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.

Mostly Missing ?

I recently read a news article claiming that it's a myth that college students are all wokeist do-goodies. This it refuted quite sensibly by looking at the fraction of students who align with said wokeist do-goody policies. Fair enough.

But it also occurred to me that in some ways this is slicing things backwards. If you want to say that university life is or isn't dominated by the "woke", then it's perfectly correct. If, however, you want to understand why the popular image of the woke might fit a certain demographic, then it's the wrong way round. What you want to do in that case is analyse the fraction of those identified as woke, e.g. see if the woke are dominated by students, rather than the overall student population being political activists or whatnot. Because the numbers are small, it's entirely possible that only a few percent of students might be called woke, yet all the woke are bobble-hat wearing students.

Or, in other words : not everyone who voted for Brexit was a racist, but all racists voted for Brexit.

But that's more than enough politics for this blog. What it reminds me of is the statistical approach used by those who claim that there's a huge disagreement between observations and cosmological models of galaxy. It goes something like this :

• An observation is found which is intuitively unexpected.
• Simulations are used to assess how frequently such observations should actually occur, according to the particular model.
• The fraction found showing similarities to observations is low, and therefore the observations are proclaimed to be incompatible with the model.

What they do not do is ever look at those rare cases which do show similar features do the observations : though sometimes very rare, it hardly ever happens that the models never show such features at all. And that would be much more interesting, because by examining how such features form in simulations, you could potentially infer the mechanism at work in the real universe.

In today's paper (press release here), the authors return to the old favourite - the missing satellite problem. Simulations generally predict about ten times as many satellite galaxies around the Milky Way as are actually observed. Or rather... they used to. Actually that's been getting a bit better thanks to improved observations and far more sophisticated modelling. But certainly back when simulations generally only used dark matter (to keep things computationally cheap), that was indeed the case. It isn't the case any more, but on the other hand, adding in realistic gas and stars causes all kinds of other horrendous complications.

The important point is that the simulations tended to only use dark matter, whereas what we observe are stars and gas (for a long read on why this matters, see this). So claiming a stark disagreement was entirely warranted at the time, but provisional on better simulations. And this is still the case, though simulations have progressed significantly.

Anyway, what the authors are looking at is how recent the satellites of the Milky Way have arrived in our vicinity. The basic model predicts that they should be the leftover remnants of the Milky Way's assembly process, so they should have been orbiting our galaxy since basically forever. While the missing satellite problem might have alleviated in terms of pure numbers, if in fact most of the satellites are recent arrivals, then the problem is just as bad as it ever was - if not worse.

Now you can't just go an observe something like "arrival time" directly. What they have is data from the Gaia space telescope that gives the proper motions of the galaxies across the sky. Combined with line-of-sight velocity, this can be used to give clues as to the three-dimensional orbital of the satellite around the Milky Way. However, it does not give the full orbit directly : for that, they use independent measurements of the mass of the Milky Way. That's rather tricky, because (ironically) being inside the disc makes such estimates much more difficult than when measuring other galaxies.

Their most important result to me are the phase diagrams they plot for different components. This is just a plot of speed against distance, combined with models to show what speed is expected where. At small distances, a satellite galaxy can be bound to its parent even if it's moving at high velocities, whereas at high distances it must have a much smaller motion to remain bound. So you get characteristic curves, depending on the parent mass, showing whether galaxies are likely to be bound or unbound from the parent host. But remember, the mass of the parent galaxy - the Milky Way in this case - isn't very well constrained.

Still, they show convincingly that the satellite galaxies are generally all at higher velocities than other components in the galactic "halo" beyond the disc (individual stars and globular clusters). While only a very few require the largest possible parent mass to be in a bound orbit, they point out that this doesn't imply the galaxies moving at lower velocities have therefore been present for all that long. Rather, most of them appear to be in a regime where it's much more likely that they're recent infallers.

This is a nice, intriguing result. But as usual, it would be premature to throw out the standard cosmological model just yet. 

For example, one parameter they measure is the tangential velocity compared to the radial velocity, finding that this is in excess compared to expectations. Early authors claimed that such an excess happened in just 1.5% of the simulations, but this is problematic for two reasons : (1) they used pure dark matter, which doesn't have the massive baryonic galactic disc that can cause markedly different tidal effects; (2) it's using the wrong selection criteria - as above, why not look at those 1.5% which did show similarities, and see if those show any other similarities to the observations ? That would potentially be much more informative.

Furthermore, they cite other authors who postulated that maybe this is a survivorship bias. In this scenario, satellites on highly radial orbits are destroyed by the tidal effects of the parent galaxy. But they reject this hypothesis as the observable galaxies don't show the expected distance-based trend in tangential/radial velocity ratio, but I would (naively) assume destruction to be a highly non-linear process, so I'm not sure this is such a big problem.

Now one might expect that an encounter with another massive galaxy would play a huge role in the formation of its satellites. So they also select more advanced simulations, which do employ gas physics, containing pairs of giant galaxies resembling the Milky Way and M31. But they only have ten of these, so this already restricts the possibilities and doesn't let them select similar objects by design - which I think would be a much better approach. One should start by assuming that the mechanism for this velocity excess is unknown and see how the simulations reproduce it, not by seeing if any particular mechanism can explain the results or not. 

It also seems strange to me that they don't try and use the orbits of the galaxies to try and constrain the mass of the Milky Way. While they say there are other reasons to prefer a smaller halo mass, this rapidly escalates in complexity. It definitely feels to me like there is a large aspect of personal preference at work. If you prefer the standard model, you could use the evidence to infer a large halo mass with some anomalies; if you don't, you could say there's a small halo mass with some different anomalies.

Finally, the elephant in the room is that the Milky Way's satellites are in a plane. Long-term readers will know that I'm deeply skeptical about claims for similar features around other galaxies, but that of the Milky Way is virtually certain. So knowing that its satellites are atypical in their positions, should we really expect them to be typical in other parameters ? Probably not. 

All in all, it's an intriguing result. But as usual, I don't think it's anywhere near enough for the standard model to have anything much to worry about.

The farting dwarf is a lie !

Today's paper is another one that combines my favourite topics of dark galaxies, ultra diffuse galaxies (UDGs), and ram pressure stripping. There have been a few of these lately, for example, this one from May where the authors postulated that "fake" dark galaxies could form as a result of ram pressure stripping, and in particular this one from April claiming that they'd found an UDG doing something similar.

I would strongly recommend reading that second link before reading this post, as today's paper is a direct response to that, with this being a saga that's been running since 2015. In brief, there's this large, very faint galaxy (a UDG candidate) in the Virgo cluster that's close to this other faint, disc-like galaxy-thing. Both of these are embedded in a common envelope of atomic hydrogen gas (HI). So the previous authors said that maybe we were witnessing RPS occurring from the UDG - that is, the gas in the UDG being pushed out as it moves through the hot, thin gas that fills the cluster. 

In that case, the edge-on discy thing would probably be stars that formed out of the stripped gas, and not really a galaxy in the conventional sense. This would all be pretty neat, especially as seeing RPS from a UDG would give important clues about how such things form and evolve.

The connection to truly dark, starless galaxies is a bit tangential. With earlier optical data, it wasn't at all obvious if the HI had any optical counterpart at all, whereas now it's clear that it does - it's just very faint.

This paper is partly a re-analysis of the HI data and partly new optical data that gives independent constraints on the distances to the objects. One of the nice things about working with cluster galaxies is that they're all at about the same distance, or at least similar enough that the minor variations don't make a jot of difference to any calculations. And Virgo is especially nice because there don't seem to be any significant numbers of foreground galaxies to mess things up. So once you've got a galaxy in this general area and you know it's velocity, that's enough to give you a pretty solid distance estimate, and then of course if you spot alignments like this one, you know it's physically meaningful and not just a projection effect (i.e. a coincidence).

... Or so the legend goes. These authors disagree. They note that not only are the disc and UDG embedded in a common envelope of HI, but that both are connected by a much longer stream to a pair of other Virgo members. One of those already (VCC 2037) has independent distance measurements saying it's actually much closer. Whereas Virgo is at about 17 Mpc distance, new measurements say it's not more than 10 Mpc away and possibly as close as 5 Mpc. But a note of caution needs to be sounded here : there are two different papers, each using the same data but different methods, giving these two quite different results. That's a bit worrying in itself - and not at all uncommon either.

The authors of the current work say that as well as this, the UDG candidate is also likely to be much closer, maybe 7 Mpc away. So it's not forming from RPS at all, since it isn't even in the Virgo cluster.

What's actually going on here then becomes much more complicated. According to the new measurements, the faint disc does seem to be a Virgo member, but it doesn't have the normal scaling relations expected of a galaxy of similar stellar mass. So it might still be the result of RPS after all, just from a different galaxy (VCC 2034) : there's no reason that clouds of stars formed in this way should in any way resemble conventional galaxies.

In this case, the very good alignment between the HI stream and the other galaxies (the UDG candidate and VCC 2037) is just an unfortunate coincidence. Hence their title "two deceptive dwarfs"; the farting dwarf is indeed a lie.

But I am not at all sure about this. Given the big discrepancies in the distance estimates, particularly given the ping-pong distance estimates we've seen recently with galaxies claimed to lack dark matter, I think it makes sense to be very cautious about this. It seems a heck of a coincidence for an HI stream - of which only a handful are known in the cluster - to be superimposed so neatly on not one but two faint, unusual and interesting objects, again with very few foreground objects known. Both of these would also have to have quite extreme peculiar velocities, easy to explain if they're in the cluster, but not at all obvious if they're in the foreground where there's nothing much to disturb them. So this scenario seems to me to be very anti-Occam : possible, yes, but I would want a lot more data to be fully convinced of it.

To make matters worse, they also note that RPS is not the only possible gas removal mechanism, though it is the most likely. The alternative is a tidal encounter, but in this case narrowing down the possible perturbing galaxy is very difficult : there are many candidates, but none of them are much good. Saying anything very firm about this is even more difficult than usual.

There are two bits of good news to end on though. First, they already have more data themselves for at least three other objects, so additional papers are in progress. Second, the objects studied here should all fall within our very own WAVES survey, which is much more sensitive than the HI data used here. Having done a similar analysis of another object, these ones are natural things to take a look at. Watch this space.