Dynamically Different Dwarfs Defy Definitions

This paper is another page in the long-running saga of the dynamics of Ultra Diffuse Galaxies. Many of these large but faint objects have been claimed to have a curious lack of dark matter, in contradiction to what we might guess from standard cosmological models. That by itself does not challenge the dark matter paradigm, but might even, perversely, support it. 

How ? Well, the existence of dark matter at least allows for the possibility of this separation of components, whereas theories of modified gravity basically don't. In the CDM (cold dark matter) framework, galaxies may or may not contain dark matter, and this mass strongly affects the motions of their stars and gas. Indeed, we know that not everything is dominated by dark matter : globular clusters and tidal dwarf galaxies certainly don't have any dynamics that suggest any significant amount of extra mass.

In contrast, with modified gravity theories (or Modified Newtonian Dynamics at any rate), if you have two systems with the same distribution of mass, they should always have the same dynamics. These theories say it's all dynamics is due to the visible material, so if this is the same, so should be the motions of the stars and gas. A major caveat is that the systems must be isolated and in equilibrium, but that's the long and short of it.

But the discovery of isolated galaxies apparently lacking dark matter poses a different sort of challenge to the standard model. Such systems weren't predicted; there's no obvious reason to expect them to exist. And yet they do.

This is a convoluted tale, and it's been a while since we last looked at this. So now's as good a time as any to recap the whole story so far. Feel free to skip ahead if you prefer.

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1) Ultra Diffuse Galaxies were first discovered decades ago, but not as a class of objects. They were noted as exceptional, "low surface brightness" galaxies, big and faint and fluffy, but rare. Then a few years ago they were discovered in large numbers, first in galaxy clusters, but then in other environments, even in isolation. Only when they were realised to be common were they given the UDG label to distinguish them from more general low surface brightness objects.

The obvious question after their discovery was whether, being large, these were also heavily dark matter-dominated systems, "failed" Milky Way-like galaxies only much fainter, or actually a sort of huge dwarf : very extended structures which had some dark matter component but nowhere near as much as the true giants. Although there are a few very interesting objects which may well have a great deal of dark matter, the consensus seems to have settled fairly quickly on "huge dwarfs" rather than "failed giants". I go through this in a lot more detail here and here. 

The most extreme interpretation of this is that UDGs are actually misnamed, and their large size is a misleading result. There are a lot of different ways to measure galaxy sizes, and only one of these really indicates that they're giants. Using other measurements, they're actually of the same size as typical dwarfs. They'd have unusually flat distributions of matter, but this isn't nearly exciting as the more popular claim that they're as big as the Milky Way but a thousand times fainter. Certainly it's a lot easier to account for the discovery of lots of faint dwarf galaxies than it is lots of giant ones. Cosmological models in fact demand we find extra dwarfs, but would suffer horribly if there were lots more giants than predicted. A few would be okay, but a whole population would lead to some serious head-scratching.

So this general picture seems to point to UDGs as being interesting for the details of galaxy evolutionary theory, but not a challenge to anything more fundamental. But then things took a left turn.

2) Measuring the dynamics of these systems is a challenge because they're so bloody faint. Some early results did show that at least some were indeed dark matter dominated, but not to the extent of being giants. But then came the discovery of two UDGs, in a nearby group, that apparently lacked any dark matter whatsoever. Controversy dogged this one from the start, with initial claims that they'd just measured the velocity dispersion wrong (I thought this one was a bit forced, and it turned out the measurement was just fine) giving way to a much more protracted debate about the distance. 

Finally it turned out that the distance was the larger, more interesting value and the claims that they lack dark matter were vindicated. This is an important lesson in not trusting to consistency too much. Had they been smaller and closer, they'd have been perfectly normal systems and it would have been a big hoo-hah about nothing, which is usually the case. But this time it was the more interesting result that was correct.

That places UDGs firmly back in the realm of being a potentially serious difficulty for the standard cosmological model. Yes, we know such objects can form in rare cases, but nobody predicted them in large numbers like these. Indeed, when people did find them in their simulations, they found they were the result of a numerical error, not a physical result at all !

But along came a new model which seemed to be quite satisfactory. It's well-known that tidal encounters between galaxies can preferentially remove dark matter, since this is their most extended component. This new one showed that direct collisions can, in the right conditions, remove enough dark matter to explain these two weird galaxies, in a rare but not exceptional event. Further observations seemed to vindicate this.

Thus was the challenge to cosmology averted once more. Initial worries that there might be too many massive, faint galaxies gave way to concerns that there might be too many faint, dark matter-deficient galaxies, but these were addressed with a convincing explanation : only a few of these latter types had been found, and those in an environment where galaxy-galaxy encounters could be shown to be plausibly responsible. Phew !

All was well with the world. Except...

3) These two cases were the tip of the iceberg. Quite separately, some UDGs had been found to have gas, which makes measuring their dynamics much easier. And a whole population of these appeared to be rotating too slowly, again implying a lack (or at least a deficiency) of dark matter. Even with really good quality data this still seemed to be the case and not a measurement error. And they were in isolation too, so the collisional model just couldn't work for these.

But then it seemed that actually maybe there was a measurement error after all, and that while the authors had put forward many genuinely excellent arguments to support their conclusions, it was more probable that they're just estimated the inclination angle wrong. It only takes a slight error in this angle to turn an interestingly-small rotation speed into a boringly-high rotation speed, to go from no dark matter to an entirely normal amount of dark matter. Though, this is only my interpretation, and the authors are sticking to their guns that they've measured the inclinations correctly, so fair play to them.

There's one other, largely overlooked part of the story : the detection of a population of normal brightness galaxies which rotate too slowly. Here, claims that the inclination angle is a problem don't look credible to me, and these probably deserve more attention.

So that's where we currently stand. The discovery of UDGs initially prompted concerns that maybe there was a population of massive galaxies that could pose a severe problem for cosmological models, maybe even for the dark matter paradigm itself. That concern has greatly alleviated. Now the question is more the opposite. Are there a significant number of isolated galaxies which have far less dark matter than more typical galaxies ? If so, how do they form, why aren't they found in our models, and why didn't we notice them before ? 

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Enter the latest paper. Previously, analysis of the gas-rich UDGS (HUDGs, for HI-rich UDGs) has concentrated on the half-dozen or so with high-resolution gas measurements. This allows us to check that the velocities of the gas follow the standard, ordered pattern expected for rotation. It also allows us to estimate the inclination angle independently of the optical data, as the HI and stellar discs might be misaligned. We need the inclination as this lets us convert the apparent rotation speed (that we directly measure) into a much more accurate value.

This paper revisits the first sample of HUDGs. I noted at the time that these seemed to show a distinct trend from the Baryonic Tully-Fisher Relation, which simply plots total mass of normal matter as function of rotation speed. Traditionally galaxies follow a very nice, low-scatter trend here, but the UDGs extended to much lower rotation speeds.

My plots were impossibly crude (for a gruelling blow-by-blow account of how to do all the proper corrections needed, see this), and here the authors attempt something much more rigorous. They restrict the sample to the best possible cases, 88 galaxies where the optical images are clear and the HI signal is bright. They don't have high resolution gas data, but they have a clever way to demonstrate that this isn't necessary.

Their main result is exactly as I found : the HUDGs demonstrate a break in the BTFR, following a continuous sequence from "following the usual relation" to "having much lower rotation speeds than expected". Although this is in no small part because they have a great deal more gas than typical galaxies, interestingly they still show the same basic offset even just using the stellar mass rather the total (gas and stars combined). 

Of course the question then becomes, well what about the inclination angle ? Here's their clever trick. They don't have any better gas measurements than the original sample, so these are all unresolved : they can't check for sure if these objects have ordered, rotational motions. So they use the optical measurements to estimate the inclination angle. Using this, when the plot the corrected rotation velocity as a function of the apparent axial ratio of the galaxies (which is what gives you inclination), they find no evidence of any systematic trends. This means that inclination angle errors might be causing some extra uncertainty and scatter in the velocities, but are very unlikely to be responsible for the overall systematic trend.

This looks like a very nice piece of work for me, quite careful to check for any possible systematic effects, plotting things in many different ways to figure out what's going on. The one trick they've missed is a reference to this recent paper, which shows that some low-mass galaxies (not specifically UDGs) which apparently rotate too slowly can be reconciled with the BTFR with enough corrections. So their claim that the deviation of the UDGs from the BTFR is not in agreement with standard models might not be correct, but on the other hand, this is all very new and it needs a lot more examination. Furthermore, that other paper was concerned with galaxies which were much lower mass than these, so its finding may not be all that relevant here.

Where does that leave us ? Well, these UDGs seem to be dark matter deficient but not totally dark matter free. They say, "The formation of them cannot be reproduced in current cosmological simulations. These baryon-dominated dwarf galaxies could be game-changer laboratories in testing cosmology models and galaxy formation models." And they might be. They're weird. But this saga is already full of many twists and turns, and this paper is just one more page in what's turning out to be a gripping read. What happens next, I for one wouldn't like to guess.