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.

Darkest tales

Another paper ! But just a short one this time.

This one involves observations with China's giant "FAST" telescope, which I'm pleased to say I have ongoing observations for. So I'm naturally curious as to how well it works in practise. While it's collecting area isn't all that much larger than Arecibo's, its 19-beam receiver coupled with high-performance electronics looks like it will essentially overcome that. I still think their estimates of their major survey's expected performance are optimistic, but I also still think it'll be a fantastic survey regardless.

Here they point FAST at the giant spiral galaxy M101 and its companions, looking at our much-loved friend the 21cm HI (atomic hydrogen gas) line. While previous observations have detected a bridge to the nearby dwarf galaxy NGC 5474, with the improved resolution of FAST this is not detected. M101 does have a tail roughly in this direction though, so they suggest that the previous claims might just have been the effects of low resolution making the HI from both galaxies appear connected.

The tail could have originated from an interaction between the spiral and the dwarf. As well as the tail, they also claim to detect three distinct clouds associated with the end of the tail. But while the data they show appears to be of excellent quality (hooray !), they don't really show enough to determine the nature of these supposed clouds. To me they look more like small perturbations at the edge of the tail, rather than independent clouds in their own right. This might be different in the full 3D data though.

In one image, there is a suggestion of a somewhat more extended tail that might help explain the previous claims for a full-on "bridge". But what's missing is any hint of an extension in the much smaller NGC 5474. If the interaction was able to so disturb the gas in the giant M101, then how come the dwarf doesn't seem affected at all ? What also seems strange is that M101 has a strongly disturbed stellar disc on the opposite side to its disturbed HI content, which isn't remarked on.

But if it's not an interaction that produced the disturbance, then what was it ? Accretion of material from the intergalactic medium doesn't seem likely, as M101 doesn't appear to host the expected massive halo of hot gas expected in such a scenario. They also say that there's no sign of enhanced star formation activity near the disturbance, so the expulsion of gas from a violent starburst doesn't seem likely. Some discussion on the timing for this scenario would have been useful here though - would we still expect such an outburst to still be ongoing ? I don't know.

I think the whole thing would have been a lot more interesting if I was persuaded that those optically dark clouds really are separated from the main disc. As it stands, it's a set of minor oddities : it certainly looks like an interaction of some sort has happened, but whether this is really responsible for the disturbance in the gas disc isn't all that clear - it's a nice example of the difficulties of distinguishing external from internal processes. Still, the data from FAST appears to be meeting expectations, which is very encouraging : at least some of Arecibo's capability has been replaced, albeit in a limited fashion for the time being. This definitely has strong potential for becoming a powerful discovery machine in the near future.

Tantalisingly tidal

Here's a paper on that ever-popular topic : those two funky galaxies which apparently don't have any dark matter.

It seems that the great distance controversy has at last been settled; to my knowledge no challenges were raised to the latest measurements on that score, despite an abundance of dispute on similar previous claims. This means the galaxies are indeed very far away, so their small velocity dispersions do indeed imply a lack of dark matter. While if they were closer they would just be perfectly normal galaxies, which Occam's Razor could imply is the more likely scenario, the data really seems to rule this out.

Recall also that galaxies without dark matter do not, paradoxically, challenge the dark matter paradigm itself. If dark matter exists, then it can in principle exist independently of ordinary stars and gas. By contrast, if it's just an illusion and it's actually our theory of gravity that's at fault, then it shouldn't be possible for similar objects to have dissimilar velocity dispersions : similar objects, in similar environments, should always be kinematically similar. What they might potentially challenge instead is the nuances of how galaxies assemble, though conceivably they may just have had their dark matter removed in ordinary tidal encounters.

Searching for the effects of tidal encounters could give important clues to how these objects were formed. One such previous study clearly shows there are tidal streams of stars in the group in which our two little galaxies are likely located, but there's no sign of such features associated with the oddball galaxies themselves. Another claimed to have found characteristic S-shaped tails associated with one of them, but I was very skeptical about this because it looked marginal at best.

Today's paper uses the Dragonfly telescope to conduct a new search for tidal features associated with the weird galaxies. They give a very nice but probably unnecessarily thorough introduction, given that they don't actually find any tidal tails per se. Ideally, if they found nice classical extended tails, that would settle the argument pretty unambiguously, but alas such features aren't seen yet.

However, what they do find is evidence of tidal distortion. Looking at their images I was a bit skeptical - the galaxies look pretty normal, and the outermost measurements look like they're almost within the noise. But their measured surface brightness profiles (which show how the stellar density changes with radius) are rather more convincing, showing a clear, coherent change of gradient in both density and ellipticity over several data points. It certainly doesn't appear that they're really probing much into the noise, although of course even more sensitive measurements would always be better.

(The only real caveat I might raise to this is that there's no discussion of alternative explanations, especially how long some mild ellipticity might persist in a galaxy consisting purely of stars as these do. But that's probably not very important.)

Using this, they can estimate the radius within each galaxy at which the change of profile occurs. And from theories of tidal distortions, they can then estimate how close they must be to the larger galaxies present within the group. The neat finding is that if the galaxies do actually have dark matter, then for them to be this distorted, they'd have to be even closer to the bigger galaxies than is possible according to the observations. That is a pretty strong result that these galaxies really do lack dark matter. You can always say a galaxy might be at a different distance along the line of sight, but the distance across the sky is a very much harder limit. They also find no evidence of those S-shaped tails I was wary about, though this doesn't really change anything.

What does this mean ? To be honest, not that much as far as understanding their origins goes - though I got a bit bored towards the end because it is a rather long paper. We can now, I think, quite definitively say that these objects are strange - we're just going to have to wait for even deeper data to say more about what the bloody things actually are. Weird ? Yes. Challenge to theory ? Not so sure, for now.

His Dark Accelerations

I've got a backlog of papers I should try and read that's so long it's preventing me from reading any at all, but at long last I managed to knuckle down and read one.

Today's paper sees the return of our old friend, the Radial Acceleration Relation. Very briefly, this is the observed strong correlation between the observed and theoretical prediction for acceleration of material in galaxies. What's supposedly odd about this is that we can predict the acceleration very well without knowing anything about the dark matter at all, which other observations show is usually dominant over the ordinary matter we have to observe. It's a bit like being able to predict the acceleration of a horse without knowing that it's on a train : the fact that the horse has four legs ought to tell you nothing about the power of the train's engine, but apparently it does.

As shown many times, however, this initially surprising result doesn't look to be anything very interesting, as it falls out quite naturally from bog-standard simulations of galaxies. It seems it's just a perfectly normal scaling relation based on the dark matter; in my silly (and flawed) analogy, it's as though there are actually very good reasons to expect a certain kind of horse to always be found on a certain type of train. True, theories without dark matter (most notable MOdified Newtonian Dynamics, MOND) predicted this in advance, whereas the standard model didn't, but that doesn't change the result that there appears to be no way to distinguish the expected behaviour from the two competing theories. Hence the RAR is not much use to anyone, really.

But hold on, say the authors of this paper*, maybe there is a way to tell them apart after all - in which case, I recant, and the RAR does potentially become very interesting again. And ironically, this may come from that much-maligned feature of MOND, the External Field Effect.

* I know two of them and indeed we collaborate. I was not involved in this research at all, which is something on which we usually disagree.

This is far beyond my level of specialist understanding, but the basic deal is that the MONDian behaviour of a gravitational system breaks down if it's present in a sufficiently strong external gravitational field. So MOND gives different predictions for identical system in a way that's much more strongly dependent on its environment than in the standard model of gravity. 

Consider a star in the outskirts of a galaxy, orbiting at a very small acceleration. If we neglect dark matter, Newton's theory says the star should be orbiting at a much lower velocity than MOND predicts. 

At least, that is, if the galaxy is isolated. If instead our galaxy is also close to a second, massive galaxy, then the speed of its most extended material becomes greatly reduced - everything fades back into ordinary Newtonian behaviour. So for MOND, the rotation curve depends not just on the individual system but also its environment. And this just doesn't happen (at least, not to anything like this level of strength) with Newtonian gravity.

This potentially gives a neat way to discriminate between MOND and the standard model. MOND implies that we should see declining rotation curves if systems are not sufficiently isolated, whereas dark matter says their environment is basically irrelevant, except for secondary effects like galaxy evolution. On the other hand, the dark matter paradigm doesn't say you can't ever have a dark-matter-free galaxy, it just doesn't say there should be any correlation with something as neat, say, a cluster-centric distance.

In short, the speed of the orbiting material of an isolated galaxy can have flat rotation curves (high speeds at high distances) under MOND, but they ought to have declining rotation curves (lower speeds at higher distances) if the galaxy isn't sufficiently isolated. In the standard dark matter paradigm, by contrast, they should almost always have flat rotation curves, simplifying slightly.

This paper looks at the velocity dispersion of Ultra Diffuse Galaxies in clusters to see if any influence of this External Field Effect can be detected. Being extremely low density, UDGs are good tests since their stars should be orbiting at low accelerations around their galactic centres. Being inside clusters, the EFE should be strong, so they should be a good test case where the EFE can most easily overcome the internal gravitational fields of the galaxies.

What they find, after some very extensive analytical modelling, is... unclear.

Or rather, it isn't. It's just that what they say they find doesn't look much like what I think the figures suggest they actually do find.

They have a sample of ten UDGs. Of these, four have velocity measurements at many different distances, i.e. with proper rotation curves. The rest have only one data point each, and I would tend to neglect these as they don't seem to show anything very much. But the ones with multiple points do look useful. For example, here's one case, comparing the observations (points with error bars) with MOND predictions that neglect the EFE (sold lines, under a few different assumptions) :

Which is a pretty clear disagreement at the larger radii. Now the outer points decay towards the Newtonian (neglecting dark matter) prediction, so that suggests maybe the EFE can indeed help here - this is exactly what the EFE is supposed to do. But of course, we need a more quantitative prediction. Here's the same galaxy after their very careful work to model the EFE - this is rarely done, so kudos to them for the attempt :

Which is I think very clearly a much better agreement with the data. It's the same story for the other three galaxies : MOND with the EFE works better than without. As you would hope, really, at least if you're a MOND supporter. Pretty neat.

The problem is that the authors say the exact opposite, that the EFE makes things worse ! If that were so then this would be a big problem for MOND. As a MOND skeptic, I'd have a field day (pun intended, oh hah hah). So I find myself in the strange position of preferring dark matter but defending these results against the MOND-advocating authors...

It gets even stranger as they suggest several possibilities to save MOND, not least of which is that the EFE could be screened in clusters by some kind of "dust" (hence His Dark Materials). How this is supposed to work is unclear to me.

This all feels very weird. A more natural narrative would be that they've shown that the EFE can quantitatively explain the unexpected dynamics of specific galaxies, hence the popular opinion that MOND can't allow declining rotation curves is falsified. Had they done that, I'd say, "Yep, you've got a point. I still don't like MOND, but this allows a testable prediction, so good job."

That they claim the opposite is very confusing. If the EFE could be somehow be screened, this would seem to be a powerfully anti-Occam argument against the whole idea of modified gravity : if you're going to allow it to work so inconsistently, that would seem to make it unphysical. Yet there doesn't seem any need to invoke anything like this, here, rather the data seems to show that MOND's EFE might not be such a get-out-jail-free, fit-anything-to-anything panacea fudge factor as it sometimes appears.

In conclusion, I don't know.