EAS 2024 : The Other Highlights

What's this ? A second post on the highlights of the EAS conference ? Yes ! This year I've been unusually diligent in actually watching the online talks I didn't get to see in person. Thankfully these are available for three months after the conference, long enough to actually manage to watch them but also, crucially, short enough to provide an incentive to bother. And I remembered a couple of interesting things from the plenaries that I didn't mention last time but which may be of interest to a wider audience.

Aliens ? There are hardly any talks which dare mention the A-word at astronomical conferences, but one of the plenaries on interstellar asteroids dared to go there. The famous interstellar visitor with the unpronounceable name of ʻOumuamua (which is nearly as bad as that Icelandic volcano that shut down European airspace a few years ago) got a lot of attention because Avi Loeb insists it must be an alien probe. He's wrong, and his claims to have found bits of it under the ocean have been utterly discredited. Still, our first-recorded visitor on a hyperbolic trajectory did do some interesting things. After accounting for the known gravitational forces, its rotation varies in a way that's inconsistent with gravity at the 10-sigma level. The speaker said that the only other asteroids and comets known to do this have experienced obvious collisions or have obvious signs of outgassing, neither of which happened here. He took the "alien" idea quite seriously.

Ho hum. No comment.

Time-travelling explosions. The prize lecture was by best-PhD student Lorenzo Gavassino, who figured out that our equations for hydrodynamics break down at relativistic velocities. Normally I would find this stuff incomprehensible but he really was a very good speaker indeed. And the main results is that they break down in a spectacular way. You might be familiar with simultaneity breaking, where events look different to observers at different speeds. Well, says Lorenzo, this happens to fluids moving at relativistic speeds in dramatic fashion : one observer should see a small amount of heat propagating at faster than the speed of light while another would see some energy travelling backwards through time. The result should be massive (actually, infinite) instabilities and the spontaneous formation of singularities. Accretion discs in simulations ought by rights to explode, and God knows what should happen to neutron stars.

The reason that this doesn't happen appears to be a numerical artifact which effectively smooths over this (admittedly small) amount of leakage. But what we need to do to make the equations rigorously correct, and how that would affect our understanding of these systems, isn't yet known.

Ultra Diffuse Galaxies may be tidal dwarfs in disguise. Another really interesting PhD talk was on how UDGs might form in clusters. When galaxies interact in low density environments, they can tear off enough gas and stars to form so-called tidal dwarfs. The key features of these mini-galaxies is that they don't have any dark matter (which is too diffuse to be captured in an interaction like this) and short-lived, usually re-merging with one of their parents in, let's say, 1 Gyr or so. But what if the interaction happens near the edge of a cluster ? Well, then the group can disperse and its members separated as they fall in, so the TDG won't merge with anything. Ram pressure will initially increase its star formation, increasing the stellar content in its centre and making it more compact, before eventually quenching it due to simple lack of gas. So there should be a detectable trend in these galaxies, from more compact to more diffuse going outwards from the cluster centre, all lacking in dark matter.

Of course this doesn't explain UDGs in isolated environments, but there's every reason to think that UDGs might be formed by multiple different mechanisms. A bigger concern was that the simulations didn't seem to include the other galaxies in the cluster, so the potentially very destructive effects of the tidal encounters weren't included. But survivorship bias was very much acknowledged : all galaxies, she said, get more compact closer to the centre, but not all survive at all. It's a really intriguing idea and definitely one to watch.

Even more about UDGs ! These were a really hot topic this year and whoever decided to schedule the session to be in one of the smaller rooms was very foolish, because it was overflowing. A few hardy souls stood at the back, but most gave up due to the poor air conditioning. Anyway, a couple of extra points. You might remember that I wasn't impressed by early claims than NGC 1052-DF4, one of the archetypes of galaxies without dark matter, had tidal tails. Well, I was wrong about that. New, deeper data clearly shows that it does have extended features beyond its main stellar disc. Whether that really indicates tidal disruption... well, I'll read the paper on that. And its neighbour DF2 remains stubbornly tail-less.

The other point is a new method for measuring distances to UDGs by looking at the stellar velocity dispersion of their globular clusters. This was the work of a PhD student who found that there's a relationship between this dispersion and the absolute brightness of the parent galaxy. Getting dispersion of the clusters is still challenging, requiring something like 20 hours on the VLT... but this is a far cry from the 100 HST orbits needed for the dispersion of the main stellar component of the galaxy itself. Apparently this work on the dispersion within individual clusters, so even one would be enough. They tested this on DF2 and DF4 and found a distance of....16 Mpc, right bang in the middle of the 13 and 20 Mpc claims that have been plagued with so much controversey.

Ho hum. No comment.

Fountains of youth and death. Some galaxies which today are red and dead appear to have halted their star formation very early on, but why ? One answer presented here quite decisively was due to AGN – i.e. material expelled from the enormous energies of a supermassive black hole in the centre of the galaxy. Rather unexpectedly it seems that most of this gas is neutral with only a small fraction being ionised, and detections of these neutral outflows are now common. In fact this may even be the main mechanism for quenching at so-called "cosmic noon" (redshifts of 1-2) when star formation peaked. Well, we'll see.

The other big talking point about fountains of ejected material was how galaxies replenish their gas. Here I learned two things I wish someone had told me years ago because they're very basic and I should probably have known them anyway. First, by comparing star formation rates with the mass of gas, one can estimate the gas depletion time, which is just a crude measure of how long the gas should last. And at low redshift this is suspiciously low, about a billion years. Does this mean we're in the final stages of star formation ? This is still about 10% or so of the lifetime of the Universe so it's never seemed all that suspicious to me.

The problem is that this depletion time has remained low at all redshifts. It's not that galaxies are suspiciously close to the end, it's that they should have already stopped forming stars and run out of gas long ago. Star formation can be estimated in different ways with no real constraint on distance, though gas content is a bit harder – we can't do neutral hydrogen in the distant Universe, but we can absolutely do molecular and ionised gas. Despite the many caveats of detail there's a very strong consensus that galaxies simply must be refuelling from somewhere.

One of those models has been the so-called galactic fountain. Galaxies expel gas due to stellar winds and supernovae, some of which escapes but most of which falls back to the disc. Now this is obvious as to how it explains why star formation keeps going in individual, local parts of the disc where the depletion time is too short, but how this explains the galaxy overall has never been clear to me. What might be going on is that the cold clouds of ejected gas (which look like writhing tendrils in the simulations) act as condensation sites as they move through the hot corona and fall back. Here gas in the hot, low density corona of the galaxy can cool, with the simulations saying that this mass of gas can be very significant. So the galaxy tops up its fuel tank from its own wider reservoir. It will of course eventually run out completely, but not anytime soon.

This is a compelling idea but there are two major difficulties, one theoretical and one observational. The theoretical problem is that the details of simulations really matter, especially resolution. If this is too low, clouds might appear to last much longer than they do in reality. One speaker presented simulations showing that this mechanism worked very well indeed while another showed that actually the clouds should tend to evaporate before they ever make it back to the disc, so this wouldn't be a viable mechanism at all. On the other hand, neither used a realistic corona : if it's actually not the smooth and homogenous structure they assume it to be, this could totally change the results.

The observational difficulty is that these cold gas clouds are just not seen anywhere. This is harder to explain but may depend on the very detailed atomic physics : maybe the clouds are actually warmer and more ionised than the predictions, or maybe colder and molecular. Certainly we know there can be molecular gas which is very hard to detect because it doesn't contain any of the tracer molecules we usually use; H2 is hard to detect directly so we usually use something like CO. 

And with that, I really end my summaries of EAS 2024, and return to regular science.

EAS 2024 : The Highlights

For the last major conference I went to, I combined the science and travel reports together. This was easy because Cardiff to me is not an exotic destination so it hardly needs much description. But this year's EAS conference was in Padova, and my explorations of the city itself and neighbouring Venice easily required their own dedicated post over on Physicists of the Caribbean. That, however, is a sideshow. Here I should say something about the main reason I was there, i.e. the SCIENCE !

This will only be brief. I have a number of things I want to look up in detail, but for now, here's what I leaned from the conference itself.

Euclid Is Mega Awesome. Like, seriously awesome. I have this chronic bad habit of not paying attention to new telescopes when they're being proposed or even during construction : who know when they'll launch or whether they'll reach their design spec ? Worst of all, and perhaps a better reason, is that their marketing is often terrible. It's all public outreach about their main target mission. For example as far as I knew Euclid was some specialist thing for probing dark energy presumably by measuring something very specific and niche, which just goes to show how little attention I ever give new instruments.

Actually it's more like Hubble on steroids. It's got comparable resolution (though not quite Hubble level) but with a vastly larger field of view and exquisite optics that gives a uniformly high quality image across the whole field. This makes it a fantastic, game-changing instrument for the low surface brightness universe. Want to find faint stellar streams, tiny dwarf galaxies and other exotic phenomena ? Euclid to the rescue ! If they'd sold it as more of a survey instrument and not this dark energy thing... well, quite possibly they did and I just wasn't listening. 

Whoops ! Luckily for me, its main survey will be almost all-sky and openly available, so I won't have to do anything except look at the data when it's available.

Cosmology Isn't Dead Yet. There are lots of press releases about the discovery of disc-like and massive galaxies in the early Universe, only a few hundred million years after the Big Bang, which is supposedly not long enough for them to have formed. The picture according to the experts is a bit more subtle than this popular description though. Some of these objects might be a problem - they really might, and not in a "but probably not" way : there's every chance there's something going on here that we don't understand. Whether that's the cosmology itself, i.e. the structure and nature of the universe, or just the detailed physics of the gas and star formation... that's where things get more suspect.

For example, it's not, it turns out, that simulations don't predict such objects. They do. It's just that it appears that they predict fewer than the numbers observed. In the Illustris simulations apparently about 10% of the relevant comparison sample are discy in the early simulated Universe, which isn't a lot but isn't insignificant either. The problem is that it's unclear if this is in conflict with the observations because the numbers are still small, and we aren't sure about the observational biases. For mass the situation is much worse : when deriving the mass using synthetic observational procedures (that is, transforming the simulations into the kind of data observers would process, and then using their methods to guestimate the stellar mass), results vary by up to three orders of magnitude away from the true value. So claims that there are too many massive galaxies in the early Universe can be safely ignored.

Except, there's a major caveat. Mass is a derived parameter with many uncertainties, but straightforward luminosity (i.e. brightness) is a direct observable. And here there does appear to be a conflict (sorry, tension) with observations.  There are potential solutions here as well though. For example a more "bursty" mode of star formation, rather than the more continuous process generally assumed, as well as accounting for nebular continuum emission and a top-heavy IMF (that is, forming more big stars in proportion to small ones than in the nearby universe, which could happen because their chemistry would be completely different), might be enough to solve all this. As some wise old sage commented, complicated problems often have complex, multi-parameter solutions rather than one big single "change this and all will be well" moment. Which unfortunately means that figuring this out is going to take time.

EDIT : I almost forgot. Back in 2017 there was a paper claiming that galaxies in the early Universe show declining rotation curves, indicating they were far less dark matter dominated than today. I was skeptical of this, as reported here with some follow-up here. And from conference results it seems that these results are heavily dependent on observation time : too short and indeed the result is declining curves, but observed for longer than they flatten considerable. Exactly why this should be I don't know, but it appears the original authors rowed back on their initial claims somewhat in a 2020 paper. It seems that the original results were something of an oversimplification, if not just simply wrong.

There Are No Dark Galaxies. New HI surveys are reaching lower column (or surface) densities, that is, how much gas they detect per unit area, than I was aware. In fact they're comparable to Arecibo but with the added benefit of much higher resolution. The penalty is that this takes enormous amounts of observing time but this is largely compensated for by large fields of view.

The results so far I think are still mainly "watch this space" except for individual objects. But one interesting finding is that there's a distinct lack of optically dark galaxies which have gas but no stars. This is something I've been working on for many years with Arecibo data, but MHONGOOSE's much improved resolution combined with its sensitivity means we would already have expected to see something if they exist in numbers of any significance. Thousands of detections of normal galaxies already (over 6,000 in fact - compare this to the 31,000 from ALFALFA which took many years to achieve !) but no hint of anything dark that isn't explicable by another mechanism... though of course, this has some caveats, but a significant population there appears none.

Gas Accretion Definitely Happens. But we still don't know when or how ! Obviously galaxies acquire gas at some point or they'd never form any stars, but while there's much indirect evidence for this, direct observations remain hugely unconvincing. Mergers have been ruled out as a significant source of the gas, there just aren't enough objects to do this. Cold accretion, where cold atomic HI falls into galaxies along streams, remains a distinct possibility but unobserved, even with the newest and most sensitive instruments.

Hot accretion (from the hot gas large-scale cosmic web) also remains a possibility but while large-scale bridges of X-ray gas have been detected, everyone was much more circumspect about claiming these as detections of the web itself than they have been in the past. And quite properly too, because any individual detection can always be challenged as an interaction. To claim a detection of the web, we'd really need to see it ubiquitously, with multiple strands connecting multiple clusters. Interestingly, HI intensity mapping has thus far got a statistical detection, but not to the point of being able to do actual imaging yet.

Radio Halos Do Funny Things. Not only does the X-ray gas trace giant, megaparsec-scale structures linking entire galaxy clusters, but so does the much lower-energy radio emission. There are different kinds of radio halos, with Mpc-scale giants to ~100 kpc scale "minihalos". And these aren't the same component, with the density profiles of the two showing a distinct change of slope. Then there are radio relics, the result apparently of shocks in the intracluster medium producing giant arcs of radio emission.

In one particularly noteworthy case, one of these giant radio lobes appears to be interacting with a galaxy. This shows a tail which is distinctly different from most. Where galaxies lose gas by ram pressure, the tails tend to be well-collimated and decrease in brightness at greater distances. This one instead gets both wider and brighter, indicating a different physical processes is at work. Like classical ram pressure, however, it seems to have caused an initial increase in the star formation rate of the galaxy. This is interesting to me because I've always assumed these features were of too low a density level to have an impact on anything, being an interesting way to trace the dynamics of an environment but being themselves no more than tracers, not interactors.

Ultra Diffuse Galaxies Are Still A Thing. There seems now to be a quite firm consensus : UDGs are two populations, one of "puffy" dwarves of low total mass that have become somehow extended, and the other of "failed", much more massive galaxies not predicted by any simulations. Several people made that last point and nobody raised any arguments, though exactly how massive they are (and how numerous this population is) wasn't clearly stated. Even so, to my mind if you want a serious challenge to cosmology, forget the attention-hogging results from JWST and look at these much nearer objects !

Likewise, the view seems to be that UDGs are indeed (following results from a few years ago) not actually all that large - but they are flatter : their light profiles are basically constant and then suddenly truncated at their edges, whereas normal galaxies have more complicated and varied profiles. There still seems to be some disagreement, however, as to whether UDGs therefore represent extreme examples of regular dwarf galaxies or are genuine outliers which are qualitatively different from the main galactic population. 

The dynamics of the more massive objects to me suggests the latter, but there's still not a clear answer to this. Their globular cluster populations are also highly diverse, with some having none at all and others having far more than expected. One very interesting set of observations by Pierre-Alain Duc shows stellar tails from globular clusters in the inner regions of the UDGs, likely merging with the main body of the galaxy - that's how good our observations have become ! Considering their whole set of properties, it remains understandably difficult to decide what the hell UDGs actually are.

To me the most interesting individual object presented in the whole conference was a UDG by Pavel Mancera Piña, he of the "UDGs have no dark matter" fame. Regular readers will know I was at first enthusiastic about this result, then became a bit more skeptical, but finally I've settled (?) back to my original stance : the results seem secure enough that they can't be attributed to observational errors or improper corrections. Now Pavel has found an object which is especially weird. Like the others, it's isolated. Its HI map shows very neatly circular contours, and its kinematics are consistent with no dark matter at all. The only way it can have a dark halo is if the concentration is very low - much lower than standard cold dark matter predicts, but explicable with self-interacting dark matter... 

And because it's isolated, explaining it with modified gravity is hard. If gravity rather than matter governs dynamics, then all isolated objects of the same mass and radius should show the same kinematic properties. That they don't might well argue that such notions are simply wrong, even as objects like this one indicate that the standard model of dark matter itself has flaws.

Well, of course much remains to be done in all those categories, especially the last. My own talk I'm pleased to say went down very well, I got lots of questions about the AGES dark clouds and had some nice discussions afterwards. My poster (should be accessible for a few months, I think) may have sunk without trace, but no matter. And the conference slogan "Where Astronomers Meet" may be the most blandest thing that ever did bland, but the sessions themselves were full of interesting stuff.

I'm old enough to remember conferences of a different era of more, shall we say, "robust conversations". I've not seen any actual arguments (except in some much smaller events) in many years now. Disagreements still arise but they're altogether gentler. Sometimes I miss the spectacle of hearing a good row from a safe distance, but perhaps, as long as we still do interesting science and still have fruitful discussions, this new way is better.

The shoe's on the other foot

My, how the tables have turned. The hunter has become the hunted. And various other cliché's indicating that the normal state of affairs have become reversed.

That is, as well as having to write an observing proposal, I find myself for the first time having to review them. Oh, I've reviewed papers before, but never observing proposals. This came about because ALMA has a distributed proposal review system : everyone who submits their own proposal has to review ten others. And since this year I finally submitted one, I get to experience this process first hand.

The ALMA DPR procedure

When you submit a proposal, you indicate your areas of expertise and any conflicts of interests  – collaborators and direct competitors who shouldn't be reviewing your proposal, either because they'd stand to benefit from it being accepted or would love to take you down a peg. It's a double-blind procedure : your proposal can't contain any identifying information and you don't know who the reviewers are. Some automatic checks are also carried out to prevent Co-Is on recent ALMA proposals being assigned as reviewers, and suchlike.

Then your proposal is sent off for initial checks and distributed to ten other would-be observers who also submitted observing proposals in the current cycle. You, in turn, get ten proposals to review yourself. Each document is four pages of science justification (of which normally one or even two pages are taken up with figures, references, and tables) plus an unlimited-length technical section containing the observing parameters for each source plus some brief justification on the specifics (in practise, in most proposals each of these so-called "science goals" are very similar, using the same observing setup on multiple targets). You then write a short review of each one, of a maximum of 4,000 characters but typically more like ~1,000 (or even less) describing both the strengths and weaknesses of each. You also rank them all relative to each other, from 1 (the strongest) to 10 (the weakest).

That's stage one. A few weeks later, in stage two you get to see everyone else's reviews for the same proposals, and can then change your own reviews and/or rankings accordingly, if you want to. So far as I know, each reviewer gets a unique group of ten proposals to review, so no two reviewers review the same set of proposals, meaning you can't see the others rankings. Exactly how their rankings are then all compared and combined, and ultimately, translated into awarded telescope time, remains a mystery to me. Those details I leave for some other time, and I won't go into the details of anonymity* here either : I seem to recall hearing that this gives a better balance of both experience and gender, but I don't have anything to hand to back this up.

* I will of course continue to respect the anonymity requirements here, and not give any information that could possibly identify me as anyone's reviewer.

Instead I want to give some more general reflections on the process. To be honest I went into this feeling rather biased, having received too many referee comments which were just objectively bollocks. I was quite prepared to believe the whole thing would be essentially little better than random, which is not a position without merit.

First thoughts

And my initial impressions justified this. It seemed clear to me that everyone had chosen interesting targets and would definitely be able to get something interesting our of their data, making this review process a complete waste of time.

But after I let things sink in a bit more, after I read the proposals a bit more carefully and made some notes, I realised this wasn't really the case. I still stand by (with one exception) that all proposals would result in good science, but the more I thought about it, the more I came to the conclusion that I could make a meaningful judgement on each one. I tried not to judge too much whether one would do better science than another, because who am I to say what's better science ? Why should I determine if studies of extrasolar planets are more important than active galactic nuclei ?

These aren't real examples, but you get the idea. Actually the proposals were all aligned very much more closely with my area of expertise. The length of four pages I would say is "about right", it gave enough background for me to set each proposal in its proper context as well as going into the specific objectives.

Instead, what I tried to assess was whether each project would actually be able to accomplish the science it was trying to do. I looked at how impactful this would be only as a secondary consideration. There isn't really any right or wrong answer as to whether it's better to look at a single unique target versus a statistical study of more typical objects, but I tried to judge how much impact the observations would likely have on the field, how much legacy value they would have for the community. But first and foremost, I considered whether I was persuaded the stated science objectives could actually be carried out if the observations themselves reached their design spec.

Judgement Day

And this I found was something I could definitely judge. Two proposals to me stood out as exemplary, perfectly stating exactly what they wanted to do and why, exactly what they'd be able to achieve with this. It was very clear that they understood the scientific background as well as anyone did. I initially ranked these essentially as a coin-toss as to who got first and who got second place; I couldn't meaningfully choose between them.

At the opposite extreme were two or three which didn't convince me at all. One of the principle objectives of one of them was just not feasible with the data they were trying to obtain, and they themselves presented better data in their proposal that they already had which would have been much more suitable for this. Lacking self-consistency is a black mark in just about any school of thought. Another looked like it would observe a perfectly good set of objects, but contained so many rudimentary scientific errors that there was no way I could believe they'd do what they said they would do. 

Again, deciding which one to rank lowest was essentially random, though I confess that one of them just wound me up the wrong way more than the other.

In the middle were a very mixed bunch indeed. Some had outstanding ideas for scientific discovery but were very badly-expressed, saying the same thing over and over again to the nth degree (I would say to these people, there's no obligation to use the full four pages, and we should stipulate this in the guidance to observers and reviewers alike. I tried to ignore the poor writing style of these and rank them highly because of the science). Some oversold the importance of what they'd do, making unwarranted extrapolations from their observations to much more general conclusions. Some had a basically good sample but claimed it was something which it clearly wasn't; others clearly stated what their sample was but the objects themselves were not properly representative of what they were trying to achieve.

This middle group... honestly here, a random lottery would work well. On the other hand, there doesn't seem any obvious reason not to use human judgement here either, because for me at least this felt like a random decision anyway. And if other people's judgements are similar then clearly there are non-random effects which probably should actually be accounted for, whereas if they are truly random then the effects will average out. So there's potentially a benefit in one case and no harm in the other, and in any case there almost certainly is a large degree of randomness at work anyway.

Reviewing the reviews

I went through a similar process of revising my expectations in stage 2, though to a lesser degree. At first glance I didn't think I'd need to change my reviews or rankings, but on carefully checking one of the other reviews, I realised this was not the case. One reviewer out of the ten had managed to spot a deeply problematic technical issue in one of the proposals that I otherwise would have ranked very highly. And on checking I was forced to conclude that they were correct and had to downgrade my ranking significantly. This alone makes the process worth doing : 1 out of 10 is not high, but with ~1,600 proposals in total, this is potentially a significant number overall.

Reading the other reviews turned out to be more interesting than I expected. While some did raise exactly the same issues with some of the proposals that I had mentioned, many didn't. Some said "no weaknesses" to proposals I thought were full of holes. One even said words to the effect that "no-one should doubt the observers will do good science with this", a statement I felt presumptuous, biased, and bordering on an argument-from-authority : it's for us the reviewers to decide this independently; being told what we should think is surely missing the point. 

The reverse of this is that some proposals I though were strong others thought were weak – very weak, in some cases. Everyone picks up on different things they think are important. There was one strange tendency for reviewers to point out that the ALMA data wouldn't be of the same quality as comparison data. This is fine, except that the ALMA data would usually have been of better quality, and downgrading it to the same standard is trivial ! I sort of wished I'd edited my reviews to point this out. Some also made comments on statistics and uncertainties that I thought were so generic as to be unfair, yes of course things might be different from expectations, but that's why we need to do observations !

What the DPR doesn't really do is give any chance for discussion. You can read the other reviews but you can't interact with the other reviewers. It might have been nice to have somewhere where we could enter a "comment to other reviewers", directed to the group, or at least have some form of alert system when reviews were altered. Being able to ask the observers questions might have been nice, but I do understand the need to keep things timely as well. On that front, reviews varied considerably in length; mine were on the longer and to be honest perhaps overly-long side (I think my longest was nearly 2,000 characters), while one was consistently and ludicrously short.

All this has given me very mixed feelings about my own proposal. On the one hand, I don't think it's anywhere near the worst, and I stand behind the scientific objectives. On the other, I think I concentrated overmuch on the science and not enough on the observational details. Ranking it myself with hindsight I'd probably have to put it in the lower third. It was always a long shot though, so I'll be neither surprised nor disappointed by the presumed rejection. One can but try with these things.

One thing I will applaud very strongly is the instruction to write both strengths and weaknesses of each proposal. All of them, bar none, had some really good points, but it was helpful to remind myself of this and not get carried away when reviewing the ones I didn't much like. Weaknesses were more of a mixed bag; one can always find something to criticise, although in some cases they aren't significant. Still, I found it very helpful to remember that this wasn't an exercise in pure fault-finding.

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How in the world one judges which projects to actually undertake, though... that seems to me like the ultimate test of philosophy of science. Groups of experts of various levels have pronounced disagreements about factual statements; some notice entirely different things from others. There's the issue of not only will the science be significant, but also whether the data can be used in different ways from what's suggested. That to me remains the fundamental problem with the whole system, that one can nearly always expect some interesting results, but predicting what they could be is a fool's game.

Overall, I've found this a positive experience. Reading the full gamut of excellent to poor proposals really gives a clearer idea of what reviewers are looking for, something it's just not possible to get without direct experience. Not for the first time, I wonder a lot about Aumann's Agreement Theorem. If we the reviewers are rational, we ought to be persuaded by each other's arguments. But are we ? This at least could be assessed objectively, with detailed statistics possible on how many reviewers change their ranking when reading other reviews. 

And at the back of my mind is a constant paradoxical tension : a strong feeling that I'm right and others are wrong, coupled with the knowledge that other people are thinking the same thing about me. How do we reconcile this ? For my part, I simply can't. I formulate my judgement and let everyone else to the same, and hope to goodness the whole thing averages out to something that's approximately correct. The paradox is that this in no way makes me feel any the less convinced of my own judgments, even knowing that some fraction of them simply must be wrong.

Other aspects are much more tricky. This is a convergence of different efforts, both trying to asses what-is-true (what science claimed is factually correct, why do experienced experts still disagree on some points), what will likely benefit the community the most, and how we try and account for the inevitably uncertain and unpredictable findings. As I've said before many times, real, coal-face research is extremely messy. If it isn't already, then I would hope that telescope proposals ought to be an incredibly active field of research for philosophers of science.

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.

Coming soon : FRELLED version 5

I had two lockdown projects. One was to develop an interactive model of Arecibo, which I more-or-less have working but just have to find the time and inclination to get into a useable format (which is tedious and boring). The second was to recode FRELLED, my Python script that imports 3D FITS files into Blender. This was originally written for Blender 2.49, released back in 2009 (!), but Blender 2.5 has a completely different Python syntax - more like using another language than making minor modifications. So it took a global pandemic to force me to re-write the bloody thing in a modern version of Blender.

After several months, I'm pleased to announce that this is done. Well, sort of.

The new version uses Blender 2.79. This isn't the very latest version, but for a very good reason. Blender 2.8+ doesn't support the OpenGL realtime shaders that FRELLED relies on, and unfortunately neither Cycles nor Eevee are suitable replacements. Apparently it will get a modern OpenGL equivalent at some point though, and the Python syntax is almost identical to that used in 2.79. This means the next update won't be anything like burdensome as recoding the entire thing again.

FRELLED version 5 looks like this :

This is the main display section with an example cube loaded. Blender's GUI now enables adjustable panels, so the user won't be overwhelmed with information. Presets are now such that loading a cube should be a matter of about five mouse clicks. This, I hope, will be easy enough to persuade people that it's worth installing and using.

Incidentally, installation should now be MUCH simpler. Blender 2.79 comes with its own internal Python and PIP kept completely separate from system Python (you can download it in a zip file, no other installation needed). So installing the modules FRELLED needs is now trivial... at least it was for me on Windows. It should even work on Linux networks.

Loading cubes is now much faster, hence there's generally not so much need to worry about which projections are being imported, so this is all hidden by default. But all that is still there for enthusiasts and those using very large or weird data sets. In particular, the "sparse sampling" option now lets you import only every nth slice of the data, adaptive to the size of the cube in different directions, so in principle even arbitrarily large data sets should be no problem. Contrary to expectations, loading in less of the data often makes the appearance better rather than worse.

Not every feature in the GUI is currently functional - the major one being multi-component rendering, but also the quick import and preview buttons. Both of these are actually fairly simple -  they just requires me to work out the most efficient way to do it (for multi-compment/volume rendering, the GUI buttons will greatly simplify what used to be a rather ugly, hacky process that worked but was unpleasant to use). 2D mode, though, is fully functional.

The Analysis menu does look a bit scary, but you can hide any panels you're not using and most of them should be simple enough. A big advance is that you can change the spectral axis units and it's no longer hardcoded to assume the data is HI, so the velocity of any molecular line just needs the rest frequency (a drop-down menu provides a few preset values and also access to Spatalogue). It's also possible to hide the axes with a single button, which used to be a much sillier process.

Region analysis tools remain much the same as in the original FRELLED but with improvements. Contours are now much faster and true isosurfaces are supported (and are fast enough that you could even show these for an entire cube, as an alternative to volume renders). You can also show velocity maps as well as much nicer-looking flux maps that use the requested colour scheme rather than only greyscale. There's a simple toggle for using a geometrical progression for contours (or logarithmic display for maps), with built-in safeguards to stop the user trying to display unfeasibly large and complex contours. And SDSS maps are now opaque, making them very much easier to see.

Isosurfaces are functional, though currently with only limited display capabilities. The mbspect section also has limited (but significantly improved) capabilities : it can only produce the input files and not run interactively, but it does allow all the options to be set directly in the GUI. The interactive version will be restored once I have access to a Linux system to test it on. Finally, users can also set some options for NED queries instead of just having it return absolutely everything.

I've tried as much as possible to test everything and test again. But as you can imagine, it's just not possible to test everything to destruction. So before making an official release, it's time for some beta testing. Volunteers are welcome ! Preferably those who aren't scared of working with FITS files. I can provide instructions and example cubes to try, but I'm especially keen to see what happens with data sets I've never tried, and with using it in anger : doing things in odd sequences and using features in unexpected ways. So if anyone out there wants to help, do get in touch. Leave a comment on social media or this blog, or contact me directly at feedback @ rhysy . net, and I'll add you to a beta-testing email list for next week.

Living in a virtual world (but I am not a virtual girl)

Last week I did exactly two things : I attended my first online conference, the EAS 2020 "in Leiden", and I spent every spare minute playing with my long-awaited Oculus Quest. More on that elsewhere. Here, let me say something about the wonders of attending a conference from one's own home.

Back in my day, online talks were a thing to quaken the hearts of the bravest of men. They were sorry and desperate affairs that were as much use as listening to the London Underground tannoy for a solid hour : totally inaudible and we'd all have been better off spending the time silently contemplating the Oneness Of All Things. Not so in the modern era, where the pandemic has made a necessity of achieving something which was already well within technological capacity.

There are, of course, both advantages and disadvantages to online conferences. On the positive sides, there's no need to travel, talks are stored online (sensibly only for a month, meaning I might actually muster the energy to look at talks I missed : if they were there indefinitely then I never would), and it's far easier to drop in and out of different sessions. On the downsides, there's no opportunity to travel, you have no sense of the audience response, there's no social aspect (they did try, but after spending so much time listening to a screen, I found it necessary to spend the break times not staring at a screen, at least not one filled with science), and it seemed to me that people were lessing willing to raise controversial topics.

Overall, the positives have the advantage. In the future I think it would be extremely strange for any conference not to move to at least a hybrid system - the convenience is too great. At the same time, it would be a loss if physical attendance became unusual - the social aspect of presence is important (and one of the perks you get for accepting an astronomer's meagre salary is an astronomer's not-so-meagre travel privileges, but I'd be happy if they converted this to salary instead !). Through body language and the more free-flowing discussion that happens in tea time, it's easier to say, "I disagree" in person without sounding like a jerk. Which is a bit strange, because plenty of people still manage to say, "I disagree" while sounding exactly like a jerk, and it ought to be easier to avoid this in an online system.


As for this specific conference, everything went almost without a hitch. Things got off to a rocky start though, when the first speaker in the first talk I went to turned out to be actually painfully dull to listen to. If he'd been on Just A Minute he'd have been out in seconds. He talked so incredibly slowly that you'd forget the start of the sentence by the time he finished, thus meaning he conveyed no information whatsoever - no, really, absolutely nothing - besides what a poor choice someone made in inviting him to give a presentation.

Although almost entirely humour-free, there were however a couple of amusing points. One speaker stood up, earning praise from the chair for making it more lively, so the next speaker said they weren't going to stand up on the grounds they were still in pyjamas. Then there was a faux pas a chair was clearly unaware of, saying, "unfortunately we have to move on to the next speaker", which I thought sounded pretty bad for the next speaker !

This raises my only serious niggle : the tendency for several people to have multiple talks in different sessions. I do find this really unfair and annoying. By all means, give as many posters as you like, but if you get to speak for 45 minutes because your research is famous and I only get a 1.5 minute poster presentation, that needlessly exacerbates inequality. There needs to be more coordination between sessions to ensure that no speaker gives the same talk twice, and has a maximum of two talks. Otherwise, lesser-known researchers get hidden in the virtual-but-not-entirely-metaphorical poster basement.

I don't know if anyone else did this or it was just me, but I didn't feel particularly inclined to check out any other posters. In a real conference, you can combine wandering around the poster room with a nice cuppa, allowing you to at least partly switch off during the tea breaks. For me, not listening to science for 30 minutes and being able to browse the pretty pictures is important to maintain sanity. I might eventually get around to it, but I felt no pressing need to do so here.

But all of these are quibbles. Overall, it was a great conference with some really interesting talks and truly exceptionally high-quality timekeeping, even if it didn't have enough jokes. So, on to the science !


It's not bug, it's a feature

First off were an interesting pair of talks. Frederico Lelli - he of the MDAR - claimed that the baryonic Tully-Fisher relation is consistent and has low scatter across a wide range of masses, whereas Pavel Pina - he of the UDGs - claimed that there's good evidence that some galaxies don't obey the BTFR at all. Or to put it another way, either all galaxies have similar dynamics that can be predicted entirely from their baryonic matter (which would be weird if they're all dominated by dark matter) or only some can. It's all very confusing and definitely not settled.

Although I'm firmly in the "dark matter is definitely a thing" camp, both had some interesting points. Lelli notes (of course) that there's a break in the stellas mass TFR, but that goes away if you add in the gas. He also shows that the tight relation is only seen with the highest quality data, and you have to have rotation curves which extend sufficiently far as to reach the flat bit. I'm in two minds about that. On the one hand, I can see why you'd do it, but on the other, how do we know the flat bit indicates stability or that the baryons are sufficiently extended ? Has anyone looked for baryon configurations that could give stable results without flat rotation curves ? Otherwise selecting only the flat curves is a potential bias that means you'll never find any deviants. Maybe.

Pina's Ultra Diffuse Galaxies show strong, weird deviations from the baryonic TFR and now he adds a few more that are intermediate. I wasn't sure if the data was really good enough to give the accurate velocity widths needed, and as I've noted here before, minor inclination angle errors can give substantially wrong velocities. I'm still not entirely convinced, but he has several very strong points in his favour. First, the rotation curve fitting software was shown in a later talk (I forget by who) that it does extremely well with low resolution data - much better than I would have expected. Second it's unlikely that all the galaxies in the sample have huge inclination errors, and third, the low velocity dispersion is inconsistent with a thick disc needed for inclinations that would bring them back into agreement with the BTFR. So these objects are, at the very least, a challenge to the idea that the BTFR is flawless, but more data could eventually settle the issue.

If these UDGs really do deviate, then something odd and potentially very interesting is going on. Lower surface brightness galaxies should deviate, but generally - as shown very nicely in Lelli's talk  - don't. So what's different about these guys ? What makes them so special ? I for one have no idea.


Magnetic Blobby Things

Dylan Nelson gave an excellent talk about the formation of optically dark gas clouds, an obvious point of interest for me. His simulations of galaxy clusters find free-floating gas clouds of similar size and temperature to the dark HI clouds in Virgo, which live for at least 1 Gyr. Instead of being supported by thermal or dynamic pressure, which we already know doesn't work, they're supported by magnetic fields. What keeps them from evaporating is the temperature gradient, with an intermediate temperature zone allowing gas to flow into the clouds.

This is super interesting to me as it would provide a potential explanation for how the Virgo clouds survive in significant numbers whilst not (yet) being found outside the cluster. What would be really interesting is to know their mass and velocity widths. Unfortunately I had minor technical issues so I couldn't ask questions, but this is definitely one I'll be following up on.


Magneticum

There is a simulation code called "magneticum". That's hilarious, but I didn't have the heart to tell them why.


The Stars Are Not 2D

Cecilia Bacchini gave a really nice look at volumetric star formation laws. Normally, for simplicity, we look at the 2D density of the gas and see how it compares to star formation, but Bacchini shows how it's possible to get a reasonable estimate of the true volumetric density without too much bother. Unlike the classical star formation law, the volumetric version has no break at the low end and a uniform tighter scatter. Their efforts also show how the classical law can be rederived form the VSFL.

A second interesting point - raised elsewhere by Luca Cortese, Amelie Saintonge and others in other talks - was that star formation is probably not governed by molecular gas alone, contrary to a great many recent claims. The good old-fashioned atomic gas likely plays some role as well - it's not just a reservoir from which molecular gas eventually accumulates. Bacchini shows that the scatter if the VSF law is actually lower if you use only HI instead of H2, which I would not have expected. A paper is in preparation.


"The AGN is strongly turned on here"

Why ? Did it meet someone nice ?


Women in astronomy

At long last I finally got to here from the legendary, Jocelyn Bell Burnell, discoverer of pulsars and slayer of sexism. Her talk was a look at the changing IAU membership by gender since they started maintaining a well-organised database 20 years ago. Back in 1990, the then-director said that this was a social issue which they weren't going to tackle, which sounds a lot like total bullshit to me : there's nothing political about trying to ensure fair representation in your organisation.

Currently the IAU has 14,000 members, of which just four were unwilling to specify gender. I have to say I was surprised to learn that the gender balance is strongly unequal, with an average of just 19% female in countries of more than 200 members. The highest is Italy at 28%, whereas the UK has a mere 13% (!) and Japan the lowest on 7%. This doesn't reflect my experience as a UK undergraduate at all, where the gender balance was close to equal - if I recall correctly, the problem is retaining female astronomers as they climb the career ladder, not so much in hiring them.

The Netherlands is exactly average, but has increased significantly in the last few years. Here was something I asked a question on, but it didn't get answered due to a deluge of other questions : what's the fastest rate of change we can realistically expect ? I agree that a change of 0.5% per year doesn't sound great, but potentially it might be. Say a country has 500 astronomers but only 10 new members per year (balanced by deaths), then if the gender balance is equal, that still leads to a very low percentage change. Obviously we can't start firing existing astronomers, nor should we deliberately hire more women to make up the existing deficit, so I would say we should look more at the changes in new members rather than the whole. But of course, looking at the point at which people renounce IAU membership is also crucial.


Best of the rest

Those were the personal stand-out talks for me, but there were plenty of other interesting talks and very few duds. Several people noted the important of including the Large Magellanic Cloud in simulations of the Milky Way formation as this will affect cosmological issues like the missing satellite problem, including the very intriguing possibility that satellite galaxies can themselves have satellites - apparently there a few good candidates for such "yo dawg" objects. There were more claims for satellite planes, which I still find unconvincing but at least having samples to test is important.

Federico Lellli gave a second very nice talk about a new way to estimate the true halo masses of galaxies, which unfortunately I took crappy notes for but the result was extremely surprising : apparently there's no missing dwarf problem but a missing giant problem. There's a big extrapolation between the measured rotation and the estimated halo mass though, but I think I really need the paper to make a sensible comment. It looked to me like this implied that previous measurements must have got the halo masses wrong somehow, but apparently this isn't the case. There were also some cool remarks about galaxies at high redshift with well-resolved rotation, for which there should be some exciting papers in the near future. And there was a nice talk demonstrating that the MeerKAT telescope is doing fun stuff by observing HI in nearby clusters.

This conference also established the Strong and Weak Frenk Principles. The Strong version says that the missing satellite problem doesn't exist, the Weak version that it's been solved. I, and probably the majority of others, don't think that either situation is the case : the missing satellite problem is indeed a problem* and hasn't been solved yet. Springel noted that the number of free parameters in the modern simulations isn't as high as is often claimed, but that needs more detail. And finally, Malhan showed how different types of progenitors could lead to distinct differences in tidal streams, potentially opening a new avenues on the core-cusp problem and the nature of dark matter.

* Can I propose the Strong and Weak Kroupa Principles ? The Strong version would say it disproves all of cosmology; the Weak version only that it poses difficulties for dark matter.


All in all, a great conference. Not quite as draining as a regular conference, and lots of double-edged swords at work : every advantage came with a disadvantage. But most importantly, in these trying times, science marches on.

Goldilocks And The Three Ghosts

On the 23rd December 2019 I received a particularly nice early Christmas present : confirmation that my paper had, at long last, been accepted. We submitted it on 18th October 2018 and it went through three(!) reviewers before it was finally accepted, so this was much, much more of a saga than I was expecting. Whether it's more like Goldilocks And The Three Reviewers, or of a visitation of the three ghosts of Christmas, I'm not sure. It's probably best told as a mash-up of the two.

Challenge accepted. Here goes.


Once upon a time, there was a nice little blonde astronomer named Goldilocks who noticed some interesting things in his data that he hadn't noticed before. He decided to write a paper about it.

"Ho hum !", he said to himself. "This should be an easy little write-up, and then I can do something else."

But then he found that there was a weird pattern in his data that didn't make much sense. He decided to run a public poll to make sure he hadn't gone mad. He got about a hundred responses on the so-called "ghost town" that was Google Plus, and everyone agreed that the findings were correct. So he thought about it very carefully and realised that it was probably just an interesting but not terribly unlikely coincidence. He wrote up his paper and sent it to his co-authors for comments. As no-one had much to add, he submitted it to a journal and slept soundly, thinking it should not be a terribly controversial discovery.

That night he awoke with a start. A chill air filled the room and there was a most terrible wailing. Suddenly a ghastly phantom rose from the floorboards and cried,
"Rhyyyyy.... I mean, Gooolldddiiiiiloooooocks ! You shall be visited by three ghostly reviewers to inspect your paaaapppeeerrrr !"
"But why ?", cried Goldilocks. "It's nothing special. I mean, it's nice enough, but it's just some galaxies with stripped tails of gas that are pretty much exactly what we expected to find. Come on, galaxies in the Virgo cluster, losing gas exactly as predicted ? Fun, but hardly a revelation. The only real oddity is the way the streams are pointing, but we've explained that well enough. Why should it need three reviewers ?"
"I doooon't knoooooooow !" wailed the phantom. "They just woooooon't belieeeeeeeve yoooooou ! Expect the first ghost quite soooooon !".

A few weeks later Goldilocks again awoke to the clanking of chains and a mysterious wailing. Creeping downstairs, he found a middle-aged man lounging on his sofa and throwing popcorn at his TV in a carefree fashion.

"Yo," said the man, "Someone's been sleeping in MY bed... ! I'm the Daddy Ghost of Utter Pointlessness. Here's yer report." And with that he vanished.

Goldilocks picked up the report and read it carefully. It was quite long, but it didn't seem too bad at first. It was pretty darn clear that the Ghost really just wanted extra citations to their own papers and didn't understand some very basic concepts from radio astronomy. That was a bit worrying, but easy enough to address. It also came with the dreaded task of "shorten the text", which was, as usual, quite meaningless as it came with no further instructions. "Too many notes", tutted Goldilocks to himself. "But everyone says that, so it's not much to worry about".

More reassuringly, the Ghost didn't ask for anything drastic or express any major scientific skepticism, and the requests to make things "more convincing" seemed quite reasonable : clarifying the improved sensitivity from the new analysis, more labels on figures, that kind of thing. It made sense that one might be a bit skeptical about detecting this many new gas streams, even knowing that the cluster was exactly the environment where one should expect such features, since they were somewhat on the faint side.

Goldilocks did as the seemingly sensible (if rather ignorant and uninformed), Ghost suggested and duly returned the paper. Before long, the Ghost came back with a new report and vanished once more, feeling even less inclined to discourse than the last time.

Goldilocks eagerly read the report and immediately fell into despondency. This wasn't so much pointless as it was downright rude. He'd carefully addressed all the points from the first report and explained things at length in the accompanying letter. Yet the Ghost's report was barely a single paragraph and, worst of all, insisted that Goldilocks hadn't done what was asked. Even on those points that were really simple, like asking for a number which was now very clearly highlighted in bold. And the Ghost had asked for more explanation on the improved sensitivity, while Goldilocks had explained several times that sensitivity wasn't the issue, it was about visualisation. Of course he'd explained the procedure in more detail as well, just to make sure, but the Ghost either just didn't get it or was being deliberately obtuse.

"What on Earth am I to do," said Goldilocks to the co-authors, "when someone asks me what the number is, I tell them 'it's six, six is the number, and the number shall be six' and they insist that I haven't told them what the number is ?"

Goldilocks was both cross and confused. The Ghost's response was hopelessly inconsistent. Whereas before the Ghost seemed a bit concerned if the admittedly quite faint tails were real, now they were wondering if they could have been produced by something other than ram pressure stripping. That was something they easily could have pointed out at the first stage, and adding it now really felt like being strung along. And the Ghost made a bizarre claim that one source, already firmly established in the literature through several other independent observations, was only "probably" real. This was a bit like saying it bricks would only "probably" hurt if you dropped them on your toe. It was pointless.

Goldilocks couldn't see the point of answering a referee report knowing that they might just ignore everything and shift the goalposts again. Especially since they insisted the paper was now longer when it was objectively shorter.

"FFS", said Goldilocks to himself.

After consulting the co-authors, Goldilocks decided to ask the editor what to do. He was a bit disappointed that the editor hadn't already intervened, because the problems with the Ghost's response weren't subtle. They were, in fact, glaringly obvious, and he'd seen editors intervene by themselves in the past with things less blatant than this. Goldilocks complained that addressing this new response wouldn't work, since the referee was so inconsistent and asked for things which were already done and stated very clearly indeed in the main text. Trying to address things raised by someone who would simply ignore you no matter how clearly you stated things was indeed Utterly Pointless.

The editor thought for a while and declared, "Hum ! So, this porridge is a bit hot, is it ? We'll see if we can find some that's a bit cooler". And with that Goldilocks waited for a brand new Ghost.

Some time later Goldilocks again awoke to hear a low moaning. This time there was a slightly older matronly figure sitting in a more dignified position and wearing a monocle. "WoooOOooo !", said she. "Behold, I am the Mummy Ghost of Undue Skepticism. Read my report, mortal, if you dare !". She shook her fist in a dramatic fashion and disappeared.

Goldilocks read the report with some trepidation, but was soon confident he knew what to do. The report wasn't without problems. This Ghost was asking for a figure to be both improved and removed, which was very confusing. However, they were very explicit about their main concerns, which made them a lot easier to address. First, they were worried that some of the streams might not be due to ram pressure stripping. Goldilocks was fine with that, he'd never thought that the situation would be otherwise. Making this clearer was no problem. Second, the Ghost wasn't sure all the streams were even real. That wasn't too big of a deal either, as it was quite straightforward to give their statistical significance and predict how many false streams should be expected in a data set this large (the answer, it turned out, was a healthy zero).

The other referees' comments being minor, Goldilocks soon found a way to measure the statistical significance objectively and clarify that the streams might have multiple formation mechanisms. He didn't really understand why anyone would be hung up on these points though, as it was hardly a breakthrough discovery and plenty of other much stupider papers were floating around in the literature. Surely, he thought, the results are at least solid enough that the rest of the community deserved a look at them. "And anyway," he said to himself, "it's well-known that if you provide enough details to recreate your results, which I bugger well have, it isn't necessary that the referee actually has to agree with your conclusions. They shouldn't reject it unless they can actually find a flaw in the analysis, or better yet they should correct it."

So Goldilocks sent off the report feeling cautiously optimistic that this time he'd succeed. The referee certainly seemed more familiar with radio astronomy, which seemed like a good sign.

Alas ! Some considerable time later, the Ghost re-appeared. "Woe !" she cried with a banshee wail, "I remain unduly skeptical ! This porridge is too cold. Thou hast not addressed my concerns, and I reject thine paper ! May it be cast into the pits of hell !"

Goldilocks was astonished and dismayed. He read the report with contempt. The Ghost had blathered about a few points that made little or no sense, but worst of all he hadn't responded to the correction on the main point - at least, not sensibly. Instead of addressing the whole new section dedicated to assessing statistical significance, which was objective and quantitative, she'd simply said she "understood" it, but thought that "the evidence should be in the images".

This didn't sit well with Goldilocks at all. "Fair enough," he thought to himself, "an objective analysis can absolutely be wrong if the wrong procedure is used or whatnot. But surely in this case someone needs to tell me what the blazes actually is wrong with it, rather than just saying they understand it. If they really understand it, they bloomin' well ought to be able to explain why it's wrong." And he was also more than a little annoyed that they wanted "evidence in the images". All this amounted to the Ghost wanting subjective proof in place of an objective one, without saying what was wrong with the method. Goldilocks was Not Happy.

(He thought about complaining to the editor but decided it would do little good. He also noted that the Ghost claimed to remain skeptical of the "majority" of the streams, but when you added up the number of individual streams they said they were happy with, found that they came to 60% of the total.)

What to do ? Goldilocks was not as despondent as you might think. He'd been working on the analysis for well over a year already, and every time someone had come up with a reason to doubt their existence, the tests had only strengthened the case for the streams. True, two ghastly shades hadn't been convinced, but both appeared to be quite bizarre. The other co-authors were all happy with the result, all of whom were more senior and more experienced than him.

"Right," said Goldilocks. "None of the objections raised by the referees make any sense. Therefore, strange as it is, the only reasonable conclusion is that I'm right and they're wrong. I'm not going to dump more than a year of work on the scrapheap because some spectral nit doesn't understand it. I'm going to submit it to a whole new journal."

Goldilocks did, however, accept that images can often be more persuasive than numbers. So he did a whole new analysis in which he injected fake sources into real data, not only measuring the very few false positives that appeared but also making the same contour plots of them as were presented for the real streams. It was pretty effin' clear that you just didn't get false positives that looked anything like the real streams, exactly as the earlier analysis had shown. And so Goldilocks submitted the paper and once again waited.

And waited.

And waited some more.

Well, actually not really, because this time the Ghost was very prompt.

"Ahhhwoooooo !" cried the spectre. "Behold, I am the Baby Ghost of Precise Instruction ! Read my report with the utmost care and all will be well !"

"Oh spirit," said Goldilocks, "I fear you more than any ghost I have yet witnessed. Can it really be true that you are indeed the Ghost of Precise Instruction ?"

But the miniscule phantom only pointed a spectral finger at the report and disappeared.

Now you must understand that Goldilocks was in a pretty strange mental state but this point. He'd been haunted by three strange spectres all questioning his spectra, and was both quite cross and trepidatious. He didn't doubt himself, but he was highly suspicious that the Ghost would actually do their dang job properly. He read the report quite nervously, and decided to avoid reaching any conclusions, knowing that you can't judge someone until you see how they respond a second time.

Still, it looked promising. There were no clear indications that this doubt thought the porridge was too hot or too cold, only that the height of the chair was a bit off and the window needed oiling. That is, the Ghost didn't seem concerned about whether the streams were real, only that the paper was too long and didn't have a good comparison sample.

"Well, fair enough really," thought Goldilocks. "A comparison sample is a great idea, but unfortunately just not practical. Hopefully the ghost will understand this if I explain in sufficient detail."

The Ghost had, however, provided very Precise Instruction indeed when it came to shortening the manuscript. Goldilocks didn't particularly want to do this, but instructions this clear were difficult to get wrong. Best of all, by the very simple direction to "concentrate on the new results", this made it trivial to extrapolate as to which other parts could be cut. Those few words transformed a task ordinarily fraught with problems into the work of a couple of a days. Soon the paper was five pages shorter and, Goldilocks had to admit, considerably more focused.

"I'm still in two minds about it," said he, "but overall this is probably better. I liked the more detailed original version, but more people are likely to actually read this shorter document."

So Goldilocks made the remaining changes and carefully explained why they couldn't provide a comparison sample, substituting this for a literature search of similar features instead. He poked and prodded his co-authors until they finally gave the go-ahead, and then he submitted the revised paper. And very soon the Ghost returned and said "this porridge is just right !", all was well, and there was dancing in the streets.

"Hooray !" said Goldilocks. "But we've learned some valuable lessons here. First, the rules of refereeing ought to be clearly spelled out and not just left to the referee to make them up however they see fit. You can't just go around saying, 'I don't agree' without providing any justification. Second, editors ought to actively check if both sides follow the rules, and not just act as postmen. Pretty much a year of valuable research time has been wasted dealing with this crap and that needn't have happened. I'm going home."

And with that he stomped off and had a lovely Christmas. The end.

IAU Symposium 355 : The Realm of the Low Surface Brightness Universe

I've decided to do everyone a favour and not combine the science and pretty pictures of landscapes from the latest conference. You can read about the travel experience of Tenerife here. Although I've already mentioned some of the outcomes, I thought I'd also give a more executive summary of the stuff I found most interesting.

The theme of this conference was the low surface brightness universe - the faint fuzzy stuff. It covered the whole range of such features, focusing on galaxies but also including zodiacal light, the challenges of observing (including a very nice talk from an "amateur") and data processing, philosophy of science issues and reproducibility (it's tricky to get really deep images, and sometimes they give remarkably different results), and was just generally awesome. So here are my highlights.


Mike Disney's introductory talk 

Mike is essentially the conference godfather, who predicted the existence of large numbers of low surface brightness galaxies way back when. He began by noting just how vicious the field can be - and he's right. The conference in Cardiff back in 2007 was the first I ever went to, and by far the most bitter and acrimonious. I'm not sure why it went down that way, but this one didn't. People said plenty of controversial things (especially Mike !) without ever seeming like they were about to start beating each other up.

Mike's major points were that there should be a large population of hitherto unseen faint galaxies, though he tried earnestly to present arguments both in favour and against this. While everyone agrees that galaxies certainly get a lot more interesting the deeper you look (in particular elliptical galaxies tend to look much more interesting), it's more controversial as to how many brand new galaxies this will pick up (more on that later). I've covered some of these arguments before, but I'll be revising some of them thanks to this conference.

One of the issues I don't think people quite got nailed down was the significance of the low surface brightness galaxies. Many good arguments were presented that such hidden galaxies cannot contribute much to the total amount of starlight. This is probably true, but misses the point that such galaxies could still be dynamically massive and dark matter dominated. So perhaps both sides are right, depending on whether one thinks of stars or dark matter as being more important.

Mike's other controversial point didn't get so much attention, probably because the conference wasn't really focused on it : the galaxies seen in the spectacular Hubble Deep Field, he says, cannot possibly be the progenitors of today's galaxies because they're too bright. I'm skeptical, but there wasn't time to go into this much.


Johan Knapen's data talk

This was essentially a philosophy of science talk from the perspective of the sheer data size that's coming our way very soon. This is a very real and serious challenge, with the SKA expected to produce exobytes of data per second. While Mike raised the point about defining the scientific method being very difficult, Johan took this a but further. He listed four possible paradigms of science that have changed over time :

1. Experiment-driven, as in the days of Newton and Gallileo
2. Theory-driven, as in Einstein and other analytic theoreticians
3. Numerical simulations
4. Data exploration

Other suggested that the fifth paradigm could be A.I. while the sixth would be letting Facebook do everything.

Johan's point (if I remember correctly) was that astronomy was going to have to move away from the traditional hypothesis-testing method we learn in schools and towards this "fourth paradigm" of being a data-driven approach. He's not wrong, but those who I talked to seemed to agree that this already the case - and maybe always has been. I've made the point before at length : there's more than one right way of doing science, and the data usually tell you something very interesting but utterly unrelated to what you were interested in. I mean this very literally. And I believe it was Simon Driver who, in discussions afterwards, said that in experiment proposals it should be absolutely legitimate to describe which area of parameter space you wanted to explore and why it was new, without needing to say exactly what you expect to find there. With this I fully agree. It's largely a waste of time describing observational results before you've got them.

Mohammad Akhlaghi's reproducibility talk

Mohammad doesn't like the fact that papers often contain highly vague instructions as to how experiments were carried out and how data was analysed. He's trying to tackle the latter issue by developing a package that makes it very easy to document the full details of the software used, by including the exact software name, version, and all its associated dependencies (libraries etc.), in a way that makes it simple to include in a paper. It'll also let you automatically update any numbers if you change the software without having to redo the calculations or edit the paper yourself, and it doesn't require any special software modules to install : the point is that the user has full control over what packages they use.

I was a bit skeptical listening to this, being acutely aware that many aspects are entirely subjective, but I came around to it afterwards. You'll have to take care that if you make changes, your new numbers are still consistent with your original conclusions. And I'm a bit doubtful that there are that many cases where changing software actually changes the answer. But the basic idea that papers should be as reproducible as possible is something I can definitely get behind - provided we remember that objective, repeatable measurements can still be absolutely wrong. Reproducibility means you can find the errors, not say, "my method is objective and therefore objectively correct", which is an easy trap to fall in to.

Thomas Sedgwick's hunt for dark galaxies with supernovae

This was one of the most interesting and novel methods proposed for finding very faint galaxies. As long as the galaxy is forming some stars, it will have a few supernovae, and these can be detected. This is not something I would ever have thought possible - even though supernovae surveys are now decades old, I just don't think of stochastically exploding stars as something you can do a survey of. But you can. And you can even work out how many galaxies your survey implies, given some very reasonable, justified assumptions about the survey completeness and star formation rate.

Interestingly, it turns out that these corrections imply a galaxy distribution that's fully compatible with the standard model : that there are indeed large numbers of very faint galaxies out there, just as models have predicted but observations failed to find. This is a really cool result, but it relies on an enormously large statistical extrapolation, so it's probably safe to assume the problem isn't solved just yet (and kudos to the speaker for saying as much).

A somewhat similar talk was given by Raja Guhathakurta on looking for faint galaxies by searching for their globular clusters. The difference here is it should be possible to get much better completeness of the sample. We're planning to do a search for such features for the Virgo clouds I've been working on.

Nushkia Chamba's new definition of the size of galaxies

Nushkia knew me as "that guy with the hilarious blog", which absolutely made my day. She's come up with a new parameter for the size of galaxies, which is extremely interesting. If correct, it'll dramatically change how we think about ultra diffuse galaxies. But she asked that this not go on twitter, and I assume that includes other social media so I'll say no more about it. Expect to hear a lot more when it's published.

Daniel Prole's talk on the abundance of UDGs in the field

Are ultra diffuse galaxies a very common galaxy component that until recently went largely undetected, or are they just a smattering of exotic objects ? Several people made the point that UDGs were already know but it's their abundance in new surveys that's got people excited about them again. Daniel (who has the same PhD supervisor as I did) is attempting to estimate how abundant they are in the field, which is much harder than in clusters and has a far large volume. He's got a number, but rather surprisingly hasn't compared this to other numbers so it doesn't mean much yet. I would have thought there are already numbers for more typical galaxies, but perhaps getting a fair comparison (e.g. correcting for survey biases) is harder than you might think.

Pavel Mancera-Piña's talk on the dynamics of UDGs

I was so glad to see this talk. You may remember that I've commented several times on the weird line widths of UDGs with HI detections, which tend to be much less than expected (scroll to "things are getting weird" in that link for a plot). I emailed a couple of people about it - I got a cautious response from one and nothing from another. I've shown several people and they all think it's interesting, but I never have time to work on this myself. Thankfully Pavel does, and he's done a much better job than me of demonstrating that this weird result probably isn't due to observational constraints : these galaxies do seem to be weird. While it looks unlikely that certain famous candidates are not actually galaxies without dark matter, some of these UDGs might soon resurrect this possibility.

My one major concern, which I think should be relatively easy to address, is survey incompleteness. At any given mass, galaxies of low line width are easier to detect. If galaxies are rotating discs, it so happens that means we'll preferentially detect ones which are close to face-on from our perspective, which makes it hard to estimate their true line width (details here). So it might be that the survey is biased towards nearly face-on galaxies - and because they're so damn faint, it's hard to measure their inclination angle directly. In principle one could test this by calculating the line width they'd need to escape detection and the corresponding inclination angle required to reach this. However, I very much doubt this will explain all the objects. Many of those galaxies with reasonably clear detections look quite convincingly close to edge-on, implying a negligible velocity correction.

Freeke van de Vort's talk on simulations the circumgalactic medium

Freeke has done some spectacular simulations of the gas structures around galaxies. Normally I think of this as probably very diffuse, fluffy stuff, interesting but not particularly photogenic. But Freeke's simulations really are spectacular - they look like the sort of thing you'd get if you told a Marvel CGI artist to "make some pretty gas - really go nuts with this". She notes that the simulations aren't yet converged. While they get the major galactic structures right, increasing the resolution keeps changing the results for the CGM. She's also found some gas clouds without stars or dark matter, but of course the resolution dependence makes the significance of this hard to assess.

Honourable mentions

There are too many to mention properly but I can't avoid a few others :

• Eva Grebel made the point that there are a few red, isolated UDGs known in the field, while several people (but particularly Anna Ferré-Mateu) noted that at least some UDGs may indeed be giants even if they aren't the majority. Most people seemed happy with the notion that there may be several different ways to make a UDG. 
• The Dragonfly team defended their data reduction procedures in the face of their failure to detect the double arms of NGC 5907, although no-one seems to know what happened. Another talk showed us that the double arms had been detected independently so they're almost certainly real.
• Gaspar Galaz said that there's a large linear stream extending from Malin 1, which is just weird. How you get a linear stream intersecting a stellar disc, I just don't know.
• Everyone agreed that galaxies look much nicer with deeper imaging but that it's jolly hard to do. 
• Bärbel Koribalski gave a very nice overview of extended optically dark gas features. Very nice to know that it's not just me working on this !
• Sarah Pearson showed how we'll soon have detections of large numbers of globular cluster streams around galaxies. I don't think of streams in statistical terms, but this could be an interesting way to constrain the behaviour of galaxies and their dark matter content.
• Anna Saburova, a collaborator of mine, sounded like she was about to kill everyone (it's the Russian accent) but described how difficult it is to explain giant low surface brightness discs. They most likely have different formation mechanisms - some by catastrophic collisions, others through slow accretion.
• Sebastiano Cantalupo explained dark galaxies at high redshift, which I was surprised to hear may be not all that dissimilar to the candidates at low redshift, with similar masses and dynamics. Definitely one I need to read up on more as I'd assumed the high redshift objects would be very different.

Which just about wraps it up. Plenty of background reading to do until the conference proceedings are released.