

For now, my guess remains on the tidal encounter scenario. There are so many parameters in simulations like this that the possibilities are vast; I would not be inclined to take a lack of observed tidal features in this case as being decisive. But what continues to interest me more are those similar objects in isolation. DF2 and DF4 may be emblematic, but as members of a whole wider population, they themselves no longer matter quite so much. Regardless of their own particular origins, there are plenty of other fish to fry.