Nothing to see here

The last few years have seen a lot of hoo-hah about galaxies apparently lacking dark matter, but this may at last be drawing to a rather boring conclusion.

First, we had a whole kerfuffle about two dwarf galaxies, NGC 1052-DF2 and DF4. These seemed to have such a low velocity dispersion that they wouldn't require any additional matter apart from their stars to hold them together. There was some dispute about the magnitude of the velocity dispersion (in my opinion not a very credible one) which went away quite quickly because yes, their velocities really were very low. A much more protracted and complex dispute arose about their distance, which was only settled quite recently and found that yes, they are far away (and lacking dark matter) after all.

Those first discoveries were pure starballs. Then, we've also had the saga of galaxies with gas, for which we can much more easily examine the rotation velocity. They were also so far away that there was no prospect of a distance controversy. But just the other day, despite many arguments in favour of these being a population of weird objects, simulations showed that such objects can't be stable. It's far more likely that there's been a slight measurement error in their inclination angle. I wouldn't personally call that one settled just yet, but it's definitely going in that direction.

There was also bad news for those of us hoping the original objects would turn out to be something truly weird : simulations found a plausible way of producing objects like this through unusual but entirely plausible encounters, specifically requiring direct collisions between galaxies, which are relatively rare. Which is great news for the boring old standard model of cosmology.

Today's paper feels like another nail in the coffin for anyone hoping these objects would turn out to be truly strange. 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.

It's a nice little paper but I do have to wonder why on earth it made it into Nature. Noting the simulations showing that collisions are a viable explanation, they go out and have a look at the context of DF2 and DF4 on a larger scale. They show they're in a neat line of maybe 11 objects in total, stretching over 2 Mpc. This is nicely consistent with the collisional origin hypothesis.

And honestly... that's it. Yes, it's a nice discovery, but they don't go on - as I assumed they would - to do numerical simulations to back this up. They don't have any new observations of these additional galaxies to see if any of them have anomalously low velocity dispersions. They just report the basic discovery along with their (undeniably) very interesting hypothesis. I mean, I'm glad they reported this, but it feels much more worthy of a letter to MNRAS than to Nature.

As I said though, this is not the final nail. More observations of those new galaxies are essential to determine their distances and velocities, otherwise this is just a chance alignment. More simulations to examine exactly what we expect in a collision similar in quantitative detail (major kudos to the authors for proposing this, which largely offsets publishing it in the wrong journal) would also help, a lot. But it's awfully tempting to declare the mystery... well solved is too strong a word, but shall we say, addressed then ? Yeah, that'll do. Mystery addressed !

There is also perhaps some prospect of a weird afterlife for these oddballs. If they do form through good old-fashioned collisions, they're like teeny-weeny versions of the famous Bullet Cluster. This has caused a lot of controversy because Standard Model enthusiasts have pointed and laughed at MOND believers by saying, "Hahah ! Look, we found a case where dark matter can be separated from normal matter, and you can't do that just by modifying gravity !". To which MOND devotees responded by saying, "Yes it can, you don't understand our theory at all, and look the collision velocity you need is just too dang high." But then the Standard Modellers retorted with, "Yeah, but you don't have a unique relativistic solution, and anyway, the collision velocity is way lower than we though, so pfffffp !".

Or something like that anyway. I tend to heavily favour the Standard Model, but if we had more cases to compare, especially on such dramatically smaller scales like these galaxies, this might give us a much more rigorous comparison. The thing about MOND though it's that it gives such similar results to the dark matter paradigm that one wonders what the point of it is. So, we'll see.

Curiously cloudy

Today's paper is one that fell through the cracks of time from March 2021 all the way to the present day. Meaning, I should have read it a year ago, but didn't.

Returning to my favourite topic of optically dark gas clouds, this is an early science project of one of the SKA pathfinder telescopes. Naturally they chose a nearby galaxy group as a target. It's not an especially interesting group as far as I can tell, but looking at any galaxy group with the higher sensitivity and resolution of a new instrument is always a good idea.

What they found is... not much. None of the galaxies seem to be doing much of anything. Except, there are these two quite hefty HI clouds with no optical counterparts floating around and no obvious reason how they got there. Hooray !

Unfortunately though, God appears to have been very cruel by aligning both of these with optically bright sources. One is right behind a bright foreground star, which must be within our own galaxy and cannot possibly have anything to do with the cloud. The other is in front of a more distant galaxy, likely too distant to have any relation either. But of course this doesn't mean that the clouds don't have their own optical emission and it isn't simply being wiped out by the brighter intervening sources. They try to model and remove these, and don't find anything obvious, but I'd be very cautious about drawing any strong conclusions about this. There's just too much other optical crap in the way to be sure they've removed it all.

There's still a lot the data can say though. One of the clouds looks quite a lot like a standard rotating disc. The other is an elongated blob, but here the velocity gradient is across the shortest axis which is frankly just confusing - a spinning cylinder ? Doesn't make a lot of sense to me.

Combined with their size of a few kpc, the velocity gradients aren't very high. In fact they're low enough that the objects could be gravitationally self-bound with their HI alone. And their HI mass is pretty substantial, comparable to dwarf galaxies. So are these the long-sought "dark galaxies" of popular radio astronomy myth and legend ?

Maybe ! But no explanation is very satisfying, which is why these are really neat objects.

First, the standard go-to explanation for all HI clouds is that they must be tidal debris. This is a perfectly sensible default option. The problem in these cases is that there aren't many galaxies around, and the only others with HI are many hundreds of kpc away and don't show any signs of disturbances. Where are the long HI tails ? You'd expect to see these even if much smaller amounts of gas were removed, but that we see substantial amounts of gas in just these two clouds implies a factor of several more must have been removed - tidal encounters invariably produce streams, not just isolated clouds.

But given that the clouds appear to be self-bound, this may not be such a problem. Perhaps the encounter just took place a long time ago, so that most of the rest of the gas has dispersed. Without some dedicated numerical simulations, and even more sensitive observations to look for streams, we can't really rule out this scenario yet.

On the other hand, if they were dark galaxies, their self-bound nature is very surprising. The clouds we found in Virgo had such high line widths that they directly imply the presence of a dark matter halo to keep them stable, just like for normal galaxies. These objects don't fit that. The authors search existing numerical simulations for analogues, and they do find a few, but those tend to have higher line widths.

I would also note (though the authors don't plot this themselves) that these clouds have such narrow line widths and such high amounts of gas that they deviate from the standard Tully-Fisher relation for normal galaxies. Actually this looks very similar to those controversial galaxies without dark matter, which have been quite a challenge to explain. So it does seem a bit strange to invoke a dark galaxy for objects where there doesn't seem to be much indication of a dark halo. On the other hand, they do fit the standard mass-size relation for normal dark matter-dominated galaxies. 

What's going on ? I don't know, but this opens up quite a bit of parameter space for HI clouds. The Virgo clouds I've spent so much time on really can't be self-bound by their HI because of their high line widths : they'd have to be so compact that they'd have densities so high we'd expect explosive levels of star formation. These clouds, though they have ten times as much gas, are in a regime of size and velocity where they could indeed sit around quietly doing nothing. That they're so massive is really interesting : why haven't we found more objects like this already ? They were in fact already detected with much less sensitive instruments. That kindof implies a special explanation, rather than them being part of a larger population.

My preference would be old tidal debris. But I can't see any good reason why such clouds are found only in this group and not elsewhere, so this is more instinctive than rational. Those bloody intervening stars and galaxies make things especially tricky... yet even if they do have optical counterparts, that wouldn't necessarily make the situation any clearer : objects which are self-bound by ordinary visible matter are just strange, regardless of whether they're purely gas or purely stars. This is definitely a case where the rallying cry of "we need more data !" is absolutely legitimate.

Is it a plane ? No, it's a... uhh... orbital anisotropy, or something

We haven't heard from satellite planes from a while, so let's re-open that old wound and fill it with hornets. 

My stance on satellite planes calcified quite some time ago. Claims that satellite galaxies orbit their parents in narrow planes are, in my view, mainly due to an over-reliance on numerical estimates of statistical significance and an apparent disdain for actually looking at the bloody data. Most such structures don't look like anything very much at all to me, let alone the insurmountable challenge to the standard models of cosmology they're supposed to be. See the previous link for a much more in-depth and angry examination of this.

The major exception is the plane around our own Milky Way galaxy. This one is visually unmistakable, and regardless of whether such planes are indeed common or not, this surely deserves an explanation. Of course, it's possible our own galaxy just happens to be unusual, but this isn't very satisfying.

This paper takes another look and says there's nothing remarkable about this at all.

Frankly, I am not sure I either agree with or fully understand their arguments here. They examine the plane only using the 11 brightest "classical" satellites, noting that the anisotropy is dominated by just one or two objects. Move them around a bit and actually the "plane" becomes nothing very remarkable.

Hmm... maybe. It would have been nice to see an example, but more fundamentally, they don't seem to consider that basically all the more recent satellite galaxies also follow the plane. I think it's unfair for people to add other features like stellar streams as independent evidence of the plane (because if the galaxies are in a plane, then all features resulting from their interactions must be there too), but ignoring the other galaxies seems strange. To be fair, this seems motivated by previous studies which also limited themselves to this subset.

Their other claims feel on firmer (or at least clearer) ground. Earlier studies couldn't reproduce the planes, they say, because of numerical limitations in their simulations. Apparently some satellite galaxies are disrupted through purely numerical effects (though I am not sure what exactly), but they are able to account for this and reproduce systems more closely resembling the Milky Way.

What seems to me their strongest argument is new data from Gaia for proper motions of the galaxies. If the plane was indeed long-lived and stable, as claimed, we'd expect there to be little velocity dispersion perpendicular to its axis of rotation : in fact, they say, the dispersion both parallel and perpendicular is about the same. That's not at all consistent with rotation, and indicates the plane is just a chance alignment. So while people have generally concentrated on what fraction of galaxies in simulations show planes at any one moment, it might be interesting to consider what fraction ever show a plane at any point in their history.

All this is nice, but I can't help looking at the structure and still thinking, "really ? that's normal, is it ?" Just as it would take something dramatic to convince me of the counter-claim that "planes are everywhere !", so it's going to take me a bit more than this present work to convince me that "the Milky Way's plane is nothing special."

Un-missing matter

When you find that the missing matter of a galaxy is itself missing, and then you realise that actually it isn't missing missing matter, it's just the regular missing matter, you're in a right proper linguistic pickle.

Readers are advised to consult this previous post for a full summary. In brief, there's this galaxy which doesn't seem to have the usual dark matter (unseen and hence missing by direct observations) needed to hold it together. Based on the mass of its visible stars and gas, its rotation speed is so slow that it's apparently gravitationally bound without needing any of this additional matter at all. And this just doesn't fit the general picture of galaxy evolution : yes, some systems like this can be produced in interactions, but this object is nowhere near anything else. And a generally overlooked detail is that the slow rotation speed means the galaxy wouldn't have had much time to settle into its apparent stability.

My major worry last time was that the rotation speed of the galaxy might have been underestimated. This is dependent on estimating how inclined the object is along our line of sight : if it's directly edge on, we'd be able to measure its full rotation directly, whereas if it's face on, we wouldn't be able to measure rotation at all; at intermediate inclinations we can apply a correction factor. The observations indicate a low inclination angle, which makes this correction already uncertain, but the very ragged edge of the gas disc used for estimating the angle makes this process extra difficult.

To be fair, the authors of the observational studies have had many good arguments in favour of the higher inclination angle they say is preferred by the data. Indeed, it wasn't until the last paper that I would even have questioned this. And they do admit that the inclination angle is the biggest uncertainty.

Today's paper is by a different group coming at the problem with numerical simulations. They cite a lot of private communication with the original team, so it's nice to see that this dispute is being addressed cooperatively.

You might remember my own extensive efforts to model a galaxy without dark matter that met with assorted dismal failures. Only when I added a nice big dark matter halo, to ensure stable circular orbits, did everything magically click into place. This is nothing very new, as the author's of the latest paper explain - in fact, disc stability was even one of the motivations for dark matter in the first place.

What they do is much as you might expect. They model the galaxy as according to the original observations and see what happens. Which, as I found myself, is that the object tears itself apart in short order. Even if they use a very peculiar dark matter halo the original team say the observations do permit, it just doesn't work - it's a bit better, but not much. And then, lo and behold, if they allow a lower inclination angle and hence higher velocities and a big dark halo, everything works out nicely.

This is an awfully tempting explanation. It's just possible that there is some combination of parameters that would be compatible with the observations where a dark halo is not required, but this needs a lot of fine tuning and feels contrived. In contrast, accepting the inclination angle is wrong and there actually is a dark halo after all is a hell of a lot simpler and more satisfying. There is one other option that the object is not in fact stable, and is in the process of disintegrating, but this would seem to be incredibly unlikely in the case of an isolated object.

Not that we should discount these alternatives altogether, mind you. Remember one of those other claimed galaxies without dark matter, which had a prolonged controversy over its distance. A lower value would have made it a very satisfactorily normal galaxy after all, but in fact it was the more puzzling higher value that won out. So galaxies without dark matter are still, for the time being, very much a thing. But in this particular case, my money would now be quite firmly on the "inclination angle measurement" problem. And given that those other cases may well be tidal objects, this particular controversy just might be wrapping itself up.

Not the One Ring but the wrong ring

I have to confess that I read today's paper by mistake. The title is about "HI debris in the NGC 7232 group", which I misread as NGC 7332. And we have our own observations of that group, so I wanted to see if the new data found anything that we didn't. Well, I suppose technically it does, but only because it's of another group entirely...

This meant I was at first disappointed, and then intrigued, then annoyed, then intrigued, and then mildly annoyed again. But overall I'm interested. In fact I think the authors potentially under-sell their main result, which is always better than the more common opposite case.

Their target is a relatively nearby little group of galaxies, using radio data to study gas loss. Why look at groups ? Massive clusters tend to get all the glory because there are so many galaxies crammed in that there's always tonnes of stuff going on, plus you can observe lots of targets in a single observation - but it's small groups where by far most galaxies tend to hang out. Clusters may be sexy, but that's like saying Instagram influencers are sexy. Perhaps they are, but if you wanted to study a cross-section of humanity, looking on Instagram would be about as poorly representative as selecting people randomly from the public library. Galaxy groups, on the other hand, they are much more like the great thriving mass of Joe Public - it's them you want to study if you want to know about more typical behaviours.

The authors use the fancy new MeerKAT array of 64 antennas to get higher resolution and sensitivity HI observations than has been possibly in this group before. And this does help, substantially, though I should add that the column density sensitivity is still about two orders of magnitude worse than Arecibo was capable of. But such is life.

Anyway, at the heart of the group is a triplet of galaxies - two spirals and one lenticular. They find... a great big HI ring (see their figure 3). This is already really strange, but for some reason they don't comment on the morphology at all. I find that to be a very strange omission, especially considering that the ring isn't small - it's about 100 kpc across. Polar rings, which orbit directly around the plane of a galaxy, are at least sort-of understood, but this isn't one of those : the three galaxies are found all on one side of the ring, embedded within its HI gas - there's no galaxy in the centre of the ring. Not mentioning that the gas is a giant ring is a bit like finding a giraffe and neglecting to mention that it was purple.

Actually, they point out that only two of those galaxies may be associated with the ring material. At the velocity of the third spiral, which is quite a bit different from the other galaxies, they don't detect any obvious HI extensions. So probably there are just two galaxies involved here. But the ring isn't anything remotely like the tail and counter-tail structure seen elsewhere and in umpteen numerical simulations of interacting galaxies.

Connected to the ring is a long HI tail terminating in a big blob. A bit further away in the same direction is another, smaller tail, orthogonal to the first and pointing directly towards (almost intersecting) a lenticular galaxy. So this also, they say, might have been involved in the interaction. Okaaay... yes, it might be, you're not wrong, but this is glossing over a lot of interesting stuff ! How do you get a ring-shaped structure with a neat linear tail and then another tail that's orthogonal to it ? That's just weird ! And lenticular galaxies don't often have gas, so that's of note all by itself.

And the mass of the gas present doesn't really help. They estimate the deficiency of the galaxies and find that overall this system has more gas than expected. While the galaxies themselves have lost significant amounts of gas (though I would dispute the use of the words "vast majority" here), overall, the total mass of HI present is actually somewhat more than the whole system would be expected to contain if it was a bunch of normal, isolated galaxies.

What's going on ? To my mind this is one case where accretion ought to be considered a serious possibility. We know that galaxies in general have star formation rates that require external replenishment of gas, but claims for detecting the accretion material are always fraught with the difficult question as to why we don't see such structures everywhere. Most such circumgalactic material (as it's called) can indeed probably be explained as gas removal through tidal interactions. This is the interpretation the authors exclusively employ here, but I don't think it fits well at all. The morphology is all wrong, the kinematics don't fit (though their velocity resolution is low), and the total mass of gas doesn't fit the picture for gas loss. Surely, this deserves some more exotic considerations.

So yeah, this is a really interesting little system. I'm slightly annoyed that they don't cite any of my papers, more annoyed that they restrict themselves to a single interpretation, but it's a very nice result indeed and clearly we need more MeerKAT.

Not interesting but important

Well... it's probably unfair to say it isn't interesting. But when I read this paper I was expecting some cool scientific findings, of which there are none. Instead, this paper presents a new method for analysing HI spectra in a robust, reliable, objective way, providing quantitative ways to describe things like the shape of the profile and whether it's asymmetrical or not. They apply this to the enormous ALFALFA data set, so describing this method and giving the full catalogue on such a large sample is very important. It's just not interesting yet because they don't do any scientific analysis of the results.

Anyway, I've long thought that there's potentially a lot of unexploited information in an HI spectra. For example, the classic double-horn (a.k.a. Batman) profile arises because of flat rotation curves : most of the gas in a galaxy is moving at a single, constant velocity, irrespective of its distance from the galactic centre - the very discovery that led to the idea of dark matter. So could there be other such important discoveries to be made by further exploiting the data ?

Perhaps not. But it's been difficult to examine the spectral shapes in a systematic, objective way, because the measurements are surprisingly hard. Typically, we measure the width of the line at 20% and/or 50% of the peak flux (known as the W20 and W50 parameters) , and that's about it. Nobody does much with such nuances as the shape of the profile because objectively describing the shape is rather hard. 

There's one exception. People do try and measure the asymmetry, by comparing how much flux is found either side of the central velocity. Unfortunately this is the exception that proves the rule, as here the results prove controversial. You'd expect to see wonky profiles for galaxies in dense regions, where there are lots of gravitational interactions to disturb the gas. Some people find this, but some don't, and some find there are anyway strong asymmetries even in isolated galaxies, meaning there could be internal process causing disturbances as well. It's all very confusing, the numbers vary considerably, and not at all satisfying. Nobody can agree on what constitutes a significant level of asymmetry, which is a problem.

What the authors of the present study do here is actually better explained in their previous paper (see section 3.2 and especially figure 1). They use the curve of growth method to measure the HI spectra, something which is actually quite common in optical astronomy but not much used in HI analysis.

Let me explain the problem of the existing measurements a bit more. Usually, the W50 and W20 values are in good agreement. W50 is generally better because it's measured at a higher flux level, but in a strongly asymmetric profile, this can give an estimate of the line width that's much smaller than the true value - you can end up measuring only one of the horns, for example. W20, measured at a lower flux level, is often confused with noise, so can give width estimates which are too high. Although most of the time the two measurements do agree quite well, it'd sure be nice to have something more robust.

The curve of growth method is a pleasingly simple alternative. Starting from the central velocity, the flux is integrated incrementally over larger and larger velocity widths. And all you do is then plot the cumulative flux over these increasing widths, and what you get is roughly a linear slope which then quite suddenly flattens when you hit the edge of the galaxy. Et voilà, a robust, objective way to determine the true velocity width of the galaxy, irrespective of the shape of the profile.

Marvellous ! But there's more. You can parameterise the width in different ways, e.g. by looking for the width that encloses, say, 85 or 90% of the total, giving objective criteria for measuring both width and total flux that account for the signal to noise. And you can do the curve of growth for each half of the spectrum separately, giving you robust flux ratios for measuring asymmetry. 

*Importantly, though the details are boring, the authors also correct for how much the signal to noise level affects the measured asymmetry, meaning you can now reliably quantify the significance level of any measured asymmetry and thus settle any arguments about how much of a flux ratio is really needed to quantify as asymmetry.

This also gives additional parameters. For asymmetric profiles, the slopes of the two curves of growth are different, so you can also compare the slopes as a measure of asymmetry as well as the flux ratio. And more complex parameters can quantify the actual shape of the profile. They favour a parameter they call K, which measures how much the integrated flux compares to the case of a linear increase : the linear case (K=0) corresponds to a flat-topped profile, while K < 0 (less flux than linear) corresponds to double-horned profiles and K > 0 happens for single-peaked profiles.

What this means is they can create reliable, quantifiable, continuous parameters for measuring both asymmetry and shape. What they stop short of doing is any sort of analysis on the results. Are double-horn profiles more common in certain environments, or dependent only on galaxy morphology ? Are there any galaxies with strong HI asymmetries but without optical disturbances ? Does K vary linearly, or does it fall into nice neat groupings ? Do these new parameters work well at low S/N levels ? Can they help with source extraction or this is strictly only for analysis ?

This paper very literally raises a lot more questions than it answers, which is a good thing in this case. Though I'd like to really have a good thorough look at how this technique works in anger, it's got definite potential for being a classic case of, "why hasn't anyone done this already ?". Even if it isn't likely to lead to another discovery as profound as dark matter, it's still likely to give rise to a veritable plethora of spin-off papers and a horde of citations so large the Mongols would be jealous. Well, maybe.

That's what he said

Or something. I mean, there just must be a dozen brilliant innuendos regarding any claim for the largest radio galaxy in the Universe. There just must be.

That's the topic of today's paper. Low frequencies aren't my area of expertise (then again, neither are high frequencies...) but these very low energy emissions are another good way to trace out gaseous structures not visible at other wavelengths. In this case, the aptly-named giant radio galaxies. These are - wait for it - galaxies... which emit at radio wavelengths... and are gigantic.

Ahem.

Anyway, such objects generally seem to arise from supermassive black holes. If you haven't come across radio galaxies before, you've almost certainly seen spectacular images of enormous jets emanating to enormous distances from the cores of comparatively tiny host galaxies. You bloody well ought to, at any rate, since I went to all that trouble to show just how gigantic these features can be. This one, though, is even bigger : a whopping 5 Mpc (15 million light years and change). 

I'm probably going to have to update the size chart for that one.

Not being expert in this area, the first question that came to my mind was : are you sure you're measuring it right ? You didn't, say, just take a close-up picture to make it look bigger, did you ? DID YOU ?

No, they didn't. Unlike a lot of spectacular discoveries, it turns out giant radio galaxies aren't all that uncommon. Over a thousand are known, of which a hundred are longer than 2 Mpc and ten exceed 3 Mpc. One reaches to 4.9 Mpc. So finding one which is a bit longer again is very cool, but not unprecedented. It's like finding the largest elephant, not like discovering elephants for the first time.

The second question that came to mind was : how did it get so big ? Did it see a pretty galaxy and it just couldn't control itself ?

Look, the world is still in the midst of a pandemic but seems determined to ignore it, the UK government can't bring itself to tell the truth about a piece of cake, and Russia thinks the year is closer to 1822 than 2022. If ever there was a need for really, really stupid jokes, it's now.

Somewhat surprisingly, there's nothing particularly special about the optical galaxy. Neither the galaxy itself nor its supermassive black are exceptionally large - indeed the authors describe them as "suspiciously ordinary". So it's not an outrageous level of power being generated. More promising seems to be the environment. Although it's not in a full-on void, it's not in a cluster either - it's just in a large-scale galaxy filament, like most other galaxies. Filaments do have their own gas, but at very low density. The most likely explanation for Alcyoneus' enormousness, then, is probably that there's just not much around to stop it.

Alcyoneus ? Yes. Quoth the authors :

Alcyoneus was the son of Ouranos, the Greek primordial god of the sky. According to Ps.-Apollodorus, he was also one of the greatest of the Gigantes (Giants), and a challenger to Heracles during the Gigantomachy — the battle between the Giants and the Olympian gods for supremacy over the Cosmos. The poet Pindar described him as ‘huge as a mountain’, fighting by hurling rocks at his foes.

Seems appropriate to me.

And there's also scientific value, beyond the obvious cool factor of finding anything superlative. The density of gas in the filaments is hard to constrain because it's very thin, but it might well be important in galaxy evolution. In clusters, the ram pressure from a galaxy moving through the external gas can be strong enough to rapidly remove even the densest atomic gas, but a similar effect might be at work in filaments too. Here the surrounding gas probably wouldn't be dense enough to remove the star-forming material, but it could potentially remove the more extended gaseous halos - the reservoirs of fuel for future star formation.

The giant radio lobes of Alcyoneus are a way of testing this. Being at such low densities, they might be approaching pressure equilibrium with the surrounding material, allowing at least in principle a way to estimate the density of the thin filamentary gas itself. So far as I can tell, the author's don't actually do this - seems like something that needs additional work - but they say that Alcyoneus "represents the most promising intergalactic barometer of its kind yet". Well, if nothing else, kudos to them for the lively metaphors. And not an innuendo in sight.

Escaping the dark

We now return to those dark galaxies without dark matter. No, not those ones, the other ones. The originals.

This is a topic so controversial that even those claiming to have explained them say that the original discoveries were wrong, which is very confusing. But the best bet seems to be a tidal encounter. Since dark matter, like gas, is generally much more extended than the stars, it's easier to remove. Indeed, unlike gas, dark matter is collisionless, so if it happens to be in the centre of a galaxy on part of its orbit, there's nothing stopping it from moving out to greater distances a little later. Whereas the innermost gas gets trapped in the office forever, dark matter prefers a hybrid model : sometimes it's in the city centre, but sometimes it's in the vulnerable suburbs.

Topical metaphors aside, today's paper is the most convincing argument yet for a tidal origin. The authors re-examined an existing simulation that predates the whole hoo-hah, so this absolutely counts as a prediction. True, it would have been even better to predict these things ahead of the discovery itself, but there was no reason for anyone to look for them.

They find seven objects in their 21 Mpc-on-a-side simulation that lack dark matter. Most of them live in groups (three are all in the same group), and all are associated with a massive parent - more massive than the Milky Way. Now the properties of all but one are not all that similar to the notorious NGC 1052 DF2/DF4 (they have a few times as much stellar mass and, correspondingly, rather higher velocity diserpsions*), but this would probably be asking too much of a simulation. The main point about lacking dark matter is clearly satisfied, and they're not orders of magnitude different to the observational sample.

* These two parameters are intimately related though. The higher the stellar mass, the higher the velocity dispersion must be for the systems to be in stable equilibrium.

What I find most convincing about this is that they explain their objects as a rare but not exceptional process. Since there are at least two objects known in one group, and more dark matter free objects elsewhere, any explanation that relies on a freak encounter might have problems. It's a very tricky balance to find a mechanism that is routine enough that you actually expect to stand a reasonable chance of detecting the end results, but not so common that they should have been spotted everywhere and known about for decades.

The author's answer to this is that the satellites need to be very small in comparison to their parent galaxy (0.1% of its stellar mass) and come extraordinarily close - within the stellar body. And this seems to fit the bill quite nicely. One would expect most such encounters to result in the satellite merging with the cannibalistic parent or being utterly destroyed by it, but every so often one could just manage to survive. And another satisfying point is that while the after-effects of the encounter are still visible in some of their simulated objects, some show no visible signs of disruption at all : the main body of the galaxies survive long after the tidal tails have dispersed.

If anything, I worry that this explanation might overpredict the number of dark matter deficient satellites : around 30% of the most massive galaxies ought to have at least one such companion, they say. But such massive galaxies are themselves quite rare, and the smallest satellites are the hardest to measure, so this is not necessarily and outlandish fraction. Importantly, it's testable. We probably shouldn't get too hung up on the exact numbers given the limitations of observations and simulations alike - so long as it turns out these systems are not unique to NGC 1052, that's probably sufficient.

So this is very good stuff. What remains now is to look for analogues to those other galaxies without dark matter : the isolated, gas-dominated Ultra Diffuse Galaxies. Those, they say, might exist in their simulations as well... but that's another story.

Chained in darkness

This is a very interesting paper about my favourite topic, optically dark gas clouds.

As far as I can tell, the discovery is purely serendipitous. They pointed the shiny new MeerKAT telescope at a galaxy group as part of a larger survey, and happened to find this linear chain of seven clouds. Hydrogen clouds without optical counterparts are pretty rare things, but these ones are among the most unusual. They're spread over an enormous extent (400 kpc !), are each pretty massive (around a billion solar masses, roughly the same gas mass as a chunky dwarf galaxy), have high line widths (~130 km/s, again dwarf galaxy territory), and have no obvious association with anything optical.

Somewhat oddly, they begin with a quite protracted introduction discussing why HI clouds are in general unlikely to be dark galaxies. While I agree, they erroneously cite my papers as supporting this, which of course they don't - rather, we found that certain objects are more likely to be dark galaxies than others. Even so, the introduction is a nice overview, though it feels a bit strange that they don't much discuss the possibility later on.

The paper is largely a discovery and description affair, avoiding much speculation as to the origin and nature of the objects. This is very sensible. From what we found, objects with high line widths, say > 50 km/s, are very difficult to explain as tidal debris in clusters. But these objects aren't in clusters, they're in a small-ish galaxy group (18 members), where the speeds may well change the kinematics of any HI clouds produced in galaxy-galaxy interactions. It's also impossible to ignore the neatly linear alignment of the clouds and the fact that part of the complex forms a sort of V-shaped feature. So debris of some kind does seem to be at least a plausible explanation.

Unfortunately they don't provide optical imaging over a large area, so it's difficult to see which if any galaxies these might relate to. But within the area they examine, there's no galaxy which looks like an obvious source of the gas. It reminds me quite a lot of the Kent Complex, which sadly they don't present as a comparison object (though they do compare it with HI1225+01, which is also similar). And there are certainly no big nearby galaxies with HI tails that one could point to and say, "there, that's the bugger !".

It also seems to resemble the Leo Ring in some aspects, which has comparable mass, size, lack of optical counterparts, and environment. But the kinematics and morphology of the Ring are very different. Although it lives in a different environment (a rich cluster), and measures only about a quarter of the size of this feature, to my mind the Kent Complex is overall more similar : comparable in mass, linear-ish morphology, and with several distinct "clumps" of similar kinematics. The latter is especially important, since line widths this high are extremely unusual.

No other feature is a perfect match though, and importantly, the origin of the Kent feature also remains a mystery. Dynamically, the major source in the MeerKAT object would require some dark matter for it to be stable, though not nearly as much as a typical galaxy. So neither "dark galaxy" nor "tidal debris" is wholly satisfactory in either case. Surely, it can't be a coincidence that this feature is so close to a galaxy group, but what's especially strange is that the group is HI-poor. How do you remove so much gas - nigh-on ten billion solar masses - without much affecting the optical component of the galaxies ? How do you remove the gas so completely from the parent galaxy ? At least some ought to be left in the galaxy itself, deep in the potential well.

All in all, a very interesting read about a fascinating object. Unusually, further observations here require not better spatial resolution (they already have that) but better kinematics - a resolution of 45 km/s is pretty awful, really. Even so, it's not at all obvious if better velocity resolution would help. My bet is that it will remain mysterious for quite some time to come.

Yo Dawg, Herd You Like Missing Matter

So we took your missing matter away so you can miss your missing missing matter.

Ahem.

Galaxies lacking dark matter are a reoccurring topic these days and they're back in the headlines again. Time to take a look at the paper behind the headlines.

To recap, there have been a few different galaxies reported all showing the same basic trend : their stars and/or gas are moving much more slowly, at any given radius, than normal galaxies. Whereas most galaxies rotate so quickly that they require a huge amount of extra, invisible dark matter to hold them together (or equally, their rotation can be said to imply that there's something wrong with our theory of gravity), these ones don't. Apparently, in these handful or peculiar cases, just ordinary stars and gas and good ol' Newton are more than enough.

Thus far, Six galaxies present especially nice examples. Unlike some other, more prominent cases (we'll get back to those later), these are too far away to attribute this rotational oddity to distance measurement errors. To determine the amount of dark matter, we need to know both the speed at which any part of it is moving and the distance of that point from the centre of the galaxy itself. In essence, it's the speed as a function of radial distance, not speed alone, which is the important number. We don't ever measure total dark matter content directly - instead we get the amount of dark matter enclosed within a certain radius.

Measurements of the speed don't directly depend on knowing the distance of the galaxy, but the radial (galactocentric) distance does. And in these six cases, the uncertainty on the distance is far too low to be a possible explanation for their unusual properties. Which is very helpful.

But, there's an added complication. We don't really get the speed directly either. Well, we do, but not the true rotational speed. We get how fast things are moving along our line of sight. To convert this to rotation, we need to correct for how the galaxy is inclined towards us : for edge-on galaxies the measurement is directly equivalent to rotation; for face-on galaxies we can't measure rotation at all. For galaxies somewhere in between, knowing the inclination angle allows us to make a correction. And that, arguably, is a potentially big difficulty.

In the current paper prompting the latest press release, the authors revisit one of their old favourites, using new high-resolution data of the atomic hydrogen content to estimate the inclination angle. They find good agreement with their old data, essentially supporting their original inclination angle estimate and conclusion that it lacks dark matter. They also go much further in their efforts to establish the inclination, looking at a wide variety of possibilities for the true nature of the galaxy, e.g. considering different thicknesses, orientations on the sky, and find a model which minimises the residuals : that is, when subtracting the model galaxy from the original data, it leaves behind nothing but noise.

All this seems to indicate that the galaxy is well-fitted by a model disc of pure gas and stars at an inclination of about 32 degrees. They go further, and try at some considerable length to find if there's even any dark matter model at all which could account for the observations. In short, there isn't : the dark matter would either have a bafflingly low concentration, or just gets the velocity measurement all wrong. The disagreement is so low that it could even be called non-physical.

Interestingly, this galaxy also poses difficulties for alternative theories of gravity. Now Modified Newtonian Dynamics does allow for galaxies which rotation curves like this, but only if they're near to massive galaxies. This one apparently isn't. It seems to be isolated and in nice stable equilibrium. And so MOND, just like the standard model, predicts this galaxy should be rotating much faster than it actually is.

(It's worth remembering that this is ordinarily an advantage of MOND in that it does just as well as the standard model in predicting rotation speeds. But whereas the standard model doesn't absolutely require the dark and normal matter to always be associated with each other, MOND says that in most cases, the same mass distribution should give rise to the same rotation speed. So galaxies like this one ought to be if anything more problematic for MOND than the standard model of dark matter.)

What's the catch ? Is this just a really, really weird galaxy, or is something more pragmatic at work ?

I've favoured different possibilities on this one in the past, but this paper unwittingly swings me (slightly) in favour of the latter. Here's their money figure. It shows an optical image of the galaxy with the hydrogen gas as a white contour. Their best-fit inclination (32 degrees) is shown by the black line, with the blue dashed line at the much lower 11 degrees needed for this galaxy to have a more normal sort of rotation speed :

And for all their hard work and sophisticated modelling, I'm just not convinced that they can really rule out the lower inclination angle possibility. They use the gas disc to estimate the inclination, but look how ragged that edge is... is it really such a stretch to imagine that deeper observations would reveal a better fit for the lower angle model ? Observational errors have a nasty habit of being larger than we would like (estimates of errors should always be treated with caution), and it seems like the galaxy might need to have only a marginally unusual morphology for it to be well-fitted with this lower angle. After all, the inclination angle of the gas and stellar discs can be different, and the angle measured is already very low.

But, this is not to say the case is settled. Far from it. Recall the similar case of NGC1052-DF2, which was also thought to lack dark matter. A lower distance would have settled all the anomalies of that galaxy, so from that perspective a lower distance seemed far more likely... but the best, most recent available distance measurement does appear to be the larger, weirder value (at least, no-one has challenged the claim yet, as far as I'm aware). So I'm going to be very cautious indeed about reaching any kind of firm conclusion here.

To credit the authors, they admit that the inclination angle is the most crucial measurement. And I remain persuaded that their previous arguments about how the low velocity dispersion variation (the random motions of the gas distinct from the rotation) are much more consistent with a nice stable disc than any large-scale disordered motions. So my instinct to say that this oddball galaxy is probably due to a weird inclination measurement problem is not a very strong one; maybe 60:40 odds. As they say, if they could get rotation estimates at even larger distances, this would be really interesting : their interpretation clearly predicts that here we should see declining rotation velocities. How they might get these, though, is hard to say.

Obviously, then, this isn't mystery solved. Rather, we're still at the stage of determining if there is even a mystery at all*.

* Which is a lot better than the recent approach of the Metropolitan Police in investigating crimes.