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.

Taking the galactic paternity test

Even though my backlog of unread papers is a mile long, this one from today's arXiv was so pertinent I had to read it immediately. 

You might remember that a few years ago I was co-author on a paper that re-examined an enormous gas cloud in the Virgo Cluster that looks a bit like a rhino. This was already known from ALFALFA observations, and with no clear parent galaxy but an enormous amount of gas, and no obviously-associated optical emission, it was undeniably weird. Most bits of atomic fluff floating around in galaxy clusters tend to be small, but this was was enormous : almost 200 kpc long and with an HI mass of over a billion times the mass of the Sun. Originally seen as a complex of HI clouds, our more sensitive observations showed that there was even more gas present and all the individual clouds, which originally looked like separate objects, are actually connected by fainter gas structures.

We showed that this made the already-suspicious candidate galaxies for the origin of the Complex even more implausible. To lose that much gas implies a very large galaxy indeed. Now galaxies can and do, of course, lose huge amounts of gas in clusters through ram pressure stripping : as they move through the cluster's own, much thinner gas, pressure builds up and eventually pushes gas right out of the galaxy's disc. But this process is also prone to dispersing and dissolving much of the gas, leaving it undetectable. This means that the gas we see today might only be a small part of that which was originally present, requiring an even more massive parent to supply the gargantuan amounts of gas needed.

We also suggested a new, but unlikely, possible parent : NGC 4522. This is one of the most famous examples of ram pressure stripping. It's got a nice clear gas tail, and even in the optical you can see signs of disturbances in the dust. Our observations showed a second gas tail, with different kinematics to that which was already known, lining up quite nicely with the Kent Complex (which Brain Kent prefers to call the boring name of the ALFALFA Complex 7). But the velocities of the galaxy and Complex are so different that it's difficult to believe the one could be the source of the other.

This latest paper changes the game. They find that there is actually some faint optical emission in one (small) part of the Complex, comparable in many ways to the putative ram pressure dwarfs the same authors proposed a couple of years ago. In those objects, it seems the stars form directly in the gas stripped due to ram pressure, forming stellar structures that might be gravitationally self-bound or might just be passing, transient features that will soon disperse. As with the others, the stars in this object are all young, with no evidence for a stellar population more than a few tens of megayears old. That's basically instantaneous for these sorts of features. It's worth also pointing out that this little star-forming region is very small in comparison with the rest of the Complex, maybe ~10" across compared to the 40' of the whole Complex (a factor of 240 difference in size !).

What they get with the new observations is not just the discovery of this faint patch of starlight, which by itself would be unconvincing : such little smudges aren't that uncommon, and it's just too small to be obviously related. No, they go much further than this, getting very nice resolved observations with Hubble but also a stellar velocity which is a perfect match for the Kent Complex. That makes the association about as secure as it's going to get. And they also measure the chemical composition (a.k.a. metallicity), showing that it's similar to the other potential ram pressure dwarfs but also allowing for comparisons to the other galaxies in the vicinity.

And they estimate, however roughly, the stellar mass. This is small, a few tens of thousands of times the mass of the Sun. Even only considering the particular clump of gas nearest the stars, the gas content is at least three thousand times higher than the stellar mas, which is extraordinary even in comparison to the other ram pressure dwarf candidates.

What does this mean for the origin of the complex ? Well, like our earlier paper, they conclude there aren't any fully convincing candidate galaxies, but they open the door a little. While we largely dismissed NGC 4522 because of its huge velocity difference with respect to the Complex (1,800 km/s, which is high even in a chaotic place like the Virgo Cluster), they're a bit more charitable. This extreme velocity with respect to the cluster means that it could have experienced incredibly strong ram pressure stripping and very recently too, recently enough that most of the cloud would remain intact and detectable. That would make NGC 4522 a plausible candidate because it wouldn't have needed to have had such a huge gas content, as not so much would have become undetectable yet. 

It's also a good match in metallicity, though they're careful to point out that limited metallicity data for other galaxies in the vicinity makes this not such a strong diagnostic tool as we might like. There's also a large gap between NGC 4522 and the Complex, but this could simply imply two stripping episodes, and they say that simulations have shown that such features can indeed happen.

Of our preferred candidate, NGC 4445... well, "preferred" is too strong a word. It was really only the one we disliked the least. Anyway they raise the same objection that we do, that other observations show that NGC 4445 has a tail pointing in exactly the wrong direction. So again, two stripping episodes would be needed, with some weird geometry, but this one is kinematically closer to the complex, and could more easily account for enormous gas lost.

My immediate impression is that their arguments are quite persuasive : NGC 4522 could be the parent after all. We also objected because of the particular kinematics of the structure (one of the sub-clumps is at the wrong velocity), but without a dedicated numerical simulation, this is a weaker argument. The huge kinematic difference of the Complex and galaxy remains, as they say, problematic, especially given the rather low velocity dispersion in this part of the cluster, but it might not be fatal. Still, I wouldn't rule out NGC 4445 either.

What's really interesting about this object, I think, are three things besides the obvious how-did-the-damn thing-form in the first place. First, why is only part of it forming stars, and why has that part only started forming stars right now ? What's special about this particular section of the Complex – why aren't other bits of it forming stars as well ? What happened recently to trigger star formation ? Secondly, more generally, how does this relate to the other ram pressure dwarfs, given that it's so much more massively gas rich than all of the others ? Does it point to a similar origin or is this one a coincidence ? And thirdly, why is this structure so damnably complex ? Why does it have all these fiddly little details rather than being one big long stream ?

The most likely avenue of progress at this point would seem to be detailed, dedicated numerical simulations. More observational data might help of course, but I think if we could show that certain passages of galaxies in this part of the cluster would (or more likely, would not) produce even vaguely similar structures to this one, we'd be able to formulate a convincing argument for and against individual candidate galaxies. Not at all easy to do, but possible. 

Regardless of its origin, it's a spectacular and fascinating object, and I'm gratified to see someone not only having properly read our previous work but also genuinely understanding it too. Were I there referee, this one would likely have gone through on a nod. In fact the other real question I'd have would be how it can be submitted as a letter when it's fifteen pages long and pretty fully fleshed out – may as well skip the letter phase and make it a full paper at this point.

Another one for the collection

Another paper about so-called "almost dark" galaxies, a term I intensely dislike. Just call them dim ! "Almost dark" sounds lame, like being an "almost professional" snooker player or something. Although I suppose being called a "dim" snooker player would be even worse...

With a stellar mass of about 400 million times the mass of the Sun, it would be a bit of a stretch to call this one especially faint anyway. But what it is is quite dramatically extended to compared even to other, otherwise similar Ultra Diffuse Galaxies. Its stellar mass profile is much more extended than the famous DF44, emblematic of these especially fluffy systems, and this one clearly deserves some attention.

But I'm getting ahead of things. From the outset, this paper is about how this galaxy can help inform us about the nature of dark matter rather than merely saying, "look, here's a system almost entirely dominated by dark matter, isn't that neat" rather than addressing any actual science. Kudos to them for that, this is much needed.

They seem to have found this object accidentally and then gone and got some seriously impressive follow-up data : with an optical surface brightness magnitude of 31 magnitudes per square arcsecond, this is a sensitivity I don't think I've heard quoted outside of Hubble papers before. Their 12 hours of integration on the GBT, however, has only given at best a marginal HI detection (even though it's detected in both polarisations), and it's somewhat annoying that they don't seem to quote its linewidth clearly anywhere. With single-object papers I think it's always a good idea to put the major parameters in a nice clear table, but never mind. I wouldn't have too much confidence in the measurement anyway given how weak the signal is. This doesn't affect their main analysis in any case.

[See edits below for major corrections about these points !]

From their optical data, they find that the colour profile is flat, not varying at all from the innermost regions to the outskirts. So just one single stellar population everywhere with no regions being particularly susceptible or resistant to star formation, which would seem to fit with UDGs in general. The stellar density profile does show a steady decline, indicating that they've likely reached the edge of the object and even deeper observations probably wouldn't reveal anything else.

There seem to be multiple possible ways to form faint, extended galaxies in clusters, but in isolation nobody (so far as I know) has come up with a convincing explanation. Here they ask the obvious question as to whether it could be the result of tidal interactions but the nearest other galaxy is bloody miles away so that seems unlikely; it also has a chemical composition much more typical of larger galaxies whereas tidal dwarfs tend to be even more metal-rich. Diligently, they concede that it could be a very old tidal dwarf that's had billions of years to enrich its metals and could probably have lasted this long quite happily given its isolation, but its extremely high dark-to-light ratio (several tens) and rather low gas content all but rule this out.

What they do next is a pretty neat thing to dry and I'm glad they did it, but personally I wouldn't have had the audacity. They speculate that because such objects don't appear to be compatible with the Standard Model, they maybe the point to something different about the nature of dark matter. They say that something called ultralight axions might be responsible. Now here I want to say YES ! Thank you for trying this and using these objects to probe fundamental physics, that's what's interesting about them ! And then I immediately want to say NO ! You can't provide any meaningful constraints from just one object !

I'm exaggerating somewhat. By no means do they claim to have overturned the Standard Model or anything else, they just venture an intriguing idea. Good for them. But a couple of paragraphs about how incompatible this object is with simulations seems to me to be a bit of a stretch to then go immediately for exotic physics as a viable explanation. I think what's needed here is much more about context. What are the other galaxies in the vicinity like ? Is there deep optical data for any of them ? Can we really be confident about its isolation ?

I don't think there's anything wrong with posing more radical alternatives, but I'd need a lot more persuading that the other possibilities can be so firmly dismissed. To my mind there's still enough uncertainties in baryonic physics that we shouldn't be overly-concerned that simulations aren't predicting objects like this. Regardless, it's good to see someone firmly pointing out the continuing weirdness of isolated UDGs. 

EDIT : After a group meeting it's clear that I've given the authors too much credit here. True, they did include everything in a nice clear table, at which my eyes glazed over so I didn't notice their velocity width measurement. Fair enough, they do state all the major parameters then – my mistake ! But their line width estimate is just 34 km/s, with an error bar of 11 km/s... after smoothing their native velocity resolution from an exquisite 0.7 km/s to a ghastly 25 km/s ! Lots of numbers, but the bottom line is that their velocity width measurement isn't terribly meaningful. And the line width dictates total dynamical mass, so what they can really say about the dark matter mass here is... not much.

Let me break that down a bit more. Almost certainly, they smoothed their initially very precise resolution to increase sensitivity. Nothing wrong with that, and in any case, the HI line itself is seldom less than 10 km/s in width due to the temperature of the gas anyway. So you don't really need 0.7 km/s resolution. But 25 km/s is getting pretty bad, and to try and claim any sort of accuracy that the true width is 34 km/s which is barely higher than the resolution, especially with a signal which is anyway marginal... nah. This is really stretching credibility. Any claims that this galaxy is dark matter dominated a la DF44 need to be viewed very suspiciously until somebody obtains better data.