This really isn't the law you're looking for

Remember how the good ol' radial acceleration relation was going to solve all problems in extragalactic physics ? No ? That's okay, it's been a while, so let's briefly recap.

Galaxies have some pretty odd dynamical relations, two of which I find particularly interesting. One is the oddly simple Tully Fisher relation. The TFR is the tight relation between rotation speed and baryonic (normal matter like gas and stars) mass content. That's not a problem in itself - things which spin faster are more massive, big whoop. What's odd is that theoretical predictions show it should have a lot more scatter than it does. The second weird relation is that there's a neat correlation between the "wiggles" in a rotation curve and the baryonic density, which is unexpected because the mass - and therefore rotation speed - should be dominated by dark matter, not the baryons.

Why is this interesting ? Well, the neat relations between baryons and their own dynamics suggest a direct connection between the two, and that (arguably !) doesn't fit well with the idea of dark matter at all.

To test this idea of a direct mass-velocity link, it'd be nice to have something more fundamental than either the TFR or wiggly rotation curves. Both are similar in that they're relations between mass and speed, just on different scales, but both are also subject to a host of implicit assumptions that have nothing much to do with gravity or dynamics. That's where the radial acceleration relation is supposed to come charging to the rescue.

It turns out that there's a neat relation between the observed acceleration and the expected acceleration due to visible baryonic matter. This relationship isn't linear, but it does appear to be pretty tight. It's been said quite often that this is the fundamental relation from which the TFR and wiggly curves can ultimately be derived.

This latest paper looks at the wiggly aspect. In short, it says, "actually no, this isn't what's making things all wiggly"*. And that could well be a problem for anyone trying to rid the universe of dark matter... but it also has problems for everyone else too. Everyone's a loser !

* Sadly not in these exact words.

The paper starts with the best introduction on the problem I've ever seen. According to the standard dark matter paradigm, galaxies of similar rotation speed should have rotation curves of very similar shapes. But as they show, they don't. Some galaxies do get it about right, but others, despite having the same outer rotation speed, can have curves which rise too quickly or too slowly, but in either case are well outside the standard model's predictions.

What could be going on here ? They note four possible explanations. (1) Since there's never very much dark matter in the innermost regions of galaxies, all the baryonic material slooshing about could be disrupting the dark matter there by dragging it around gravitationally, thus mucking up the rotation curves. (2) Or, more radically, dark matter could be self-interacting and so change its own distribution. (3) On the other hand it could be something as mundane as observational errors in extracting the true rotation velocity. (4) Finally, it could indeed be there's a more direct relationship between baryons and dynamics, as RAR enthusiasts generally seem to prefer because that would be extremely cool.

They investigate all these possibilities by using a whole suite of the latest all-singing, all-dancing, super high resolution simulations. And not just one specific model either, but four of the darn things (they also comment quite a lot on real observations, which is nice).

INTERMISSION

... but first, a word on terminology, because it's truly barbaric. As in the literal, original meaning of the word : "bar bar" nosies that don't make any sense. Galaxies which have slowly rising rotation curves are called "cored", because it looks like their inner dark matter has a fixed density within some "core" region. Those with curves matching theoretical expectations are deemed to be "cuspy" because the density continues to rise right down to the very centre (blowed if I know when "cusp" became a synonym of "spike"). And those which rotate too quickly in the centre don't have a special term at all, presumably due to racism or something.

Let's sort this mess out before we go any further. Sod convention, it's confusing because it's stupid. I shall use the following :

• Galaxies with inner curves that rise too slowly, and therefore seem to lack dark matter in their centres, are to be known as "lazy buggers".
• Galaxies with inner curves that match theoretical predictions I shall call "boring bastards".
• Galaxies with inner curves that rise too quickly, so having more inner mass than expected, are to be known as "greedy bitches" (I ran out of profanities beginning with b, although at the time of writing Wikipedia assures me that "Brexiteer" is also vulgar slang).

Got that  ? Good.

WE NOW RETURN TO OUR FEATURE PRESENTATION

1. It's due to the baryons doing funky things in the centre of galaxies

This doesn't work well at all. As they say, only (real, observed) galaxies with low overall baryon densities show slowly-rising rotation curves, but not all low density galaxies show this : some are perfectly normal, others rise too fast. Those with the slowest rises - the laziest buggers - imply they have the least inner dark matter, meaning they need the strongest movement of baryons (through supernovae and suchlike) to disturb it. That in turns means they should have the lowest overall baryonic density... but they don't. So baryonic density alone can't be the direct cause of these relations.

While we're at it, this also means that it the oddities can't simply be due to the sheer mass of baryons either - except for the greedy bitches. These, they say, are relatively easy to explain as being galaxies which have more inner baryons than normal, rather than there being anything weird about the dark matter (presumably they have the same central spike as the boring bastards, but with extra baryons thrown in as well). It's the lazy buggers which are the real problem.

What about simulations ? None of them are able to reproduce the full range of observed properties. Some do better than others, and it does depend on the resolution and the properties of star formation and feedback that's used. But in the end, it seems that either all simulated galaxies are lazy buggers or boring bastards (and I guess greedy bitches as well) : they can't get a mix of both.  Maybe there's a sweet spot if they were to fine-tune the simulations really carefully, but this wouldn't be a natural solution.


2. Dark matter is self-interacting

If dark matter could collide with itself, this would stop it from building a massive central spike. The problem is that this should lead to all galaxies becoming lazy buggers, and greedy bitches would be harder to explain : they'd have to have much stronger excesses of baryons than normal galaxies. And too many baryons means extremely high star formation activity, so the resulting supernovae and stellar winds should eventually prevent further baryonic infall. Worse, there are many greedy bitches which don't have many baryons, as well as lazy buggers which do have lots of baryons in their inner regions.

Using both analytic and numerical models, they quantify that this idea doesn't work. Oh, it can work, they say, if you very carefully choose the properties of the dark matter halos. But it's not at all likely to work given the distribution that naturally falls out of simulations. I'm not sure this is terribly convincing, given how many parameters there might be to play with for self-interacting dark matter, but never mind.


3. We're interpreting the velocities incorrectly

We infer dark matter by assuming that the measured velocities correspond to those of stable circular orbits. But maybe it's not as simple as that, since the orbits might not be neat circles - especially given the shape of the dark matter halo. That couldn't mean we've got the whole dark matter paradigm itself wrong, but it could mean we've interpreted the wiggly curves in a silly way.

They make mock observations from the simulations and this idea does seem to basically work. Individual galaxies could be completely misclassified according to the measurements, and it reproduces the scatter in the rotation curves very well. So this could indeed by a major contributing factor. What spoils this is that many lazy buggers show well-ordered velocity fields where non-circular motions don't seem very likely, and the fact that all lazy buggers are of low density - and there's no obvious reason, they say, why low density galaxies should be more prone to non-circular motions than dense ones. I'd have thought the lower density makes them more vulnerable to internal effects, but that needs a lot more examination. Anyway, this seems like the best bet so far.


4. It's an effect of the radial acceleration relation

The big one. This relation was touted as a "new law of physics", much to everyone's disgust, but it does have an appeal : it gets back to fundamental physics instead of all that wishy-washy baryonic stuff. As they say, if this is truly a law, you should be able to derive the rotation curve from baryon distribution alone.

(Of course, it isn't truly a law at all - it's a rather subtle effect of galaxy scaling relations, and is very well reproduced in perfectly normal simulations that use standard physics. But let's indulge the speculation for the sake of interest)

If that's true, then knowing the baryonic distribution for any galaxy, you should be able to predict if it has a wiggly curve of any type, from boring bastard to lazy bugger. Can you ? No, you can't. Using the RAR to predict the dynamical relations gets a much lower scatter than the observations. In particularly, baryon-rich lazy buggers should not exist according to the RAR, but they patently do.

Another issue is that RAR-enthusiasts have this tendency to plot the data with 2D histograms, so that more densely-populated regions on the expected-observed acceleration plot are more visible. This is good for discerning the possibly underlying trend, but it reduces the visible scatter. Here, they don't do this trick, and show the data as raw points. Although most galaxies do follow the trend well (rather better, I think, than the "only approximately" the authors state), it's clear the scatter is pretty large. And the outlying galaxies are, very clearly, all lazy buggers.

Couldn't we combine this with the the idea that the velocities have been misinterpreted ? In fact, we'd have to, since the RAR doesn't predict the observed deviations. But they say that if the same velocity measurements used to derive the RAR are deemed to be wrong, that would undermine the basis for the RAR in the first place - essentially, unjustly throwing away data points that don't fit the trend. At the very least, the RAR is quite definitely not enough by itself to explain things... if you're allowed to say that these galaxies have errors, you need some reason to say why these galaxies in particular and not the others.



What's both nice and frustrating about this paper is that they don't come to any firm conclusions. Practically any scenario could probably explain the trend with enough work, but none is entirely satisfactory. So something of a mystery remains, although to my mind the problem of interpreting the velocities looks by far the most promising.

As to their main point about RAR not being a law, this feels a bit like a highly ingenious way to say what we already knew : there's very high scatter at the low accelerations. Although I'm not at all convinced RAR is any kind of fundamental breakthrough, I don't think they here sufficiently explained why it couldn't be that the outliers had problems with their velocities. It's a really nice piece of work, but it left me scratching my head and going "hmmm" more than I would like.

Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves

We use a compilation of disc galaxy rotation curves to assess the role of the luminous component ("baryons") in the rotation curve diversity problem. As in earlier work, we find that rotation curve shape correlates with baryonic surface density: high surface density galaxies have rapidly-rising rotation curves consistent with cuspy cold dark matter halos; slowly-rising rotation curves (characteristic of galaxies with inner mass deficits or "cores") occur only in low surface density galaxies.

Six strangely slowly spinning spacey starry systems

Remember those galaxies which were rotating weirdly slowly ? Of course you do. The damn things are bizarre.

We last saw these six little guys in a letter describing how they rotate so slowly that they deviate from the Tully-Fisher relation : their velocity is much slower than expected given their baryonic mass. They're not quite so slow that they lack dark matter, but they are slow enough that we've likely found all their baryons (in most galaxies, a large fraction is missing, usually thought to be very hot, thin gas). Unlike other cases, these are too isolated to be the result of galaxy-galaxy interactions and too far away for distance uncertainties to mess thing up. This latest paper is the follow-up full article to the original letter, wherein they show all their data and not just the sexy money plots.

These particular galaxies are extremely faint, fuzzy little "ultra diffuse galaxies". Since they're extremely faint, and also since the letter didn't show the full maps of each object, it's only natural to wonder if the inclination angle hadn't been computed incorrectly. If they're actually face-on, for instance, we wouldn't be able to measure their rotation speed at all. Similar claims have been made for much brighter galaxies that are spinning more slowly than expected.

Looking at their figures, first I thought, "ooh, they show ordered motions, so they're definitely rotating !", then I thought, "ahh but hang on, the gas is at totally different angle to the stars, so maybe the inclination angle is wrong after all", and then I noticed, "hey, that galaxy's rotation is going the wrong way !". Finally I did what I should have done from the start and read the blasted text. In short, I think this paper reasonably settles any concerns about the inclination angle measurements, though I won't say it's unquestionable.

There's not much doubt that these objects are rotating : one side is clearly at a different velocity to the other. And the velocity axis aligns very well with the morphological axis, except in one case where the resolution is a bit funky. This alignment, they say, means they can dismiss outflows or inflows. Although they don't have much resolution, they have enough to employ some fancy modelling software to extract the true rotation curves of the galaxies. They've tested this on similar simulated galaxies and it works well. And although the stars are all over the place, they point out that the gas in these galaxies is ten times or more massive than the stars, so it's the gas they should use for measuring inclination, not the stars.

Even so, I'm still a bit skeptical about three cases : one looks a bit disordered, one looks too close to face on for reliable measurements, and one is rotating in the wrong direction. The other three cases I'm satisfied with, but no matter how sophisticated the modelling is, I'd still be cautious.

They then compile other samples of galaxies to search for any other possible TFR-deviants. Besides their own sample, there's not much more than a hint of any deviation from the trend. This, as they note, is well-known oddity, since low surface brightness galaxies should deviate from the TFR but generally don't. In fact, so far there haven't been indications of any trend in those few galaxies which do deviate. Here, they find one : the greater the stellar scale length, the greater the deviation (restricting the sample to galaxies with the lowest rotation speeds).

The hell does that mean ? I don't know. They mutter something about the velocity-mass plane not being a well-defined distribution, but that doesn't clear anything up for me.

How do such objects form ? Buggered if I know. They note that the gas discs here are of normal size given their mass, so they're not of exceptionally low gas density. Obviously these particular UDGs can't be "failed" Milky Ways like some other UDGs are reputed to be, since their dark matter content is too low (perhaps the UDG label just doesn't have much physical relevance). Interestingly, if their gas density is normal, their stellar density is low, so stellar feedback may have been weak, preventing outflows from driving their gas away, resulting in these gas-rich, diffuse objects. But of course that doesn't really help explain much : what keeps star formation low in galaxies of normal gas density ?

Just about much dark matter they have remains to be seen. They say the objects are consistent with the average cosmological baryon fraction, meaning we've likely found all their baryons. In that case they'd have about ten times as much dark as visible matter, but this halo would have to be unusually diffuse. But they can't be sure, and given the available resolution, it's possible they actually have no dark matter at all. So higher resolution might yet pin down the little blighters. The research continues.

Robust HI kinematics of gas-rich ultra-diffuse galaxies: hints of a weak-feedback formation scenario

We study the gas kinematics of a sample of six isolated gas-rich low surface brightness galaxies, of the class called ultra-diffuse galaxies (UDGs). These galaxies have recently been shown to be outliers from the baryonic Tully-Fisher relation (BTFR), as they rotate much slower than expected given their baryonic mass, and to have baryon fractions similar to the cosmological mean.

Bringing wide open spaces indoors

My lockdown side-project is reaching the stage where I feel confident enough of finishing it that I can show what I've got so far.

Seven years ago (!) I made a model of Arecibo observatory, which became a pretty glass cube. Some time later this got heavily updated to the standards of having enough detail for a passable rendering (even if the materials were never that great) for a pre-rendered VR video. But wouldn't this be much cooler if it was an interactive game-like thing you could walk around in ? Answer : yes, yes it would.

After playing around with a few realtime data-based experiments of the amazing Blend4Web plugin, I decided it was time to take on the more laborious task of converting Arecibo to something people can explore for themselves. This is a lot more work than the previous experiments, which are largely a matter of tweaking Python scripts - essentially they're automatic except for some manual window-dressing. With Arecibo, everything has to be done manually.

A quick test proved that the plugin was easily capable of handling the mesh at an extremely high frame rate. But the materials were designed for the world of raytracing, and for various reasons they mostly look pretty horrendous in the realtime view. The easiest solution would be to remove them all and replace them with very simple plain colours, but this would look meh. So instead I learned about texture baking, which essentially does the rendering and stores the image on the mesh so it looks just like the rendered view but in real time.

Unfortunately this can't easily be scripted. Meshes have to have clean geometry and unwrapped in a reasonably decent way. Large parts of the mesh are, for reasons best known to my younger self, not clean at all. Large parts have had to be remodelled completely just so texture baking will work. Even then it isn't perfect, but I've decided that version 1.0 of Half Life Arecibo (working title) will be a quick(ish) learning experiment. For version 2 I'll learn the nice new materials available in Blender 2.8 and make everything look way better.

Anyway, screenshot time ! Here's the whole site. The landscape will be extended using a Blender plugin that automatically extracts textured terrain meshes from Google Earth (I actually already did that but I seem to have misplaced the landscape somewhere, as one does). The grey sky is what you see in Blender internally, although Blend4Web can replace this with something sensible.

The ground screen (that protects the beam from receiving the hot ground) was a particular challenge to re-texture. The baking isn't perfect, but I think it's probably good enough. The supporting struts do not yet have correct materials so they don't display nicely yet. 

The towers are all fully textured, though one of them has wrong material settings on the ladders and suchlike. Again, the cable materials are not yet correct.

The Gregorian dome was surprisingly easy to convert. For now, all textures are 4k resolution, even the really small objects, but this will probably change to save memory.

Having found a texture baking solution that generally works well, I was rather annoyed to find this method didn't work at all for the triangle. Then I found out the mesh has - lord knows why - far too many faces, so I completely remodelled it and it worked (there are a lot of minor defects, though I think they don't notice much). The same needs to be done for the azimuth arm. 

You can see a few rendering artifacts in a few places. These can probably be fixed, but for this first test I won't bother. In general they don't cause any serious problems.

Getting this building to work took up a good few hours and it still has some nasties. And the signs (not yet shown) are being particularly uncooperative. I don't know why this little shack is being such a pain, but I'll get there in the end.

That's for now. Most parts of the mesh are relatively straightforward. It's the few bits that don't work so easily that slow everything down. Still, if I can do a little bit every day, it shouldn't be too long before everyone can recreate Sean Bean's infamous death-by-telescope scene... well, maybe.

To twirl or not to twirl, that is the question

There's two ways to stop a galaxy from collapsing. Either it can spin around, which flattens the gas and stars into a disc, or everything can be on chaotic, random orbits. The latter doesn't really work for gas, which is collisional and tends to do unpleasant things like shock and lose energy.

What about all those recent ultra diffuse galaxies ? If we want to know if they're dwarfs or giants, which will affect theories of galaxy formation, we'll need to know if they're spinning or supported by dispersion. This paper looks at a sample from simulations.

The paper gives a very good overview of the current state of play for UDGs, noting the interesting shenanigans going on with deviations from the Tully Fisher relation, the diversity of the sample, the different ways to measure their dimensions, and the different formation mechanisms proposed. Essentially there are two main ideas : they could form due to internal processes, or they could form in clusters (where most UDGs have been found) due to unique environmental processes. The later doesn't seem likely, or at least it certainly isn't sufficient, since we now know of isolated UDGs that have never even see a cluster.

The one major thing I don't like about this cluster is that they're unclear about their simulations and sample selection. I don't even like reading these bits of simulation papers and I normally just skim them, but even I found this too brief. It's important to know what the mass resolution is and what they mean when they say they selected an "isolated" sample - how you define isolation is basically arbitrary. They don't even say how many galaxies were in their simulation in total or how large a cosmological volume they simulated. Sure, I could consult the original simulation papers, but I shouldn't have to : these parameters are fundamental to this paper.

That aside, they find that almost exactly half of their sample of 38 isolated UDGs are dispersion dominated and the other half rotation supported. They tend to be gas rich (though "rich-most" is an ugly phrase indeed), though dispersion dominated galaxies are more gas poor. This is partly a selection effect, in that rotation is better at supporting more extended systems at any given mass. Also, their objects are not part of the high-spin tail of normal galaxies, as one of the most popular early papers claimed.

Interestingly, at these masses, supernovae feedback is expected to be highly efficient, but they find that this plays only a secondary role in determining whether a UDG is rotation or dispersion supported. The major driver appears to be how the gas accretes. If it happens to align with the existing rotation of a proto-galaxy, the accreting gas helps increase the rotation. This lets it maintain a steady accretion of gas without any destructive starbursts. If, however, the accreting gas is misaligned, then chaotic accretion leads to bursts of star formation and supernovae feedback that quickly reduces any further gas inflow. So rotationally supported systems tend to have later gas accumulation.

Prior to the lockdown I was writing a paper on gas observations of UDGs. Although the deviation from the Tully-Fisher of some objects is weird and interesting, quite a few don't show this : they look like normal rotating discs. So this paper makes a lot of sense to me, but I would have liked more information. How extended are these galaxies using different radius measurements ? What's their typical star formation rate - do they have much molecular gas ? How are they still accumulating gas when they're in isolation ? On the other hand, if they'd given all that I might have dismissed the paper for being too long and not read it at all...

NIHAO XXIV: Rotation or pressure supported systems? Simulated Ultra Diffuse Galaxies show a broad distribution in their stellar kinematics

In recent years a new window on galaxy evolution opened, thanks to the increasing discovery of galaxies with a low surface brightness, such as Ultra Diffuse Galaxies (UDGs). The formation mechanism of these systems is still a much debated question, and so are their kinematical properties.

The slightly surprising survival of spirals

Galaxy evolution is very similar to that of large, ruthless corporations : it's driven by merger after merger, until what started off as a small family shop becomes a huge international conglomerate that sells weapons specifically designed for killing babies living in third-world countries. Or something. A slightly more popular analogy is that galaxies are cannibals, but this is no less grisly as it still involves fat, bloated monster galaxies that mercilessly devour their smaller, weaker siblings.

Look, I've been stuck inside for a month. I've got to make my own entertainment. Stop judging me !

All that horror and questionable politics aside, there's a bit of a puzzle here. Merging is thought to be the most common way to turn spiral galaxies into ellipticals. But if that's so, how can there be any massive spirals around today ? Shouldn't their formation necessitate lots of mergers, making them far more likely to become ellipticals ?

The authors of this study look at one of the all-singing, all-dancing, "everything on" cosmological simulations to trace the evolution of the most massive spiral galaxies. This simulates a large cosmological volume, so unlike smaller simulations (which have to be focused on specific environments) it includes a whole range of evolutionary processes. It's also of extremely high mass resolution, enough to simulate even very modest dwarf galaxies. This means that from the perspective of the most massive spirals, which they concentrate on here, it should be complete.

They find that giant spirals are indeed rare. Of galaxies with more than 100 billion solar masses in stars, about 11% are spirals, in rough agreement with observations.

There are two different ways they can form. About 70% result from mergers... but not just any mergers. The mass ratio of the progenitors doesn't seem to matter much, but their gas content does. Since the gas is collisonal, the merger causes it to shock and collapse to a disc supported by the angular momentum imparted by the merging galaxy.

Even more persuasively, 98% of all massive spheroids which experience a gas-rich merger subsequently become discs, so gas-rich mergers appear to be a highly effective route for disc formation. And the discs tend to have had their last major merger more recently than spheroids, meaning there hasn't been much time for other, unsuitable mergers (or other processes) to destroy them. In addition, as the gas content of the simulated universe decreases over time, so does the fraction of galaxies which are massive discs. It's pretty clear that it's gas what done it.

Apart from these "rejuvenated discs", the remaining 30% are always discs. Presumably they initially form by mergers, but subsequently they get lucky and have a very quiet merger history. They mention an interesting idea was that such galaxies could be near to even more massive galaxies whose enormous gravity could protect their smaller brethren from incoming would-be mergers, but this doesn't appear to be the case for their sample : they seldom have a more massive companion. So for the most massive objects, at least, it's simply a matter of luck.

Whether this has any bearing on runaway capitalism or the culinary practises of uncontacted jungle tribes is left as an exercise for the reader.

Why do extremely massive disc galaxies exist today?

Galaxy merger histories correlate strongly with stellar mass, largely regardless of morphology. Thus, at fixed stellar mass, spheroids and discs share similar assembly histories, both in terms of the frequency of mergers and the distribution of their mass ratios.

Filament finding fun

Where do galaxies get beaten to death ? Is a galaxy falling into a cluster about to be torn limb from limb, or is it likely to be half-dead already ?

Of course, it probably varies considerably. We know that the processes at work in clusters - mainly tidal harassment and ram-pressure stripping - certainly can cause enormous damage. And we know that at least some galaxies falling into clusters look, for the time being, perfectly healthy. But of course the real question is what happens in a typical case, if there even is such a thing.

Since we can't wait around for billions of years to track individual galaxies, like most extragalactic problems we have to tackle this statistically. As we generally have rather small samples - a few hundred galaxies per cluster - we plot radial trends in galaxy properties (colour, gas content, morphology and so on) as clues to the general trends at work. The problem is that this biases us to looking for cluster-induced changes. What about processes at work more locally, or even occurring to galaxies before they've fallen into the cluster proper ? Such "pre-processing" might be very important, but would be easy to miss in radially-averaged plots.

Being outright annoyed by such seductively simple but potentially misleading techniques, my postdoc* Boris Deshev decided to try something more sophisticated and plot Voronoi maps. We can't avoid the need to bin the data due to our small sample size, but this method allows us to really see the 2D map of various properties instead of the 1D radial average. It's a sort of adaptive-gridding that's good for dealing with points that vary widely in density from place to place, whereas a uniform gridding would wipe out a lot of fine details.

* Can one own a postdoc ? Native Americans say you can't own land, but as far as I know they don't say anything about postdocs.

The main map in the paper is a Voronoi map of the fraction of star-forming galaxies. This does not show the neat, circular trend that you might expect if the cluster was dominating galaxy evolution. In earlier drafts, we saw a clear north-south filament of galaxies dominated by non star-forming objects, but sadly this largely faded from view with more accurate star formation measurements. But it re-appeared when looking at the specific star formation rate, that is, the star formation activity accounting for the mass of each galaxy.

It's even possible to show that those objects haven't had their star formation reduced recently - it must have happened in the relatively distant past (> 500 Myr ago). So it does appear that pre-processing is having a significant influence : part of the cluster is assembling along a filament in which galaxies have already had their star-formation activity reduced. This Voronoi mapping is a neat way to show things that radial plots cannot, though the disappearing filament is an important reminder that small differences can sometimes make a big difference to the result.

This cluster is also one of only two at this distance (about 2.6 billion light years) which have HI measurements. At that distance, the resolution of the observations isn't great. Even so, there are some intriguing hints of galaxies being caught in the act of gas loss, with their HI noticeably displaced, stretched, and shifted in velocity from their most likely parent galaxy. The main problem is identifying the parent galaxy : we can't be sure that it isn't actually some barely-visible little blob that's just about visible within the beam of the telescope. But a few cases look intriguing enough that it was certainly worth reporting, as no-one's detected this at this redshift before (two of them look pretty darn convincing to me). They'll be good targets for future observations at higher resolution, and there might be more candidates hiding in the data. Developing new ways to dig them out is ongoing.

Mapping the working of environmental effects in A963

We qualitatively assess and map the relative contribution of pre-processing and cluster related processes to the build-up of A963, a massive cluster at z=0.2 showing an unusually high fraction of star forming galaxies in its interior.

The dirt in the discs

What, another paper on dust ? "But Rhys", I hear you say, "you said you hated dust !". Well, I do - it's awful. Outside of His Dark Materials, dust is not the least bit interesting. I just can't bring myself to become interested in where it comes from or what it does. Come on - it's feckin' dust. Dust is something you get rid of. Every time someone mentions it, I immediately remember an advertising slogan for a hoover : "compress your dust, compress your worries". Almost as good as a jingle on a children's magazine, "I love horses, they're my friends !"... but I digress.

Anyway, what caught my eye about this paper was not the dust, but the detection of spiral structure in elliptical galaxies. Since such structures have been found in early-type galaxies in Virgo, this is a potentially interesting avenue for exploring galaxy evolution as a function of environment.

This paper looks at a sample of field ellipticals, using two deliberately similar populations : one with and one without gas (both HI and CO). They use the MegaCam instrument on the CFHT to produce colour maps, using reddening as an indication of dust content after controlling for the underlying stellar population (I also find myself wondering why both this and the previous paper never mention Herschel or other direct dust measurements). This shows a variety of different dust morphologies. Here's their main figure using the SDSS optical images :

And here's the same sample (with no attempt at all at keeping the scaling constant) using their dust maps, taken directly from the paper :

N = no dust; D = disc; R = ring; Ir = Irregular

Now to be fair, you can see some hints of these structures in the conventional images, but they're far clearer in the dust maps. NGC 3626 I find particularly interesting : although it certainly doesn't look like a typical elliptical in the optical map, the spiral dust structure is quite different again. And if you'd just shown me it's dust map, I'd have assumed it was a quite typical spiral shown with a weird colour scheme.

The main trend they note is that there are clear differences in dust morphology depending on gas content. The presence of any gas anywhere in the galaxy seems to be an excellent predictor of dust in the central regions, which it would be nice to know a bit more about... what's the physical connection between outer gas and inner dust ? Perhaps they say something in the appendix, but that's a billion pages long so I'm not going to read it. They also show that gas-rich galaxies are dominated by dust morphologies of spiral and irregular types, with no rings and a very few discs. Gas-poor galaxies have dust morphologies dominated by discs, rings and irregulars. Gas-rich galaxies tend to have large dust structures, whereas small dust structures are found in gas-poor galaxies.

Understandably, they put a lot of effort into comparisons of their study with others, both in terms of similarities and differences. In particular, their sample only deals with field galaxies, so cluster galaxies (which are almost unanimously gas-poor) may be different. But what does this tell us about galaxy evolution ? When some apparently early-type galaxies look so strikingly similar to late-types by looking at their dust, I think we have to wonder if this indicates a connection between the two. They speculate that the origin of the dust could be due to internal production (by stars, despite the low rate of star formation) and brought in externally, but honestly, that's boring. I think this result is sufficient to warrant asking bigger questions about the connections between galaxies of different morphologies. Does one type evolve into the other ? Do they have more similar formation mechanisms than previously suspected ? I dunno, but that to me is way more interesting than any amount of bloomin' dust.

Cold gas and dust: Hunting spiral-like structures in early-type galaxies

Observations of neutral hydrogen (HI) and molecular gas show that 50% of all nearby early-type galaxies (ETGs) contain some cold gas. Molecular gas is always found in small gas discs in the central region of the galaxy, while neutral hydrogen is often distributed in a low-column density disc or ring typically extending well beyond the stellar body.

The Virgo Cluster is a dirty, dirty place

And now we return to regular boring old science with nary a space vampire in sight.

One of the many, many wonderful things about the Virgo Cluster is that it lies behind a great big hole in Galactic dust. So we can measure the galaxies there without worrying too much that the foreground dust has made them appear significantly redder and fainter than they really are*. That's especially nice for me, because I can't stand dust. I just cannot bring myself to get interested in it. Sure, it may be an important component of star formation, but... come on, it's dust. Don't expect an insightful commentary, is what I'm saying.

* This is called extinction, presumably just to be confusing.

Anyway, through some very sophisticated modelling it is possible to correct for the reddening due to foreground dust. Once that was done, the authors looked at the extinction variation of Virgo galaxies that must be caused by extragalactic sources. Normally I'd probably be skeptical, but the trend is so darn clear - more reddening near the cluster centre with a very rapid drop off with radius - that
it looks to be pretty convincing, at least to a naive dust-aversion person like me.

They're even able to make a map of the extinction, albeit at low resolution, but you can clearly see something. Dunno what it is, but it's definitely there (the issue is you can only measure the dust by looking at galaxies which have been reddened by it, rather than detecting it directly). Broadly, they say it seems to follow the same distribution of the intracluster light, thought to result from stars that have been thrown out of their galaxies by tidal interactions. They estimate the total mass of dust at just 3 billion solar masses, which is about the same as the gas mass of a single large dwarf galaxy. Being able to detect this when spread over such an enormous area is pretty darn impressive.

The main thing I wonder about is survival. They say the expected lifetime for dust in the cluster environment is about 100 million years, which is not all that long. Are there likely to be enough tidal encounters pulling out sufficient dust to explain the observations ? There's not that much of it, but I would expect it to be pretty hard to remove since it should be mainly found in the inner regions of galaxies. I dunno. I might just be too naive on this, but it would certainly be interesting to know how much dust can be removed in an encounter and then get a handle on how often such encounters must occur. Which would likely end up as an exhausting project involving all kinds of ghastly physics with a result that may or may not be interesting to anyone. Such is life.

The GALEX Ultraviolet Virgo Cluster Survey (GUViCS) VIII. Diffuse dust in the Virgo intra-cluster space

We present the first detection of diffuse dust in the intra-cluster medium of the Virgo cluster out to $\sim$0.4 virial radii, and study the radial variation of its properties on a radial scale of the virial radius.

Behold April

In the words of the great Jeremy Paxman when he was forced to present a weather forecast, "Well what did you expect ? It's April", so it is for arXiv. Here's a brief round-up for those who missed it.


The Really Habitable Zone

Who cares if liquid water can exist on a planet orbiting a star ? The important thing is whether conditions are suitable for making a gin and tonic, a region "which might actually be worth existing on." Astronomers, they say, need alcohol, and the presence of astronomers is a good definition of civilisation. And a lack of gin might explain why a planet has a terrible atmosphere.

(They note also the possibility of "ginspermia", in which juniper bushes propagate through interstellar dispersal, but suggest that efficient harvesting for maximum gin productions means there are no spare bushes flying through space.)

How to define the parameters for the RHZ as opposed to the old BHZ (Boring Habitable Zone) ? They followed standard practise and made them up. This all involved the consumption of multiple "gins and tonic", which is not a typo : "You want multiple gins, not more tonic." Adorable.


Searching for Space Vampires

"It is a truth universally acknowledged, that a single human in possession of a good space telescope,
must be in search of a space vampire." A strong opener indeed. Apparently inspired by an xkcd webcomic which notes that reflecting telescopes can't see space vampires, the authors realised that not all telescopes these days use mirrors. The Transiting Exoplanet - sorry, Exovampire Survey Satellite uses lenses, so is an ideal tool for searching for spaceborne undead.

They note three prospects for space vampires : free-floating, to be examined in a forthcoming paper by Van Helsing et al.; already landed on Earth; tidally locked around M-dwarf stars. Why M dwarfs ? Because while vampires are susceptible to sunlight, they are clearly unaffected by firelight, which is typically not too much cooler than M dwarf stars. Presumably daylight on planets around such stars holds no fears for vampires there...

They then drew a profile of a vampire and a bat in Microsoft Paint and fed it through some transit photometry modelling package to see the expected dip in the light. Comparing to observations from TEvSS, they find there could be between 0 and 394400933 possible space vampire transit events, i.e. between 0 and 100% of all observations. They consider this to be a major breakthrough.


Conspiratorial Cosmology

Reality is just a conspiracy and "generally misleading", say the authors of this the most incoherent of the suggestions. Loosely based on the idea that if anyone figured out the meaning of existence, the Universe would disappear and be replaced with something more complex, and this has already happened, the authors develop the concept of inflationary imbecility. This is like regular inflation, only for stupidity instead of space.

But who are They who are behind it all ? Following the Chuck Norris theorem - that there's an easy way and a hard way - they suggest that They started with a simple Universe and have been gradually ramping up the complexity. Recent events comprise such things as the challenge of electing an unelectable president and controlling an uncontrollable virus.

And why is the Universe controlled by so much dark matter and dark energy and the like ? They solved this by employing occultism, in which a medium declared that it's due to the Fertile Neutrino (as opposed to sterile neutrinos, of course). This is, apparently, very fertile up to several "guinea pig units", and the ongoing production of fertile neutrinos causes the expansion of space. There follows a lengthy rant about the anthropic principle, philosophy, string theory, the importance of simulations, eventually concluded that we ourselves are the grand conspirators behind it all, somehow. I got as far as, "In order to make sure that we need all the energy we harvest, we will build our spaceships in form of giant SUVs which are constructed such that they have a decent wind-resistance even in the interstellar medium, and we will cause a greenhouse effect in the Galaxy so strong that Dyson-trees (Dyson, 1997) start to grow on molecular clouds" before deciding that the words, "please stop" had never been more appropriate and gave up.


And finally, overheard elsewhere :

Normal services will be resumed as soon as possible.

Slowly does it

And now back to our regular service on galaxy dynamics and whether or not MOND is a thing.

Galaxies typically have rotation curves which are approximately flat in their outer regions. They usually rise steeply in the innermost regions, often going a bit crazy at first, but then settle down to something that's basically flat and boring. Dwarf galaxies are an exception, often showing curves which are still (slowly) rising, possibly because the stars and gas don't probe the full dark matter halo. But even a few larger galaxies also show curves which are weakly rising or declining.

Recently we looked at massive galaxies which are rotating more quickly than expected, and whether these challenge alternative theories of gravity. It seems to me that they probably do, but critics are right to point out that it matters a great deal as to which value you use to define the rotation of a galaxy, i.e. the peak or the flat part of the curve.

This latest paper looks at galaxies with declining rotation curves. They choose their sample so as to avoid the usual causes of this, like interactions or having a strong bar or bulge in the centre (such that the mass of the innermost baryons becomes significant compared to the extended dark matter). They also select them to have well-resolved rotation curves, so it's not a data artifact or anything daft like that. And unlike the previous paper, they show all of their curves, rather than coyly hiding them away like in the last one.

What I cannot for the life of me understand are their quoted ratios of peak to outermost velocity. Looking at their rotation curves, I'd guestimate the ratios to be no more than a factor of two at the very most, but they give values of 10-40 ! I'm tempted to email them because I can't make sense of that, but the text has been translated from the original Russian so that might make things difficult.

When it comes to the inevitable Tully-Fisher relation (a comparison of rotation speed and luminous mass), they show that their galaxies agree with the standard relation if they use the peak velocity, but are significantly slower than the standard relation predicts if they use the flat velocity. This is exactly the opposite of what Milgrom said when criticising the fast rotators : these authors have done exactly as he suggested, and find the opposite sort of problem ! So the claim of MOND enthusiasts that the Tully-Fisher relation actually has some miniscule scatter if you get the measurements right looks extremely suspicious to me : galaxies actually seem to deviate in all directions, even very massive ones. And furthermore, since the peak velocities here do agree with the standard relation, that makes it rather unlikely that there's been some systematic error in the velocity estimate.

The nice thing about rotation curves, though, is that you're not limited by global relations like Tully-Fisher. You can directly compare a galaxy's observed and predicted rotation throughout its whole disc. For this sample, they find most galaxies agree with MOND's predictions, but not all*. A few require the galaxies to have significantly different stellar mass from the measured value, and/or a different acceleration constant - in the worst case by a factor of six. And it would be a pretty stupid theory indeed if you had to change a fundamental constant for each galaxy.

* And they're interesting curves in their own right : some look flat to me, while others are very clearly and consistently declining, and still others show sudden decreases and then remain flat further out.

There is some scope for a MOND rebuttal here. The galaxies are not all truly isolated (one is even in a cluster). The authors say they have no nearby massive companions capable of creating a significant external field effect, but it could be that they've interacted in the recent past and are still out of equilibrium (I don't know if anyone's modelled how this would affect rotation curves, but I'd be a bit surprised if this caused them to rotate more slowly though). There's enough scope of the complexity in the modelling of disc mass and the structure of the curves and so on that the findings could be challenged. Their most deviant galaxy is interesting in its own right : visually it looks disturbed, but also lonely. MOND or not, something interesting is definitely going on here.

Galaxies with Declining Rotation Curves

A sample of 22 spiral galaxies compiled from published data is studied. The galaxy rotation curves pass through a maximum distance of more than $\sim 1$ kpc from the center with a subsequent decrease in the rotation velocity.