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.

No stacks please, we're simulationists

Back in 2017 there was a Nature paper claiming to have detected declining rotation curves in galaxies at high redshift. This would mean that galaxies in the distant early Universe have significantly less dark matter that contemporary nearby galaxies, whose flat rotation curves are one of the principle signatures of dark matter in the first place.

I was rather skeptical of this. None of their individual measurements looked the least bit convincing to me, with the fitted curves highly dependent on single data points : the slightest error could have thrown them off (and some curves just don't go through the points at all). True, the stacked curve was much more convincing, but any systematic error in estimating the rotation velocity will only compound this error rather than averaging it out.

On the other hand none of this actually would be evidence against dark matter. As with the now-plethora of Ultra Diffuse Galaxies and the like (in the nearby Universe, with relatively precise, sensitive data) which seem to have a significant and sometimes total deficit of dark matter, all this really indicates is that dark and visible matter can be separated. This is very much harder to do with modified gravity theories. If the rotation curve arises only as a result of baryonic matter, then if you have two systems in which the baryons have similar distributions (and are suitably isolated), then they should always have similar rotation curves. The only reasonable way in which they can differ is if one has dark matter and the other doesn't.

The real question is whether, according to cosmological theory, we expect galaxies in the early Universe to be less dark matter-dominated than today's. Ethan Siegel simply says "yes", that these declining rotation curves are indeed expected in standard models of galaxy formation.

The author's of today's paper, however, say No. And this is certainly more intuitive. If dark matter is mass-dominant, then it seems odd that it would actually become more important over time. Surely it should be gas falling into dark matter haloes that describes the process of galaxy assembly, and if that's the case – if dark matter makes up the bulk of the mass of galaxies from the word go – then they should always have similar (though not necessarily identical) rotation curves to those of the present day.

Now I should mention that the first author was my Master's project supervisor. You can read about this in some detail here, but in brief, we ran a galaxy formation simulation without dark matter and found that it just didn't work (see also my latter efforts failures to build a stable disc without dark matter). Anyway, the arguments that seemed quite compelling to younger me no longer have the same appeal; I'm delighted to see that others are still pursuing this line of inquiry and I wish them well, but I don't see this as likely to be anything more than a dead end. An interesting one to be sure, but I'm not convinced there's light at the end of the tunnel, so to speak.

What they do is run a series of simulations of the monolithic collapse scenario of galaxy formation. This is far simpler than the standard paradigm of hierarchical merging, in which galaxies assemble from a multitude of mergers in the early Universe (which can be a surprisingly efficient process, but then the early Universe was a lot smaller than the modern one). They consider different initial geometries, in which the initial dark matter halo is the same size, smaller, differently-structured, or spatially separated from its associated gas cloud.

They get the same result in all cases. The collapsing monolith experiences violent relaxation which essentially wipes out its initial conditions : the collapse proceeds at ever-increasing speed, the gas shocks and triggers star formation, and the galaxy forms during the re-expansion phase. This means that it doesn't really matter how you start, you get the same thing regardless.

And in this scenario you always get a galaxy with a flat rotation curve. The only way they could get a declining curve is to take out the dark matter altogether*.

* I'm intrigued that this is much more successful than my attempts at the same scenario, which gave something completely unphysical. I'll have to try and follow up on that.

This, as they've argued before, suggests that the initial conditions are irrelevant. But as I understand it, the hierarchical merging scenario is just more radically different than they give it credit for. There's just no reason in modern cosmology to assume the presence of any sort of collapsing monoliths, certainly at the very least not at all as the norm for galaxy formation in the early Universe. I don't think you even get much or any in the way of violent relaxation in this scenario, but rather a continuous assembly of tiny, already stabl-ish- proto-galaxies. So I really don't think it's fair to compare the failure of reproducing declining rotation curves in a monolith collapse scenario with the expectations from standard cosmology; this is comparing apples with oranges.

I also wonder just how similar these objects are to the observations. They don't compare the gas masses at all. Their gas radii are in some gases huge at 70 kpc : not outlandish by any means, but definitely on the larger side. They don't give this comparison to the observations, preferring to normalise their results to more accurately compare the shape rather than the size of the rotation curves.

This isn't a mistake in and of itself. But as in the press release that accompanied the original claim, there are a multitude of different factors in play. The galaxies detected might not actually be the progenitors of modern-day spirals but rather more spheroidal systems, which are anyway known to have less dark matter. There may indeed be significantly less dark matter in early galaxies overall, and those earlier systems might be more dominated by turbulence than rotation – which significantly affects the interpretation of the rotation curve.

This is not to say that there aren't questions to answer. The authors of this study point to other findings of more typical simulations saying that the turbulence isn't enough to account for all this, but they also note other observations of individual galaxies showing flat rotation curves (though they dispute this result) at earlier epochs. And they note that some galaxies might form in dark halos and others not, which seems very likely given all the hoo-hah about UDGs.

All this is very reasonable. The whole thing is a mess with many different variables to juggle. It just seems to me that we're a very long way indeed, further than ever in fact, for needing to invoke other physics like magnetic fields (as the authors do) to explain modern flat rotation curves. It seems far more likely that a combination of different effects are likely to explain early declining rotation curves than it is that they undermine the whole paradigm, which is otherwise supported by a vast array different and independent considerations, both in theory and observation, on a whole set of enormously different scales. 

Do we fully understand galaxy assembly ? Absolutely not. But it's way to drastic to point to a complex result like this and say it calls the whole basis of modern theory into question, and it's just not fair to compare the results of a monolithic collapse scenario with the radically different processes postulated by mainstream cosmology.

Theoretically dark

How do objects which don't form stars remain so dark ? That's a question I've often asked, especially of dark galaxy candidates : objects which have gas that looks like it's rotating (implying a dark matter halo to keep it bound together) but lacking any detectable stars. The big problems with such objects is that it's very hard to know if seeing is believing, if they just look like dark galaxies or were instead formed by different processes. Maybe such objects are just bits of gas ripped off perfectly normal galaxies. I've covered this in detail umpteen times before, with the basic conclusion being "some are, some aren't".

In the 2000's there was a period when a few different groups ran numerical simulations looking at whether specific candidates could be explained in this way. This has largely died off, so I was very intrigued by this paper which examines dark galaxies in the context of the latest and greatest numerical simulations. These are completely different beasts from the n-body simulations run on the desktop machines of 20 years ago : instead of a few thousand SPH gas particles, now they have billions or more particles and included all kinds of fancy gas physics that would have had us all foaming at the mouth in an ecstasy of delirium back in the day.

It certainly starts in a promising way, reviewing the major candidate objects and studies (and yes, they cite me, so thanks for that) as well as some other more recent work I wasn't aware of. So I've got a couple of other references I should check up on, which is good. But I have to say that after that it's all rather more theoretical than what I was hoping for. Not that it's difficult or unimportant, but that it never makes any comparison between theory and observation. It deals with the dark galaxy candidates in the simulations very much on the terms of the simulation alone, making little or no comparisons with the observational candidates.

In some ways this is quite novel, at least to me. Normally I look at the missing satellite problem from the perspective of the galaxies, because those are what we actually observe. But the problem itself is all about how simulations predict too many dark matter halos that never light up, so examining those halos as interesting objects in their own right is a good idea.

What they find isn't terribly surprising though. The vast majority of the halos in the simulation do indeed remain dark, for what seems to be due to a combination of factors more than any one in particular. And they form a continuous sequence from the truly starless to the merely very dark to the brightest and most luminous objects of all; dark galaxies are not special, but normal. Indeed, perhaps we should instead be asking instead not what keeps some halos dark but the exact opposite : what allows such extreme levels of star formation in the apparently "normal" galaxies ! For comparison, in their simulation they identify 5.6 million halos, of which 5.5 million are completely starless, 47,000 are dim but not totally dark, and the rest – a mere 100,000 or so – are luminous.

If we stick with the standard question, "what keeps them dark ?", though, then the answer seems to be : isolation, spin, and mass. Isolation prevents them from experiencing as many mergers as the brighter galaxies, which compress the gas and trigger star formation. Isolated objects avoid this. Spin keeps the gas more extended and its density lower, thus reducing star formation. And mass prevents much gas from getting into the halo in the first place, again keeping density low. While some dark galaxies do form stars briefly early on and then lose their stellar population, it seems that most just never form any at all. 

There's an additional effect from mass. Being small means that objects are more vulnerable to the effects of reionisation : when the first, highly energetic stars light up, they ionise all the gas in the smallest halos* and drive it out, and being so small they don't have the gravitational strength to recapture it. 

* These population III stars are thought to have been true behemoths, much larger and more energetic than any stars around today. So these wouldn't necessarily have had to form inside the halo that would later become a dark galaxy, they just had to be in some reasonably-nearby larger galaxy.

And that's really all there is to it. They cover this in great quantitative detail, much of it having long been examined before but here all at once and in some depth. But how does one go about verifying this ? How many such dark halos should have enough gas to be observable with current HI surveys ? How do the line widths of the candidates compare with the theory – how many should we expect to see according to the model ? How does this quantitatively address the missing satellite problem ? What testable predictions does it make ?

Frustratingly, none of this is mentioned. It's great to see dark galaxies being used as a mainstream term but it feels like a cliffhanger ending, stopping at the point things get interesting. And I seem to recall other people having problems with making the reionisation ("squelching") solution fit the observational data, so more comparisons to earlier works would have been nice. 

Still, the idea of dark galaxies, being a once openly-derided solution to a major problem in cosmology, now seems to have transformed into an inescapable inevitability, not a problem but simply reality. Specific candidates, I suspect, will always remain problematic, but the notion in principle now appears to be greeted nor with mere tolerance but actually embraced : yes, these halos do exist, it's just that we can't see them directly. So the wheel turns.

The faintest galaxy is getting even fainter

Today's paper is about a galaxy so faint that after reading the discovery paper back in 2018, I must have immediately forgotten all about it. Which was a mistake, because it's in Leo. Since I did a whole paper on optically dark gas clouds and stuff in Leo but didn't cite these authors, I now feel a little less annoyed that they don't cite me either.

Anyway, this little galaxy's claim to fame is being bloody dim. It is in fact, they say, the faintest object every detected by its optical emission. It's embedded right in the heart of the Leo I group, home to the extraordinary Leo Ring, which is just a little to the north of this object. In fact, the galaxy is in a bridge of HI connecting the Ring to the giant spiral galaxy M96, whose own HI emission is rather distorted.

How does such an incredibly faint, diffuse object form ? They suggest two scenarios. It could be a really extreme but basically "normal" galaxy, having managed to maintain an incredibly low star formation rate over the whole lifetime of the Universe. This would be really interesting because it would then be unclear how it ever formed any stars at all, with models not predicting stuff like this : it seems to be just too diffuse to do anything (this might be a fun connection to other Ultra Diffuse Galaxies found elsewhere). Or, perhaps less excitingly, it could be a tidal dwarf galaxy, having only just recently formed from the gas stripped away from M96.

If it was the first scenario, then it should be expected to have an old stellar population. So here they present Hubble observations indicating pretty unequivocally that it doesn't. They can detect the occasional older star but nothing above background contamination levels, with the stars in this object clearly dominated by younger ones. It all suggests a brief starburst took place about 300 Myr ago and then stopped, with pretty much nothing happening today at all based on the Halpha emission. All their measurements emphatically support this : the distribution of stars, their metallicity, modelling of these parameters by different techniques, everything. Circumstantially, the distortion of the gas in M96 also supports this interpretation (though we still have no idea how the Leo Ring itself, which is connected to M96, actually formed).

So this is pretty cut and dried. The tidal dwarf model means that unfortunately this doesn't really tell us much about galaxies more generally, since this would be enormously atypical of the galaxy population in general. And in fact they describe this galaxy as "failing", being unable to sustain further star formation and likely to be tidally disrupted. What seems to have happened was that the initial compression of the tidal field triggered a brief burst of star formation, but as the whole larger system expands, this has already stopped.

This all raises two questions for me. What does this imply for the optically dark clouds in the same region ? Remember one such "cloud" is actually optically bright, and not especially diffuse by any standards, either optically or in terms of gas. It's an open question whether that object is in any way related to the other, truly dark clouds or just a coincidence that it's so close to them and so similar in terms of its gas content. Could it be a similar case to this galaxy but far more extreme ? I dunno.

The second question is, why isn't there a third option ? Why can't it be a hybrid of the two ? Rather than a normal but extremely diffuse galaxy "recovering" from a tidal encounter, could it not instead be a once-dark galaxy that has only just lit up thanks to the tidal encounter ? I can't think of anything here that would rule this out, but nor can I think of a good way to test this. Still, maybe things aren't quite as clear-cut as they at first appear.

El Gordo plays ping-pong

We've encountered "El Gordo" (The Fat One) a couple of times before. First there was a claim that this merging cluster is just too big, too early in the Universe, and colliding at a speed too high to be compatible with Standard Model predictions. I noted that such claims do seem legitimate, but I wouldn't jump on the bandwagon just yet : the chance of finding such a cluster seemed to be very non-linearly dependent on the parameters, so I'd bet observational errors could have quite a part to play here.

Next there was a rebuttal which said that everything was fine. The mass was actually only about half the earlier estimates, and the infall velocity way smaller. But while I'm strongly in favour of the Standard Model and naturally suspicious of these claims that it's been debunked, I have to say I didn't like this particular paper. Their mass estimates seem fine so far as I can tell, but I found the justification for their much lower collision velocity very badly expressed.

Along comes a counter-rebuttal (hence the ping-pong) that, oh joy, does the same thing but for the opposite side of the argument.

The authors of this latest study don't seem to have any beef with the revised lower mass estimate. Instead their attention turns to the infall/collision velocity. They maintain there's also still a problem with the more famous Bullet Cluster, but the studies I read a while back found that there isn't, so I'm going to ignore that aspect.

They start with quite a nice overview of previous studies but immediately fail to learn any lessons from them. Noting that different authors have come up with radically different values and interpretations, they seize on the latest measurements as being of unimpeachable accuracy and precision. Inclination angles have varied from 30 to 75 degrees or so, infall velocity estimates have varied from 1200 to 2500 km/s, the current stage of the merging has been interpreted differently... all this to me suggests that there's really quite a wide margin for error here, and we just don't have good enough observational data or numerical simulations to constrain anything very much,

I also dislike their whole interpretation of the low probabilities of finding such objects in the Standard Model. It's a one-in-ten-billion, they say. This is a big red flag by itself. It doesn't make any sense to me that the Standard Model could get things so fabulously right in such a plethora of circumstances and then fail so utterly miserably in others, where the forces at work are supposedly the same. Of course, to give them their due, they would say that the Standard Model fails miserably elsewhere too, but they're wrong about that.

On a more pragmatic level, I wish they'd define the velocity terms more clearly. What exactly is meant by the infall, peculiar, and observed velocity ? Oh I'm sure they're simple enough (I can certainly make an educated guess what they mean), but without having them spelled out and rigorously defined, this becomes just as confusing as the previous paper.

And where this really becomes critical is their underlying methodology. The previous authors, they say, deliberately disregard high infall velocities because that contradicts the Standard Model. They say this means we can't use that analysis to say how likely it is that such a feature arises in the SM, but I think this is daft. If we can show that such a scenario works, that this agrees with observations, then the circular nature of the argument doesn't matter : "we looked for SM-compatible solutions and found one" is a perfectly valid approach, and while it might not tell you about probabilities, demonstrating compatibility is much more important.

And then they do the exact same bloody thing but in reverse, insisting that they must search simulations only for objects with infall velocities which are incompatible with the SM ! This is rank hypocrisy and exasperatingly silly.

They make similar daft and blunt claims about other previous analyses too. They could have just said "we improved the analyses" rather than stating that the others were outright wrong. Saying there's no reason to assume the cluster components are gravitationally bound in no way implies that they're not. And sure, it could be that LCDM is incorrect and it could be that this would result in a high infall velocity, but what exactly is the reason to assume with such certainty that this must be the case ? When is a factor of a mere 1.5 grounds for an unsolvable discrepancy ?

It's all just weird and I don't like it at all. If you're going to say that some infall velocities must be excluded, then say why. Saying that you exclude them just because they're compatible with the SM makes not a lick of sense. That's committing exactly the sin they were trying to avoid. I liked their earlier paper much more than this. As it stands, I'm now far less convinced that El Gordo is the CDM-killer they think it is.

You should have gone before we came out

Where do galaxies lose their gas ? Galaxies in clusters have been shown to have much less gas than in the general field. In Virgo, for instance, on average they have ~50% or less than comparable field galaxies of the same morphology and brightness. 

Now we know there are lots of processes at work inside clusters themselves than can do this, especially ram pressure stripping. That's almost certainly responsible for the majority of the gas lost. But there are many hints that galaxies elsewhere can lose gas too, not usually as much as in clusters, but enough to be significant. This is important because most galaxies don't live in clusters, so if we really want to understand galaxy evolution, we should probably stop spending so much time on the sexiest 1%* of the population and look at all the others from time to time.

* Well, it might be a few percent, but not more than this.

This is another paper I wouldn't normally read because reprocessing existing data tends not to accomplish much. But in this case I think they're on to something... ironically by looking at clusters. Though I have to say, their sample definition seems convoluted in the extreme and they gave such a detailed breakdown of how they divided everything I wanted to slap them and shout JUST TELL ME THE DAMN NUMBERS ! Which they eventually do, and it's a few thousand per object type.

Key to this is that they can distinguish between different sorts of galaxies in clusters. I was racking my brains because this seemed curiously familiar. In fact this idea was mentioned a couple of years ago in a conference, but disappointingly the presenter isn't on the author list or cited. It may, of course, be a completely independent discovery.

Anyway, most galaxies in the field are in small groups, often very small : two or three members, though sometimes more. So this means that clusters tend to assemble by absorbing whole groups of galaxies rather than individual ones. In some cases it's possible to identify subgroups within a cluster, which are therefore likely recent arrivals. Individual galaxies within clusters, not part of any subgroup, could be individual field galaxies falling in for the first time, but they're more likely to be older arrivals whose original constituent group has been broken apart by the chaos of the cluster. 

By analogy, nightclubs. It takes a while for groups of people to get broken up by the general throng, so if you see a group of people all together, chances are they just arrived. And equally, not many people go to nightclubs by themselves, so solitary people looking lost and confused have probably become unfortunately separated from the group that originally dragged them in there.

But... galaxies. What the authors do here is very simple. Using samples that identify these different galaxy types, they plot (see their figure 1) the fraction of star-forming galaxies as a function of cluster-centric distance. They find that for solitary galaxies in clusters, the star-forming fraction steadily increases as you go further from the cluster... right up to the level of the general field. There's no distinct break in star formation activity for individual galaxies as they enter the cluster, it's just a smooth curve.

In contrast, galaxies which are still in sub-groups within clusters also increase their fraction of star-forming members, but they reach a plateau. At high enough distances, the star-forming fraction never increases for these galaxies, whereas for the solitary ones it just keeps rising.

In other words, the gas-rich, late-type (spirals and irregulars) individual galaxies just continue losing gas as they get ever-closer to the cluster centre. But exactly the same type of galaxies which are in groups never had as much gas to begin with. They've already lost some of their gas. Not as much as they eventually lose in the cluster, but still a detectable difference compared to individual galaxies. Ergo, pre-processing definitely happens.

I think this is a very nice confirmation of something already strongly suspected. The main accomplishment here is using a substantial sample to increase the cluster-centric distance well beyond what was previously attempted, which explains why it wasn't seen before. You really need to go to very large distances indeed to see this, but when you do, it's astonishingly clear. Honestly it's rare and gratifying to see a result in astronomy which is so clear-cut as this. Hooray !

One small caveat : you might be wondering, well, shouldn't the individual galaxies still show a plateau if they too tended to be in groups to begin with ? Probably not. Deep within the cluster all galaxies will be dominated by cluster-specific processes. But at the distances where there's a difference between individual and group galaxies, it's probable that the solitary objects were never in groups : they're just too far from the general melee of the cluster to have been much affected by it. So there is a change in the nature of individual galaxies as you descend into the cluster, from being dominated by the habitually-solitary to those who were indeed once in a group. But both of these end up being equally affected by the cluster's ram pressure, hence the smooth, continuous change in star-formation properties.

Do legumes give galaxies gas ?

This paper is about "green pea" galaxies, which are so-called because they're small, highly concentrated and even look green because of their strong spectral line features. Green is a very rare colour in extragalactic astronomy, and indeed for stars in general. The blackbody curve over which they emit means that they emit so much light across the whole spectrum that green is always washed out by the blue and the red. Only when you get something that's not a blackbody, something emitting over a very narrow wavelength range, do you get anything green. And that's rare.

"Green peas" tend to be more distant, but there are also closer "blueberries", which you'll have guessed are much the same apart from the colour. Whether they have similar spectral line emission to green peas is not clear.

Anyway, the authors here try and figure out how the HI (atomic hydrogen) gas content of the green pea galaxies varies with their stellar content. A perfectly sensible goal given how strange these objects are; perhaps this could shed light on star formation in extreme objects. But I have to say I don't like it very much.

Like some other papers I've been reading lately, this one contains no new observations but uses entirely archival data. But unfortunately this one is more typical of its class, not actually finding anything new or at most a marginal, incremental difference. They have a sample of 19 HI detections (and 21 upper limits, which were observed but not detected), though they have to exclude a couple because of issues with the optical and/or UV data. Based on the details they go into, they seem to have re-processed the HI data, though it isn't at all clear why – was there some issue with the earlier analyses ? If so, they don't mention it.

Figure 2 is by far the most interesting. This plots the now-classic "main sequence" of galaxies showing how their star formation rate scales with stellar mass, in a nice, tight correlation. Green pea galaxies also seem to correlate but with a totally different, much steeper relation : they have far higher star formation rates than their stellar masses predict. This holds true for both the HI detections and non-detections. Unfortunately the upper limits of the non-detections are all over the place, so it's not possible to say if there's any real difference between those with and those without gas. And the main trend is not a new result.

What they instead concentrate on is finding other relations to the optical data. They say their sample is offset from a previous relation between the MHI/M* relation as a function of NUV (near UV) - r magnitude (basically a colour measurement*), but... well it might be, but to me that earlier relation, for normal galaxies, itself looks like a dodgy fit. So I don't think too much can be said here.

* One minor but quite interesting point they make is that using this UV-optical is better than using two optical wavebands for colour, since the green pea star light is much more UV-dominated than in normal galaxies.

One thing they do show which looks convincing, but I'm not sure if it's a new relation, is that green peas have excessively high gas fractions as a function of just about any parameter. New or not that's nice, but even here their plot could easily have been so much clearer. Then they try and plot gas fraction offset (measured – expected) as a function of various parameters, say there's a trend, but as far as I can tell there just isn't. This is one of those cases where if someone shows you a weak trend and you say, "I've seen worse claims"... this is one of those worse claims.

Then they try and find what sort of scaling relation does the best job of predicting the HI mass of green peas. As far as I can tell, they seem to have taken the standard practise of throwing everything against a wall and seeing what sticks a little too far. They come up with hideous relations involving different colours, stellar masses, star formation rates and surface brightness levels that to me has no obvious physical significance at all. Frustratingly, they don't discuss this. 

And that I do find strange and annoying. It's reminiscent of p-hacking, where if you search for correlations using complicated enough relations and large enough data sets, you're bound to find something. Was there any prior reason to suspect this torturous relation had some physical significance ? If so then it's interesting ! If not, if, as I suspect, it's just an empirical fit, then it's just a statistical artifact. I'm not saying that's the case, only that they needed to be a lot clearer about what this is supposed to mean, physically, before I take any further interest in it.

It's gotta come from somewhere

One thing that's never seemed terribly puzzling to me is where galaxies get their gas. Estimates of the star formation histories show that with their current gas content, galaxies should typically exhaust their gas within a gigayear or so. A lot of people find this suspiciously fast, that to maintain their currently constant levels of star formation is impossible unless the galaxies are being re-fuelled from somewhere.

Personally I've always found that one gigayear is not fast enough for a need to invoke refuelling (or accretion as it's usually known). It just means that galaxies had more gas in the past and will run out comparatively soon, but in a time equivalent to about one-tenth the age of the Universe isn't enough to set alarm bells ringing for me. Sure, if it was the next few or tens or even hundreds of millions years, then I might be concerned. Then I might say, "hang on, isn't it a weird coincidence that we've arrived on the scene just as galaxies are about to stop forming stars forever ? Isn't it more likely they're being replenished from somewhere ?"

On the other hand, the apparent constant rate of star formation does seem more legitimately odd. If galaxies are truly running out, naively you'd expect to see that reflected in their star formation histories. So people have postulated that galaxies are accreting gas from the field somehow, either from "hot mode" where gas cools very slowly and omnidirectionally, or in "cold mode" where it condenses into cooler, more distinct streams which funnel themselves into the galaxies. Claims to have detected the latter are always controversial because it's very hard to say that a stream of gas isn't just gas that the galaxy is losing by a host of much more well-understood processes.

Today's paper attempts to address this mild puzzlement. They use a sample of galaxies which is as homogenous as they can get and apply some reasonable scaling relation where necessary to calculate the change in gas. For instance, they assume that the bulk of the normal, "main sequence" galaxies today were already on the main sequence 4 Gyr ago. That is, they formed stars at a predictable rate given their total mass of visible matter. This is not at all an unreasonable assumption : 4 Gyr is enough to expect some evolutionary changes but nothing dramatic, and while of course plenty of individual galaxies might have experienced the odd burst or sudden cessation of star formation, there's no reason to think these would be statistically significant. 

And kudos to the authors for clearly acknowledging their assumptions and how complex real star formation activity can be. In public talks I sometimes go on a breathless monologue describing the various process at work and how they relate to each other; they do much the same here, except that they clearly spell out how this is likely to affect star formation – rather than my own take-home message which is only that it's bloody complicated. 

The really big assumption, the most difficult point to make reasonable inferences about, is how much the HI gas has changed. A handful of stacked observations have now managed to detect this atomic gas out to these vast distances, but these combine data from hundreds of galaxies. We currently have no real idea how it's changed in individual galaxies on these timescales; in contrast, the molecular gas can be observed directly and is much better understood. 

Add to that that the relation between atomic gas and star formation appears to be subtle. It's popularly described as the galaxy's fuel tank, the reservoir from which star formation ultimately occurs. Which is quite appropriate, since understanding how much gas is in the tank and how fast a car is going is not easy ! The actual gas in the engine itself, the stuff that's exploding in the pistons, is thought to be molecular, but there are strong hints that HI is directly involved as well at least in some cases. Most HI-rich galaxies are blue and star-forming, but some have loads of gas but hardly any stars at all or only old red ones, so it's not at all a straightforward connection.

Using their various scaling relations as best they can, their conclusion is at least an interesting possibility. They say that what seems to be happening is that galaxies are both losing and gaining gas : overall, they're running out of the molecular gas (the stuff in the engine itself actually doing the business of star formation) but actually gaining atomic gas (the stuff in the tank). This is a small change for dwarf galaxies but really quite large (70 % !) for massive ones, even to the point where the galaxies have grown significantly in overall baryonic mass as a result of this. Galaxies then, are still assembling even today, not just from mergers but from the condensation of the thinnest gas in the intergalactic medium.

What this means is that something is changing star formation efficiency. Somehow galaxies in the early universe were able to efficiently convert all their gas into stars, whereas today something is preventing the HI from cooling into molecular gas which can form stars.

Why could this be ? Frustratingly they remain silent about this, which is annoying because this is such an obvious question you can't not ask it. Especially since modern galaxies are much more metal-rich, which enables very much faster cooling and condensation of the gas : if anything they should be more efficient at forming stars, not less. On the other hand there are far more stars around today (though less energetic), so perhaps stellar feedback is to blame.

Well, I dunno. Fair play to 'em for being clear about all their many assumptions, but where we go next with this is anyone's guess. It needs a lot more independent studies before anything else can be said about this.