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.

Galactic mess gets everywhere

Today's paper is a strong contender for "least exciting discovery I've ever blogged". It's still interesting though.

A typical spiral or irregular galaxy has three basic aspects to its gas distribution. There's the huge, hot, very thin outer halo that emits at X-rays (though rarely at levels strong enough to detect, except in the most massive galaxies). This million-Kelvin cloud extends well beyond the stellar disc. Then there's the much smaller, cooler (a mere 10,000 K) disc of atomic hydrogen, which extends to about twice the stellar radius. Finally, within the stellar disc there's more atomic hydrogen but at considerably higher densities than in the outskirts, along with a heavy smattering of massively denser, more compact, much colder (few hundred K) molecular hydrogen clouds where star formation happens.

Galaxy clusters posses their own hot X-ray gas. So galaxies falling through them build up a ram pressure as they move through the intracluster medium. This can easily remove their own outermost gas, which does nothing much except starve them of gas for future star formation on very long timescales. Stronger ram pressure can strip the atomic hydrogen directly, and in extreme cases can remove some of the molecular hydrogen itself.

All this is well-known in galaxy clusters, and generally reckoned to be one of the main drivers of galaxy evolution in that environment. But galaxies well outside clusters also frequently show signs that their star formation has been affected too. Could this "pre-processing" be the result of ram pressure stripping in seemingly more passive environments than clusters ?

To figure this out, ideally you need a direct signature of ram pressure, rather than just looking for the side-effects like a change in stellar colour or whatever. A one-sided gas tail is a pretty darn good indication of this, since gravitational encounters usually produce two-sided structures. The problem is that such tails tend to be quite short, so you need good resolution to detect them. Since ram pressure is expected to be weaker in groups (they're less massive than clusters, so they have less gas and the galaxies in them move more slowly) you also need a large sample to stand a chance of detecting a significant number of candidates.

That's where this paper comes in. They use the shiny new LOFAR telescope, which has both excellent resolution and area coverage. Instead of detecting the gas directly, it's sensitive to emission from cosmic rays - which, like the gas, can also be stripped into long one-sided tails by ram pressure.

They have an enormous sample of both groups, clusters, and isolated field galaxies. They use a combination of automated plus visual inspection of the radio images to search for one-sided features, with their field sample acting as a control. And the result is, for once, as clear as day. Only about 2% of field galaxies have tails, whereas it's about 10% in groups and 20% in clusters.

There's also strong evidence that these are truly ram-pressure tails and not the result of some other mechanism. The orientation of the tails tends to be only away from the cluster centre, whereas for groups it could be either away from or towards the group centre. This is exactly what you'd expect. Simplifying slightly, a tail pointing away indicates the galaxy is still on its first infall, whereas if it points towards the centre then the galaxy is now moving back out. Since ram pressure is so much stronger in clusters, the whole gas content can be stripped on first infall - hence most few cluster galaxies with tails pointing outwards. But being much weaker in groups, it can take a lot longer to strip the gas, hence a bimodality in the tail directions.

This also fits perfectly with the position of the tailed galaxies relative to their groups of clusters. Galaxies close to the physical centre but offset in velocity from the cluster centre are expected to be dominated by recent arrivals, and indeed, this is exactly where most of the tails in clusters are found. That's not the case for groups, with tailed galaxies having no particular preferred location in this "phase space".

What of the 2% with tails in the field ? Those could be false positives, since some galaxies do have weird asymmetries from internal processes anyway (strong star formation, active galactic nuclei) or past interactions with other galaxies. That the fraction is so much lower here is a good indication that their sample in groups and clusters doesn't suffer too much from this and that they're genuinely examining the effects of environment. A more interesting possibility is that there could be some weak ram pressure happening even in large-scale filaments of galaxies, with even isolated galaxies sometimes being stripped in this way. That requires further research.

Finally, the only point they've left unexamined is the structure of the tails themselves. That could potentially give more clues to the differences in ram pressure between different environments, e.g. the different length of the tails, morphological features, etc. But for now, this is a very satisfying result, like assembling a piece of IKEA furniture and finding that you don't have any screws left over. It's not exciting. It's not unexpected. It shouldn't be controversial in any way. It's just damned neat.

Ram pressure stripping in groups versus clusters

We compare the group jellyfish galaxies identified in this work with the LoTSS jellyfish galaxies in clusters presented in Roberts et al. (2021), allowing us to compare the effects of ram pressure stripping across three decades in group/cluster mass. We find that jellyfish galaxies are most commonly found in clusters, with the frequency decreasing towards the lowest mass groups. Both the orientation of observed radio continuum tails, and the positions of group jellyfish galaxies in phase space, suggest that galaxies are stripped more slowly in groups relative to clusters. Finally, we find that the star formation rates of jellyfish galaxies in groups are consistent with `normal' star-forming group galaxies, which is in contrast to cluster jellyfish galaxies that have clearly enhanced star formation rates.

The orphan of the stars

Today's paper is unusual in that I hardly ever read anything about X-rays. Gas that's hot enough to emit X-rays tends to be incredibly hot, low density, smooth and featureless. It's useful for measuring the mass of galaxy clusters, but it's not interesting to look at and the physics sounds horrible.

The feature described here, however, is sufficiently weird for me to venture a comment. The bulk of the X-ray gas in the Abell 1367 cluster is, as you might expect, a big boring blob, filling the whole cluster and probably responsible for all the gas stripping of galaxies which fall into it. Perfectly normal. But just a little way outside the smooth  main body is a so-called "orphan cloud", a distinct, crescent-shaped overdensity of gas. Now large individual galaxies do tend to have their own X-ray gas, but this one isn't clearly associated with any galaxy.

As well as the X-rays, the cloud also has a H-alpha component : much cooler than the X-ray material, but still hotter than the cooler gas normally found in galactic discs (atomic and molecular hydrogen). The H-alpha morphology is quite complex. Within the Orphan, it's found mainly in the tips of the crescent, but nearby it's seen in a stream extending from a galaxy roughly in the direction of the X-ray cloud.

The H-alpha material is interesting because its kinematics can be measured. There's a velocity gradient across the cloud of about 200 km/s, but there's no clear pattern of rotation. The velocity gradient and the size of the H-alpha region are not that different to some of the optically dark HI clouds seen in Virgo, but given that this one is embedded in hot gas of a mass a thousand times greater, they could well be completely different phenomena. On the other hand, since the Orphan has clouds of multiple components, perhaps the Virgo clouds also have hitherto undetected hotter gas.

The most likely explanation seems to be that the Orphan has lost its parent by some result of stripping, as most galaxies do when falling through the hot gas. But exactly how this happened, why it produced this one particular and quite unique feature, is unknown. Why don't we see more features like this ? The mass and kinematics of the cloud suggest a massive parent galaxy, which should be easy to spot, but they don't suggest any particular candidate. It's also unclear how it could have survived as such an intact, coherent feature for long enough to become well-separated from its parent, or why it's such a discrete feature and not part of a nice long stream.

I wonder if the nearby galaxy with the H-alpha might actually be not the source but a consequence of the cloud. Perhaps it passed through the overdensity of X-ray gas and thus had a much stronger amount of ram pressure than a galaxy normally would this far outside the cluster centre. That would take some detailed modelling to properly answer.

What else could the cloud be ? Being optically dark and with negligible or no star formation activity, it's unlikely to be a galaxy - it would have to be a really massive feature to hold this much hot gas, and it's very hard to see how it would prevent star formation. Nor is there enough star formation occurring to explain the cloud as an excitation of the general intracluster medium. So "stripped out of a galaxy" seems the most likely explanation, even if that hardly answers all the questions as yet.

An H α/X-ray orphan cloud as a signpost of intracluster medium clumping

We present the discovery of the first and still the only known isolated cloud (or orphan cloud [OC]) detected in both X-rays and H-alpha in the nearby cluster A1367.This example shows that stripped ISM, even long after the initial removal from the galaxy, can still induce ICM inhomogeneities. We suggest that the magnetic field can stabilize the OC by suppressing hydrodynamic instabilities and thermal conduction. This example also suggests that at least some ICM clumps are multiphase in nature and implies that the ICM clumps can also be traced in H α. Thus, future deep and wide-field H-alpha surveys can be used to probe the ICM clumping and turbulence

EAS 2021 : The Conferencing

This time last year I went to my first purely virtual conference, EAS 2020. I happened to spend all my spare time that week mucking around with my shiny new VR headset on account of living my entire life digitally.

One year later, history repeats.

Well, almost. I don't have another new headset, but I did by sheer coincidence decide to spend all my spare time mucking around with Virtual Desktop to get the PC wireless working very much better than it used to. I also strapped a powerbank to the back as a counterweight that also charges it up, prolonging the battery life by (I reckon) at least threefold whilst making it a lot more comfortable to wear. So that's nice. What about the conference ?

EAS 2021 : The Zoomening

As before, let's do the experience and the science separately. First off, the niggles, which remain unfortunately much the same. There were once again too many people giving multiple talks, and too many plenary sessions. This promotes inequality and I don't like it. Yes, there are worse problems in the world, but it's still an annoyance : why do I get a mere poster (this time not even with a token 1 minute presentation slot) but other people get two or three talks and plenary sessions take two hours per day ? Seems unfair if you ask me.

In a rather different niggle from last year, I found this time that the session titles were often misleading.  "Galaxy clusters and AGN" didn't feature all that much about AGN, while "Satellite galaxies" didn't have that much even about satellite galaxies, etc... why people decide to give their sessions ultra-specific titles but then include very liberal content, I don't know. It's a minor irritation, but it makes it harder to know which session to submit an abstract to, and harder to plan which sessions to attend : you really had to check the scheduled talks quite carefully and not just go by the session titles.

On that note, I found an interesting and unexpected use for AdBlock. Here's the conference schedule as it appears by default :

Which is hideous. Probably less than a quarter of the screen is taken up with actual useful content, i.e. clickable schedule blocks. I got so frustrated with this obscene conference scheduling censorship that I used AdBlock's "hide something on this page" feature to cancel out the annoying extraneous faff and blam :

Much better ! There was actually a decent enough interface hidden under there - less is indeed more.

The final niggle I have to mention is e-posters. While they offer many advantages over traditional conference posters, their major weakness is that the interface is essentially useless for browsing. In a real conference you can casually walk by and look at the titles and/or biggest pictures, but it's bloody tedious to do that virtually. Not only because there's no good way to quickly search for potentially interesting posters, but also because they just don't look nice. Here's mine as it appears in the gallery :

Basically pointless. The poster is mainly images, but the preview shows almost nothing but text. Granted things improve if you deliberately order the images nearer the top of each panel, but this would make the content incoherent so I didn't. Even if you click through to the poster itself (the link works at the time or writing but won't last forever), all you initially see is a bigger, higher resolution version of the above. There's got to be a better interface than this. For example, if you could upload a preview image to display in the gallery, that could easily be much more eye-catching that the scaled-down view that's generated automatically. Yes, there was the option to upload a traditional PDF as well, but... the thing is, e-posters are two hundred times easier to build. So I took the lazy option and made a hideous poster. Oh well.

The Sciencening

Let's move on to the talks. There were quite a lot of good review talks but not that much in the way of interesting new discoveries. That said, I have to give an honourable mention to Heloise Stevance, whose talk on public speaking should probably be required viewing. Although there weren't any "oh god just shoot me instead" talks this year (unlike last time), everything tended towards a certain... mediocrity. Stevance's talk was the only one where the speaker conveyed real energy and enthusiasm. Granted, this is exceptionally difficult to do when talking to a monitor, especially when you don't see any of the other participants, but even in real-life presentations too many speakers are positively sullen bordering on depressed. I have to say that my own institute is particularly bad at this, as though everyone is positively afraid of sticking their head above the metaphorical parapet in case it gets metaphorically lopped off for some reason.

As well as the essential but apparently not-so-obvious points that you should be enthusiastic and not monotonic, Stevance made the interesting case for designing talks and papers very differently. Instead of trying to make a talk a micro-version of a paper, she says that instead you should build the talk explicitly around the take-home points. I think this is generally good advice, especially for short talks to specialist audiences. In a longer seminar or to a broader audience, you can and should afford to spend a disproportionate time on a lengthy introduction to get everyone up to speed. In a 15 minute presentation to specialists, you can practically reduce the introduction to a sentence or two, and spend the rest of the time discussing the science. Not the methodology, but the results. A lot of speakers seem obsessed with the methods to the point that what is is they actually found - the thing we've all come to learn about - gets almost tacked-on at the end. 

Come to that, too many talks were about planned projects or results at a preliminary stage. Frankly, that's a bit daft. Overwhelmingly what I want from a talk is an interesting new result, so if someone wants to present a new survey they haven't even started yet, they should bloody well do a poster instead. This seemed particularly problematic this year, or perhaps I've just lost patience because I feel badly in need of a holiday.

Sigh.

(Is it just me though ? Does everyone else long for detailed descriptions of upcoming surveys and planned instruments ? There doesn't seem much possibility for discussion about things that people are going to do but can't actually do yet. Compare that were data that's already been analysed - there you have the chance for actual productive conversations - which seems like something very important in a conference, to me.)

Anyway, jellyfish galaxies were much in vogue this year. In general they're the result of strong gas stripping occurring as a galaxy slams at high speed through the hot gas in a cluster, leaving behind tails that can sometimes look quite a lot like the tentacles of a jellyfish. Luca Cortese made the point that this phase is common but not ubiquitous, and in extreme cases it's not just the hot or warm outer gas that can be stripped but even the very cold, enormously denser gas as well. Annalisa Pillepich demonstrated from simulations that for a jellyfish to result in a permanent morphological change to a galaxy, the stripping has to actually remove the gas from the galaxy, not just cause a temporary disturbance - otherwise when the gas returns, the galaxy will reclaim its original structure.

There was also quite a lot about data science. Most of this was about machine learning and suchlike, but there was one very nice presentation by Lucia Marchetti about the iDaVIE visualisation tool. This is explicitly based around realtime rendering and analysis using virtual reality headsets, which is just scandalously cool. I'm hoping I'll have time to try out the code sometime this week. Though this is something that's long been on my Christmas list for the awesomeness alone, I'll confess to being just a little bit skeptical that doing analysis this way would actually offer much advantage over traditional methods. That said, some of the tools do look promising, and potentially easier to do in VR, which bodes well. And encouraging users to actually spend a long time just looking at their data is definitely a good thing. The more visualisation tools we have, the better.

A great deal was said about Ultra Diffuse Galaxies, particular by using their globular clusters to try and estimate their masses. An interesting controversy came up regarding DF44, the UDG that's the main candidate for being a "failed giant" with the mass of the Milky Way (the consensus seems quite clear now that most if not all UDGs are just dwarfs), with rival claims made using the same data that establish very different masses. The cause of this disagreement isn't yet known, so I'm going to wait and see what comes onto arXiv before commenting further. 

There was also quite a bit about "pre-processing", a term everyone seems to have become confused as to what it actually means. Certainly there are different evolutionary processes affecting galaxies inside and outside clusters, as well as conditions resulting from external influences acting on galaxies and those resulting from internal processes. To my mind, pre-processing means any external influence that acted on a galaxy prior to its infall into a cluster. Seems simple enough, and I'm not sure why this is at all problematic.

Rory Smith gave a very nice overview of the state of the art - mainly from the perspective of simulations, but with plenty of observational stuff too. Galaxies with lower star formation activity are seen too far from cluster centres for them to have experienced cluster processes, so something else must be at work. One plausible effect is that filaments - the largest-scale structures in the Universe - actually have their own hot gas which could cause ram pressure stripping. Probably not enough to remove the cool gas within galaxies, but enough to remove their outer reservoirs of hotter material and so eventually reduce their star formation. In support of this, clear trends are visible in filaments in age, metallicity and colour of their galaxies, which is pretty compelling evidence that something is happening. Less clear is whether this really is ram pressure or something else, as there's not much constraint on the gas density. 

Lyla Jung raised an interesting point about selection effects that seems obvious when you say it but wouldn't have occurred to me otherwise. That is, large clusters tend to be assembled from large groups. And large groups will cause more pre-processing, so there are selection effects as to where pre-processing is important. 

One potential example of this came from Tirna Deb, who presented evidence that within galaxy clusters it's actually the most isolated galaxies which are the most gas-deficient, the exact opposite of what you'd expect ! But from discussions afterward, Palo Serra suggested that galaxies which are still in sub-groups within clusters are likely recent arrivals, since groups are soon broken apart by the hungry cluster - and group effects are much weaker than clusters. Conversely, galaxies not in sub-groups are more long-term residents, so they've experience the full whack from the cluster already - hence the more isolated galaxies should be the most deficient. There does seem to be pretty decent evidence for this in that sub-groups tend to be found at higher distance from the cluster centres. So a very neat, elegant explanation for a counter-intuitive effect.

So that's years virtual overload over and done with. Mildly interesting, though disappointingly lacking in anything controversial. I'm almost tempted to say that the EAS should drop their official code of conduct policy just to encourage more fights to break out. Or perhaps we all need specialist training in How To Give An Enthusiastic Talk Over Zoom. Or holidays. Holidays are nice.

El Fatso The Less Magnificent

Back in March we had yet another apparently insurmountable challenge to the standard model of cosmology, and this paper from May claims that the problems have once again been surmounted after all.

To recap, last time a paper by Asencio et al. claimed that the El Gordo galaxy cluster is just too damn fat (i.e. massive) to exist. Or rather, according to the standard model of cosmology it shouldn't be possible to assemble such a gigantic behemoth in so short a time, and the collision velocity of its merging sub-clusters is too dang high. The authors looked at a huge simulation suite and found that no such objects should be formed at all, which is a pretty damning result if you take it at face value.

While the claims were made with a somewhat... robust level of conviction, the basic idea seemed reasonable enough to me. My main concern was whether the mass might somehow have been overestimated, since the frequency of such objects is very strongly mass-dependent. There's also the matter of the small area of the survey in which it was found. This means we have no clue whether El Gordo is a hideous bloated freak we can cheerfully ignore, or a representative of a much more interesting, widespread problem that we ought to confront.

Like the Bullet Cluster before it, this paper by Kim et al. circumvents the problem by saying that Asencio et al. were looking for the wrong object. They use new Hubble data to get gravitational lensing estimates of the mass, combined with simulations to figure out the most probable collision velocity whilst accounting for "radio relics" that were previously ignored. The bottom line is they say there's no serious conflict with the standard model after all.

So who's right ? 

Difficult to say. I'm not expert in the lensing techniques they consider, so this was a tough read for me. Fortunately most of the rest is easier, and there are some interesting and very stark contradictions with the Asencio paper.

First, their new measurements decrease the mass by a modest but significant 20% or so compared to previous estimates (50% compared to the value Asencio used). This, they say, is because the previous results had to extrapolate out to the full size of the cluster, whereas their own data covers a larger area so this isn't necessary. Well, maybe, but their figure shows the mass of the cluster continues to increase out to their observed radius limit, and shows no signs of reaching a plateau, so I'm not sure why they're so confident about their new value. 

And anyway this decrease isn't enough to bring it into compatibility with the masses Asencio found in their simulation. Here they appear to be in almost direct disagreement : Kim say the chance of such a cluster existing at such a distance is about 10%, whereas Asencio said it was close to zero. Even given the relatively small initial survey volume, Kim say this isn't surprising that such a monster was found, owing to the observational uncertainties on both the cluster properties and the uncertainties in the cosmological parameter values. But why there's such a stark difference in these claims is very unclear.

What about the collision velocity ? Here it gets worse. Whereas Asencio searched for clusters colliding at an enormous speed of > 2,500 km/s, Kim say the true velocity is likely to be closer to 450 km/s, which would certainly pose no difficulty whatever for the standard model. But their conclusion here is frustratingly brief. They used some simple models to find the general parameters, then ran a hydrodynamic code (i.e. something very sophisticated) to verify it. But do we get to see this fancy simulation ? Heck no. And their description of what they did is frankly confusing, apparently deliberately excluding cases they consider unphysical and then coming up with a velocity they had already pre-excluded !

What I think they're trying to say, which may offer a way out of this mess, is that previous authors began with high infall velocities and/or started with the two cluster components too close together. Kim et al. seem to be saying that actually the two subclusters started off both further away and less massive, so their initial infall velocity was much smaller. Presumably, as they approach each other, they accumulate other background galaxies and grow in mass, eventually reaching a higher velocity for the collision itself (which therefore poses no challenge to physics : more mass => higher velocity). Hence Asencio et al.'s statistics are all correct, but they were looking for the wrong sort of progenitor objects. El Gordo's parents weren't necessarily all that big or fast when they first started their doomed embrace - their romance started gradually, only reaching a frenzy at the final climax.

Ahem. Anyway, my impression is that this would have been considerably better as two papers. Most of this one is about the lensing measurements, with the simulation stuff being too tacked on and confusing. It would have been nice to have a much more rigorous examination of the Asencio result, e.g. how rare is El Gordo itself (rather than its parents) according to their simulations ?

My guess is that something like this analysis of Kim et al. will win out eventually : El Gordo will turn out to be an interesting beast, but not the CDM-slaying monster it's purported to be. But for now, Kim's result is just too unclear to be all that compelling. I suspect I'm missing something.

Head-to-Toe Measurement of El Gordo

We present an improved weak-lensing (WL) study of the high −z (z=0.87) merging galaxy cluster ACT-CL J0102-4915 (El Gordo) based on new wide-field Hubble Space Telescope (HST) imaging data. The new imaging data cover the 3.5 × 3.5 Mpc region centered on the cluster and enable us to detect WL signals beyond the virial radius, which was not possible in previous studies. Our updated mass is a more direct measurement since we are not extrapolating to R200 as in all previous studies. The new mass is compatible with the current ΛCDM cosmology.

Full stack

One of the major difficulties with observing atomic hydrogen is that the emission is very weak. In the very nearby Universe, say within the Local Group, this isn't a big limitation. But just a few tens of millions of light years away and it starts to demand gigantic telescopes and/or obscene amounts of integration time. Beyond about a billion light years it's nigh-on impossible.

This is a problem. In optical wavelengths we can see how star formation evolves throughout the ~14 billion year history of the Universe, and it evolves strongly. Galaxies today are but dim embers compared to the blazing fires they were a few billion years ago. But because of the weakness of the HI line, how that star formation's fuel supply has changed has remained largely a mystery. So we're missing a key part in the story of how galaxies grow up.

There have been a handful of attempts to go deeper. Short of building a gargantuan, all-crushing telescope, one solution is to stack observations of lots of galaxies together, getting you the equivalent of hundreds of hours of integration time. Thus far, none of these efforts have ever really looked terribly convincing in my opinion. You look at the spectra and go, "eeehhh, I mean, I've seen worse, but.... really ?".

This paper changes that. Actually the authors already have a similar paper, which I overlooked because I've fallen for a boy-who-cried-wolf fallacy and stopped reading such claims. So it was a pleasant surprise to see that the detection they present here is pretty unambiguously convincing, as indeed was that in their previous paper.

Using about 400 hours of integration on the GMRT in India, they average together almost 3,000 galaxies. Because it's averaging, all they retrieve is information on the average galaxy in the sample, which is the major downside of stacking (the alternative would have been to do a 400 hour integration on a single galaxy, but this would arguably be worse and certainly riskier, there being no guarantee that any individual galaxy contains a detectable amount of hydrogen). Even so, the results are much more interesting than I expected.

Most of the paper is understandably given to the observational details, but their main result is an average hydrogen mass of about 30 billion solar masses. There are a few galaxies known in the local Universe with masses this large, but not many. More interestingly, the gas to stellar mass ratio at this distance (about 8 billion years ago) is markedly different to what we see today. Nearby bright galaxies tend to have substantially less mass in gas than in stars, but these objects have ratios well above one. So there has, as expected, been a definite evolution in the gas fraction over time. That's not surprising, but it's important that we have concrete evidence now rather than mere hypothesising. The big, bright galaxies we see today were not the same earlier in their evolution.

(Though, as an interesting caveat, this might be a slightly misleading selection effect. We wouldn't expect today's big bright galaxies to have been so big and bright back in the past, because they need that time to build up their stellar content. But this should only be a modest effect, as the main result that bright galaxies today are different to bright galaxies in the distant past is still interesting.)

One of the other puzzles is where the gas is coming from. It's easy to see how it gets consumed in star formation and lost via stripping processes. My understanding was that while star formation has remained fairly constant in the recent past, the gas consumption timescale is only 1-2 Gyr, suggesting it's being replenished somehow (in particular, there was a lot of discussion about whether clouds seen around the Milky Way could be fuelling this, with the conclusion being quite clear that they could not). I dunno if I just missed some big development here, but they say the depletion timescale in the local Universe is more like 5-10 Gyr, meaning there's no mystery since the gas is being used very slowly, whereas at the greater distances it's only ~2 Gyr - so they're witnessing them at the peak of consumption. As they start to run out of gas, star formation activity should naturally drop as the gas density decreases, so it's no mystery that there's still gas around today for slower consumption. This is all nicely consistent, so I'm a bit puzzled why earlier results seem to have given such a different estimate for the local consumption rate.

Anyway, it's a very nice piece of work. An obvious follow-up would be to try a very deep integration on a single galaxy. The problem with stacking is that you wash out a lot of valuable information, so confirming the results on a single galaxy - which would also let you measure the kinematics - could be an interesting compliment. Though just try getting "let's stare at this one galaxy for 400 hours and hope we get an interesting wiggly line as a result" past the proposal committee...

Giant Metrewave Radio Telescope Detection of HI 21 cm Emission from Star-forming Galaxies at z=1.3

We report a 400-hour Giant Metrewave Radio Telescope (GMRT) search for HI 21 cm emission from star-forming galaxies at z=1.18−1.39 in seven fields of the DEEP2 Galaxy Survey. Including data from an earlier 60-hour GMRT observing run, we co-added the HI 21 cm emission signals from 2,841 blue star-forming galaxies that lie within the full-width at half-maximum of the GMRT primary beam. This yielded a 5.0σ detection of the average HI 21 cm signal from the 2,841 galaxies at an average redshift ⟨z⟩≈1.3, only the second detection of HI 21 cm emission at z≥1

Accept no imitations

So I've gone from being swamped with writing a grant application to being swamped with writing an observing proposal. Yay. But I had to read this paper, because a) it's just a letter so it's nice and short and b) it's all my favourite topics mashed together.

This paper looks at a remarkable object in a compact group of galaxies. There are several big, dramatic galaxies present, with shell-like structures and arms and stars flying everywhere, but that's not what the authors are interested in. Instead, they pick on an innocuous-looking blue blob, a structureless starball peeping out from behind one of the bigger beasts. It was previously noted as being at the end of an HI tail, so the obvious idea is that it's a tidal dwarf galaxy formed from of the torn-out gas from some galaxy-galaxy interaction.

Tidal dwarf galaxies are pretty interesting in themselves, but the properties of this one overlap with not one but two other interesting objects. I guess that cubes it interestingness, making it so interesting that everyone had better had a lie down lest they get too excited.

While the smooth, structureless nature of the object isn't anything special, its stellar population seems to be wholly young : 60 Myr or less. Most "normal" galaxies have at least some very old stellar component at least a few gigayears old, even those that are dominated by a younger crowd. And this little galaxy doesn't have many stars either : so much so that it easily counts as a Ultra Diffuse Galaxy.

"Ahah !" you may be thinking. "That means UDGs are probably just tidal after all, so we can put all this hoo-hah about whether they're massive, dark-matter dominated galaxies to rest and get on with our lives."

In fact you can't. For one, UDGs are diverse enough that we know we can't paint them all with the same brush anyway. For another, this particular tidal UDG appears to be dark matter dominated, by a factor 3-10. And that's not at all expected for tidal dwarf galaxies, which are supposed to be made of purely baryonic matter.

And it gets stranger still. Given that there appears to be no ongoing star formation this object, based on UV data, the authors calculate that it will become totally invisible in about 2 Gyr. So although it's merely a faint galaxy right now, eventually it will become a truly dark galaxy. The implication, of course, is that many other such objects could already have been produced, so they're floating freely around the place confusing all kinds of hapless radio astronomers. See, there's been lots of speculation about how HI clouds can appear to masquerade as dark galaxies, where the dynamics indicates a lot of extra mass present that isn't actually there. But this object shows that there could be some such "fake" dark galaxies which really are... dark... galaxies...

This is all a lot like the idea of faking the Moon landing on the actual Moon. Or if you want a more colourful philosophical conundrum, boobs. Are they still real if they're fake ? No, don't answer that.

What they really mean is that the idea of dark galaxies is that they're supposed to explain the missing satellite problem, wherein too few galaxies are observed compared to simulations. If most of them are just too faint to see, problem solved. These would be primordial dark galaxies. But now this little object suggests we may have genuine dark galaxies produced by an entirely different, much later mechanism that would have nothing to do with the cosmological difficulties at all. So that's an extra large spanner thrown in the works. Yay.

Personally I'm not entirely sure how secure their mass estimate really is. The line width of the object is quite narrow and the inclination correction must be difficult; the HI and optical centres are offset. I also don't understand their statement that the object could become self-bound even without dark matter : they say there's a mass threshold for this, but this must surely be dependent on other factors too.  And I don't understand how you could get such large amounts of dark matter in a tidal dwarf anyway; in simulations, it tends to disperse very easily. So this object potentially raises a lot more questions than it solves - as all good discoveries should. Hurrah !

A diffuse tidal dwarf galaxy destined to fade out as a "dark galaxy"

Assuming that the object is dynamically stable and able to survive in the future, its fading in time via the aging of its stellar component will make it undetectable in optical observations in just ∼2 Gyr of evolution, even in the deepest current or future optical surveys. Its high HI mass and future undetectable stellar component will make the object match the observational properties of dark galaxies, that is, dark matter halos that failed to turn gas into stars. Our work presents further observational evidence of the feasibility of HI tidal features becoming fake dark galaxies.

The pendulum swings and swings again

The saga of NGC 1052-DF2 and DF4 continues.

I'm not going to try and keep up with every development on this one any more because there are just too many. In brief, two galaxies were found that apparently had no dark matter at all. Given their apparent distance of about 20 Mpc, their size and velocity dispersion appear to indicate that their stars are moving so slowly that they could be gravitationally bound all by themselves, with no need of the usual extra mass.

This discovery has been controversial. First there were claims that the velocity dispersions had been measured incorrectly. This never looked terribly convincing, and indeed that idea has gone away. Then there were claims that the distance was wrong. If the galaxy is actually at about 13 Mpc, then the velocity measurements actually do require quite a lot of dark matter, as well as explaining other anomalies like the brightness of the galaxy's globular clusters. This has turned into a game of galactic ping-pong, with each team periodically refuting the other's distance claims with new data or analysis methods.

The present paper is the latest in a long series of attempts to decisively knock out the claim for a 13 Mpc distance. They use the "tip of the red giant branch" method to estimate the distance. Basically, red giant stars ought to be reasonably good standard candles (at least statistically if not individually), meaning that you know their true brightness so can get the corresponding distance. Previous claims using this method, they say, were not deep enough to detect stars at 20 Mpc distances, leading to all sorts of complications in the analysis. So they've gone and got Hubble data which should be more than sufficient to sort this out. If nothing else, their main image is certainly very pretty.

They say the distance to DF2 is actually even slightly higher than their initial claim, at 22 Mpc. DF4 they've previously said is at 20 Mpc, and though there are certainly some errors in all this, they say this is sufficient to say that at least one of them isn't part of the NGC 1052 group. That's significant, because it makes a tidal formation scenario all the less likely. In addition, they mention that they have wide-field deep imaging (not presented here) which does not show the previously-reported tidal tails, so those features are likely spurious.  

Finally, they'd previously made a boo-boo in claiming that these objects refute MOND, because MOND has an external field effect that they didn't account for. But now it looks like both DF2 and DF4 are too far away from their companion galaxies for this to be significant after all. Well, we'll see. Expect lots of angry responses from both standard model and MOND enthusiasts on this one.

A Tip of the Red Giant Branch Distance of 22.1 Mpc to the Dark Matter Deficient Galaxy NGC1052-DF2 from 40 Orbits of Hubble Space Telescope Imaging

The inferred distance is DTRGB=22.1 Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC1052. The new HST distance rules out the idea that some of NGC1052-DF2's unusual properties can be explained if it were at 13 Mpc; instead, it implies that the galaxy's globular clusters are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC1052-DF2 to NGC1052-DF4 is well-determined at 2.1 Mpc, significantly larger than the virial diameter of NGC1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics.

The tale of a tiny, farting, blushing dwarf

Who amongst us doesn't have the decency to look embarrassed when publicly expelling gas ? Galaxies, it turns out, are no less prone to turning a certain shade of red when things get... windy.

Today's paper combines no less than three of my favourite topics : ultra diffuse galaxies, ram pressure stripping, and dark galaxies. UDGs are these large(ish) galaxies which have very few stars, with dark galaxies having no stars at all. The connection between the two, if any, is highly uncertain, and it's possible dark galaxies are actually just clouds of stripped gas and not really galaxies themselves. And RPS is a particular kind of stripping mechanism : the process whereby a galaxy moving through hot, thin gas can build up so much pressure that its own gas can be forced out. In doing so it eventually runs out fuel for star formation, so all the hot, short-lived blue stars quickly die off, leaving behind only the small red ones.

Hence, farting galaxies turn red. Change my mind.

This phenomenon is more-or-less well known in ordinary galaxies, but no-one has yet seen it happening in UDGs. The authors here present a pretty convincing case for the detection of such an event. Just like their bigger, brighter cousins, UDGs suffer the same acute embarrassment when they too are squeezed uncomfortably.

The story actually stars with the detection of a neutral hydrogen (HI) cloud back in 2015. Cannon et al. reported that a few features, this one included, had no obvious optical counterpart. High resolution observations with the VLA revealed that the cloud was suspiciously close to, but not coincident with, a faint optical counterpart. On a quick re-read of Cannon 2015 I'm not quite sure why they thought this was all that odd (displaced gas is a rare but not unknown or unexpected occurrence); interesting yes, strange, not so much.

(Incidentally, I don't like the term "almost dark" that has become popular. There are genuinely dark structures, so I prefer to call these faint but definite systems dim galaxies. But that's by-the-by.)

This current paper reveals a nice connection between the optical object, the HI detection, and some ultra violet extended emission and blobs. The optical galaxy they categorise as satisfying the UDG criteria (which were generally not known about in 2015), albeit marginally. The UV emission (from ionised or excited gas from hot young stars) is slightly displaced from the UDG (by a few kpc, nothing much at all really) roughly on a line from the UDG towards the centre of the Virgo cluster. 

This very neatly fits the "fireball" model of RPS. Galaxies move towards the cluster centre, the ram pressure builds up until stripping begins, and then the tail of stripped gas behind the galaxy can cool and condense and form UV-bright stars. And that's reasonably well-known for larger systems, but never seen before for galaxies like this.

While the UV data gives them information on metallicity (chemical composition), showing clearly that most of the blobs are about what you'd expect from galactic material (and not pristine gas as you'd expect if this were a case of accretion rather than gas loss), their estimated ages are less conclusive. Through a series of really quite tedious modelling, they're able to establish that the peak of RPS most likely occurred around 100 Myr ago or so. They can't see a nice clear age gradient that would really seal the deal on the fireball model, but given the short duration of stripping and the small distance travelled, that's not at all surprising.

There are a lot of very interesting implications from all this. First, they say the galaxy is likely to have been originally a UDG to begin with - not something that formed as a result of cluster processes as some other models indicate. No, they're truly independent systems in their own right, a normal part of the galaxy population and not some cluster-specific thing.

Second, there's a possible connection to the truly "dark galaxies"... well, maybe. At the very least this is a nice example of displaced gas, whereas previously I believe the only other such example in Virgo was M49. 

It'd be extremely satisfying if the totally dark HI clouds could be explained by this, but the third point probably makes this unlikely. The displaced gas still isn't enough to account for how much gas the original galaxy ought to have had (somewhat surprisingly, many UDGs have pretty normal gas contents), so there's been a loss rate of around 4 solar masses per year. That's pretty much exactly the same as seen in much more massive systems. And if this applies to the dark clouds, which don't have much gas left, we're detecting them suspiciously close to the very end of their lives.

(I suppose there could be a selection effect, however. Maybe they only appear as truly isolated clouds just before the last bit of gas evaporates. But this still implies a high production rate of such systems, so this would be an awkward solution at best.)

What's odd about that is that UDGs need to have lower than usual star formation rates, otherwise they'd just be normal galaxies. So, presumably, they have more extended, lower-density gas. It's not in the least surprising that gas removal quenches star formation and make them all embarrassed red, but one might expect the evaporation of lower-density gas to occur at a very different rate to what happens for much denser gas (e.g. it should disperse more quickly and/or change phase to cooler or hotter undetectable gas more readily). But apparently it doesn't. So, is the physics of gas dissolution different, or is there some completely different quenching mechanism at work in UDGs ?

Right now, we've no idea. Interesting as it may be, it'll take a lot more than one farting dwarf to solve all the mysteries of galaxy evolution.

Formation of a red ultra-diffuse galaxy and an almost dark galaxy during a ram-pressure stripping event

We detected a few star-forming blobs in the VESTIGE survey, located at ∼5 kpc from a UDG, namely NGVS 3543, in association with an HI gas cloud AGC 226178, suggesting a recent interaction between this low-surface-brightness system and the surrounding cluster environment. We use a complete set of multi-frequency data including deep optical, UV, and narrow-band Hα imaging and HI data to understand the formation process that gave birth to this peculiar system.

El Fatso The Magnificent

I read this paper out of sheer spite. Being told, "anyone believing in ΛCDM isn't a physicist" at a recent seminar, as well as receiving an unnecessarily insulting comment on an old post, tends to wind me up the wrong way.

Anyway, "El Gordo", which Wikipedia says means "the fat one", here refers to a monster galaxy cluster in the early Universe. It's so massive that it's a bit of a puzzle how such a gargantuan structure managed to form so quickly after the Big Bang. 

But haven't we heard such claims before ? Indeed. The Bullet Cluster is most famous as an example of two colliding clusters clearly demonstrating that the dynamics of the systems does not follow the baryonic matter : i.e., while the X-ray gas clearly gets stuck in the middle during the collision, lensing measurements show that the dark matter doesn't really notice the collision much. But it's only slightly less famous because its sheer mass and the collisional velocity of its sub-clusters were thought to be a problem for ΛCDM, though that prospect receded when it was shown that the collision velocity wasn't as high as initially thought. Even the authors of the current paper appear to concede that point, if reluctantly.

This paper covers quite a bit of ground, so I'm going to strictly limit my comments here as to whether El Fatso does indeed contradict standard cosmology. The MONDian stuff they also present is very interesting, but I think it would have been better as a separate paper (the protracted other arguments against ΛCDM, by contrast, are wearing very thin, and in my view are long since discredited). But, while it's entirely possible that this is a case of boy-who-cried wolf, as it stands I find the evidence that the Fat One is worryingly large considerably more compelling than for the Bullet Cluster.

Essentially, what they do is use an enormous, pure dark matter simulation to search for analogues of El Gordo. This "Jubilee" simulation is the largest to date, covering about the same volume as the entire visible Universe at the distance in question (a fact which is annoyingly buried on page 13). What they try to do is find how often such analogues occur, and given the survey size in which El Gordo was found, estimate if this discovery is compatible with expectations or not. 

Of course, pure dark matter simulations are necessarily limited in what they can tell you. But unlike other cases, where the baryonic physics is almost certainly responsible (in my opinion) for any discrepancy between theory and observation, it's harder to see this being the case for El Gordo. Its two interesting features are its sheer mass and the infall velocity of its two major components. Both of these shouldn't be affected much at all by observations being restricted to the baryonic components. So using a pure dark matter model ought to be perfectly valid in this case.

I do have a slight quibble that their search criteria may be excessively strict : rather than search, say, for objects within some mass/velocity range, they search for objects which are either as extreme or more extreme than El Gordo. That's probably placing a bit too much faith in the observations, but this is largely negated because they show in their figures more detailed distributions of what they found. 

Which is : no clusters at all as massive as this one at any infall velocity. Given the well-defined mass and velocity distribution, they can extrapolate to see exactly how rare El Gordo is (or in other words how large the volume would have to be to contain such a behemoth), and the answer is, "very" : they expect to find of order 0.0000000001 such clusters, so finding even one is very surprising indeed.

Does this mean cosmology is all wrong then ? Possibly yes, though I wouldn't bet on it. I have a lot of issues with the language of the paper - I don't think it makes any sense to use the term "falsify" in the probabilistic sense they do, something that is "false" cannot be false with some given probability value. Likewise they take the 5σ threshold as something magical for some reason, and quote distances and timescales which seem worryingly accurate (e.g. kpc scales when describing something Gpc away; "559 Myr" - I find it very unlikely that such precision is justified). And I'm biased because I'm also continually annoyed that other people continually get away with making extremely grand claims while I get routinely pulled up on things which shouldn't be controversial at all.

But this is all by-the-by. More importantly, the mass dropoff in the simulations is extremely steep : go down a factor three and such clusters do exist in their simulations. So I do wonder just how secure that observational mass estimate really is. Likewise for the infall velocity, which was based on more detailed, smaller simulations. In the seminar they claimed that you couldn't use a much lower velocity and still get the same (quite distinctive) morphology as the observations, but I couldn't really see much difference between the high and low velocity cases in the figures they showed. But I would have to check that more carefully - perhaps I missed something. In any case, however detailed and extensive the previous simulations were, it's always worth remembering that there's more than one way to skin a cat.

What I think has the potential to become very much more interesting is that this monster was found in a relatively small survey. We can argue about probabilities till the cows come home, but for single objects this won't do much good. It is perfectly valid to posit a weird, unlikely occurrence to explain a weird object. Indeed, if the mechanism at work wasn't in some way unusual, we ought to see such oddities everywhere. So however unlikely their simulations say a giant cluster at this distance might be, so long as it is physically possible, a single object is nothing very worrying. After all, it's pretty unlikely that we happen to live on a planet with total solar eclipses, but we don't hold that as evidence against cosmology. You're bound to get some unusual features - flaw of averages, and all that.

But if you can show, as they heavily imply, that they expect such objects to actually be quite common in observations... then you've got something really interesting. Then you're really dealing with physics, not statistics : you need a mechanism that must occur quite frequently. And so far as I know, ΛCDM just doesn't have that.

If I can find the time, I'll try and do a proper paper chase on this one. To be honest, because of certain people's tendency to make exaggerated claims about how obviously ΛCDM is some kind of weirdly dull cult, I view such claims with far more skepticism than I otherwise would. It's hugely counterproductive. And that's a shame, because disproving it would be truly spectacular. I, for one, would quite like to know when their really is a wolf that's come along to gobble up the standard model's carefully-tended fluffy sheep.

A massive blow for ΛCDM - the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology

Such a fast collision between individually rare massive clusters is unexpected in Lambda cold dark matter (ΛCDM) cosmology at such high z. However, this is required for non-cosmological hydrodynamical simulations of the merger to match its observed properties. Here, we determine the probability of finding a similar object in a ΛCDM context using the Jubilee simulation box with a side length of 6h−1 Gpc.