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.