Sister blog of Physicists of the Caribbean. Shorter, more focused posts specialising in astronomy and data visualisation.

Friday, 16 October 2020

The tide of darkness ebbs away

That galaxy without dark matter is back in the news again. Avoiding the hoo-hah of the distance measurements, despite the author's obvious skepticism, this paper presumes the original measurements are correct and tries to simulate if such an object can ever form in standard cosmology. At face value, it does seem extremely weird : while tidal dwarfs are well-known to have little or no dark matter, their origins are usually pretty obvious because they're ugly, messy little things. This galaxy, however, is smooth and symmetrical, and shows no signs of any disturbance.

Now I'm always keen that papers should be lively and the need to be strictly accurate should not infringe on the need for readability. But here the skepticism is a tad... abundant. It's pretty obvious that the authors think the original discovers did just about everything wrong : "erroneously reported", "a result incompatible with", "a result orthogonal to", "once again, this value is incompatible"... so expect a robust response from the discoverers.

Anyway, what they do is try and simulate if such an object can be formed by tidal encounters. We already know that strong gravitational disturbances can strip stars and distort them into all kinds of fantastic shapes, and that's true for the gas as well. Simulations also show that the dark matter is easier to remove since it extends considerably further than either of the baryonic components. What they do here is to try and quantify this, using high-resolution "zoom in" simulations taken from larger projects.

At first glance things don't seem promising. None of their simulated galaxies, they say, show velocity dispersions (which is what we use for measuring total mass) anywhere near as low as those for the real objects claimed to be deficient in dark matter. Somewhat confusingly, they then go on to say that this is an effect of environment, and that actually yes, these things can form. It's not at all clear to me if they mean their initial search was only for isolated galaxies (and not satellites), or due to the low resolution of the large-scale simulations, or if something else is going on.

Their main result is that objects with low velocity dispersion can form as a result of tidal forces preferentially stripping the dark matter while leaving the stellar component more-or-less intact. This can happen to satellite galaxies in a "dense environment", whatever that means. They find several objects in their zoom-in simulations which are in good quantitative agreement with the real objects.

Importantly, they say the original findings overestimated the expected dark matter content for such galaxies, meaning that reproducing the observations requires removing an order of magnitude less dark matter than previously believed - it still means getting rid of a lot, but not that much. The other important factor is the nature of the dark matter. Dark matter particles are on much more extended orbits than the stars, and a particle found at one moment in the centre isn't likely to remain there (unlike the stars). And when it moves to the outer regions, it can be easily stripped by tidal forces. So dark matter can be removed essentially from anywhere in the galaxy without disturbing the stars very much.

Personally I think this sounds like a very serious challenge to these objects as being the weirdos they were claimed to be. The major remaining issue is how common these things are therefore expected to be. Large parts of this paper are very nicely explained, but other bits are a lot less polished. How does the environment in the simulations compare to the real Universe ? How close to the satellites come to their host galaxies ? Do objects like this require very special circumstances, or would we expect to see them everywhere ? I expect an interesting and hopefully angry response from the discovers.

Creating a galaxy lacking dark matter in a dark matter dominated universe

We use hydrodynamical cosmological simulations to show that it is possible to create, via tidal interactions, galaxies lacking dark matter in a dark matter dominated universe. We select dwarf galaxies from the NIHAO project, obtained in the standard Cold Dark Matter model and use them as initial conditions for simulations of satellite-central interactions.

Wednesday, 14 October 2020

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.

Monday, 12 October 2020

Stop judging the gassy dwarfs

Last year there was an interesting attempt to constrain the mass of those pesky Ultra Diffuse Galaxies by measuring their X-ray gas content. This was clever : UDGs are normally so faint that determining their rotation to get a direct mass measurement is very difficult. Measuring the X-ray content, or at least placing an upper limit on it, is relatively easy, and since there's a tight relation between X-ray luminosity and total mass, this should give a decent constraint on the mass of the galaxies. Basically the more hot, tenuous gas a galaxy can cling on to, the stronger its gravitational field must be.

Since they didn't detect any hot X-ray gas at all, the authors concluded that they're more consistent with being dwarfs (which don't have much hot gas) than giants (which do). Which is a shame because if they were giants they'd be much more interesting, pointing to some unsuspected problem in star/galaxy formation at the high-mass end, where models were thought to be reasonably okay.

Here one of the authors splits from the band to start a solo career. While still controversial, it seems to me that most people would accept that most UDGs are probably dwarfs. But what about the most famous candidate giant objects ? Here the author takes a good hard look at two of the most prominent examples using very deep data.

Again, he doesn't find anything, so concludes that even these are just weirdly large dwarfs. But the same objections I raised last time still apply. If UDGs formed in a somewhat different way to brighter giant galaxies, why should we assume that'd have the same sort of X-ray content ? The whole reason they're interesting is that they have far fewer stars than other galaxies of their size, so I don't see why we should expect them to have the same amount of hot gas. Likewise, if you assume that conventional scaling relations are always correct - even if they have very low scatter - you'll never find anything that deviates. So this is circular reasoning : if you assume they follow the standard relations, you automatically exclude them from being weird.

The author does make the good point that X-rays originate from sources than just hot diffuse gas, however. They also originate from high-energy binary stellar systems (white dwarfs orbiting black holes and suchlike). But it's not obvious that this makes any difference. If UDGs form far less stars than similar sized galaxies, why expect them to have a similar fraction of such binary systems ? I remain unconvinced. It's still an interesting observation, but I'd change the title and emphasise that these observations are only consistent with dwarf galaxies, not proof that UDGs aren't giants.

The Archetypal Ultra-Diffuse Galaxy, Dragonfly 44, is not a Dark Milky Way

Due to the peculiar properties of ultra-diffuse galaxies (UDGs), understanding their origin presents a major challenge. Previous X-ray studies demonstrated that the bulk of UDGs lack substantial X-ray emission, implying that they reside in low-mass dark matter halos. This result, in concert with other observational and theoretical studies, pointed out that most UDGs belong to the class of dwarf galaxies.

Tuesday, 6 October 2020

It's as dark as dark can be, probably

This year has been downright weird in just about every way, and writing papers is no exception. Before the pandemic I was working on a nice little paper about Ultra Diffuse Galaxies, which I got carried away with and made everything much more difficult for myself, but then lockdown hit quite suddenly and I resorted to happily recoding FRELLED. That's nearing completion, at least in the core aspects.

As working from home transitioned from weird to normal to actually desirable, observations with Arecibo, once defying the general trend by continuing to operate relatively normally, came to a literal crashing halt. But other telescopes are still managing. Michal Bilek, a colleague from Prague who's now based in Strasbourg, was fortunate enough to get some extra time on a 1.4m telescope in Serbia, and he had the idea of doing some really deep imaging of those optically dark Virgo clouds I love being the cheerleader for so much*.

*Only without any pom-poms, which is frankly disappointing.

Now you might be wondering what a 1.4m optical telescope can possibly contribute to the research of hydrogen clouds discovered with a 305m radio telescope. Well, for one thing it's a lot easier to get multiple hours of integration time on a 1.4m optical telescope for a single source than a 305m dish, for which we only had the equivalent for 5 minutes on-source. This means we can get really high sensitivity. And while for various reasons I strongly suspect there's no optical counterpart to be found, the only way to verify that is to go and look.

We chose one particular target since it was among the brightest of the Virgo clouds and has a nice high velocity width, although it's not outstandingly different from the others. During the discussion I learned just how difficult it is to get really deep optical data, which suffice to say is a lot more of a complex process than just doing a really long exposure. There are all kinds of post-observation procedures that need to be applied, most of which I don't understand (although one, injecting fake sources to estimate the sensitivity, is pleasingly similar to measuring radio tails).

The bottom line is that we can indeed get extremely high sensitivity, constraining the the object to have significantly less mass in stars than gas (by at least a factor three). We also didn't find any sign of optical disturbances in the nearby galaxies which might have suggested that this object is just another boring old bit of tidal debris. Although it's unlikely that a galaxy could have a nice stellar stream without an accompanying hydrogen tail, it's by no means impossible in an environment as chaotic as the Virgo cluster. So this adds, tenatatively, to the idea that it's a genuine dark galaxy, a scenario which has no problem explaining the observations.

I say "tenatatively" since this does rely a bit on statistics. Our previous simulations show that forming such objects is almost, but not quite, impossible. This makes it incredibly improbable - I would say even ruling it out pretty decisively - that all six such clouds are tidal debris, but we certainly can't make such a strong statement for each individual cloud. And there are a few short hydrogen tails in the surrounding region, but none are a good match, and the cloud would still be radically different from any other known tidal debris feature. So yes, tidal debris is a possibility, but in my view it's not terribly likely. Of course whether it's more likely that the cloud is actually a starless galaxy... well, that's another matter.

Basically, as usual, I'll be satisfied if people simply acknowledge that these clouds are weird. I don't much mind what they think they actually are - that is left to future research. For the time being, I'll continue cheerfully waving the metaphorical pom-poms for gas clouds that don't do anything until people give in and admit that they're interesting or everyone gets bored and gives up.

Deep optical imaging of the dark galaxy candidate AGESVC1 282

The blind HI survey Arecibo Galaxy Environment Survey (AGES) detected several unresolved sources in the Virgo cluster, which do not have optical counterparts in the Sloan Digital Sky Survey. The origin of these dark clouds is unknown.

Wednesday, 9 September 2020

Not so little ORCs

Let's follow up that post on terrible acronyms with an entirely sensible and appropriate one : Odd Radio Circles. Orcs. Truly the most evil servants of Sauron and somewhat perplexing for all concerned.

Circular structures are so common in astronomy that you might think there couldn't ever be anything odd about them. From galaxies and accretion discs (even planets if you count spheres), to supernovae remnants and stellar shells, being circular isn't especially strange in itself. Of course the problem is context. Galaxy collisions can produce some spectacular shells and truly circular rings if conditions are right, but generally they result in a big ugly mess. You don't, for instance, expect to find a neat ring lying right next to a galaxy that looks practically undisturbed, which is why Keenan's Ring is so weird.

Three of the ORCs reported here are found in a pilot survey for one of the big Square Kilometre Array precursor telescopes, ASKAP, and one from archival data of the GMRT. They're radio continuum sources, meaning they emit over a wide range of frequencies (800-1100 MHz). This, plus the fact that they don't have optical counterparts - at least nothing obvious - this makes it fiendishly difficult to estimate their distance. Three of them look like filled circular structures while one is a ring. Two of them are directly adjacent to each other while the rest are isolated. All are well-resolved by the telescopes. So these are particularly challenging things to study : very nice, clear images of strong detections, but almost devoid of any context at all that could help explain the little blighters.

Far from being absolutely total mysteries which break science, there are no less than eleven possible explanations listed here. The problem is that none of them are without issue. Since two of them are a pair, some common origin seems likely, but it's entirely possible that they're formed by a mix of different mechanisms. Likewise, two of them have possible optical counterparts, but nothing even close to secure. It's all very frustrating.

"Imaging artifact" can probably be ruled out since some were detected with different telescopes. Collisional ring galaxies also seem unlikely because there's no nice bright optical counterpart, nor do they match the typical characteristics of any of the various radio emission structures produced by AGN (active galactic nuclei) or the halo gas found in galaxy clusters.

There's a few possibilities left : supernovae remnants, planetary nebulae, or galactic winds. The first two they dismiss by the same argument : that the density of such features already known is so low that they wouldn't expect to detect any by chance in their survey. In my opinion this is the weakest part of the paper. Sometimes a weird discovery demands a weird explanation, so more discussion about the physical characteristics would have helped a lot here - otherwise it becomes very much a Bayesian vampire. And there should be some discussion on where such features are typically found; the surveys might be biased towards finding more than is typical.

The third, which they stop short of saying, "this is definitely the answer, we'll take the Nobel Prize now, thanks" but just barely is a galactic wind termination shock. The collective stellar wind from an actively star-forming galaxy could, they say, give rise to just the sort of features they see here, provided the galaxies are in homogeneous media. Okay, but why isn't the star forming galaxy clearly visible, hmm ? They conclude, "such a shock has not yet been observed elsewhere", which feels just a little bit like, "we like this one because it's cool".

I'm being somewhat facetious, of course. It's a good and very interesting piece of work and I hope we get more ghastly ORCs in the future. I'm not entirely sold on these things not having some boring mundane explanation, but I hope they don't.

Unexpected Circular Radio Objects at High Galactic Latitude

We have found an unexpected class of astronomical objects which have not previously been reported, in the Evolutionary Map of the Universe Pilot survey, using the Australian Square Kilometre Array Pathfinder telescope. The objects appear in radio images as circular edge-brightened discs about one arcmin diameter, and do not seem to correspond to any known type of object.

Tuesday, 8 September 2020

Annoying Causes of Really Obnoxious Non-readable Yet oMnipresent Nonsense

Why are scientific papers so damn boring ? Many reasons. The article focuses heavily on acronyms, which are a problem, but in my opinion this is just the thin end of a very large wedge.

1 million acronyms is a staggering number, but what surprised Doubleday even more was the fact that only 0.2 percent of those abbreviations were used regularly (meaning that they appeared at least 10,000 times) and 79 percent were used fewer than 10 times. “Not only are we creating more acronyms over time,” she says, “but we’re not even reusing them.”

Paradoxically, while scientists are not reusing new acronyms, they’re creating new definitions for acronyms that already exist. In an article about clarity in scientific writing, published in the Journal of the American Society of Echocardiography, Alan Pearlman says: “A psychiatrist knows that MS stands for ‘mental status,’ while a neurologist might take it to mean ‘multiple sclerosis.’ I am interested in valvular heart disease and am certain that MS stands for ‘mitral stenosis.’ My cardiac pharmacologist reminds me, however, that MS really stands for ‘morphine sulfate,’ while my neighbor, who works in the computer industry, tells me that it stands for ‘Microsoft.’”

Figuring out when an acronym is appropriate can be tricky, says Barnett. Generally, he says most scientists now think it should be used only when the terms are unambiguous, common in the field, not easily replaced by simpler language, and if the words are too long or complex to be consistently written out. Think of “HR” for “heart rate,” says Barnett—they’re both two syllables, it’s easy to spell out, and it’s a very simple concept that most people are familiar with.

Good advice. I've semi-affectionately given ALMA the nickname of the Amazingly Large Manufacturer of Acronyms, since it generates them at a truly frightening rate. It also has the habit of using this bizarre term "block" for everything : scheduling block, observing block, execution block... it's weird. If you were to say, "my observing campaign suffered the failure of an execution block because the SB had the wrong intent, so the DRM told me that the P2G would be looking into fixing a bug with the SPW setup so that the AOT will spot problems in FDM before running into this during QA2", no-one would think anything amiss.

Yes, some jargon is actually beneficial, and a good deal of it is unavoidable, but much of it is just plain useless.

Some of it, of course, is easily addressed. I remember being tremendously perplexed by the term "path length" when learning about wave interference back in high school. Somehow my brain got stuck and for the longest time it just didn't occur to me to take the term literally : the length of the path travelled. I was expecting jargon where in fact there wasn't any ! Some stuff you've just gotta know, and you can't expect a scientific paper to be fully accessible to a general audience. That's fair enough.

... but you can expect it to be accessible to a specialist audience. Acronyms are only a small part of why this isn't always the case. To be honest, these days I find reading most extragalactic astronomy papers relatively easy-going, but even then only within a very narrow sub-discipline, and it's taken me a long time to get to this point. When I first started (and still today if I venture too far outside my comfort zone) I found few things as tedious as slogging through what are often some of the driest, most lifeless pieces of text that some poor sod has ever had to endure writing. It's not really the acronyms so much as the whole style - or more often lack thereof - and format of the whole thing.


What's the real problem ? There's no one single cause, but the biggest contributing factor, in my opinion, is a lack of clear narrative flow. Any good piece of text ought to flow linearly, one section leading naturally to the next. In a paper this is sometimes not the case at all, with each section being almost entirely independent. What's particularly maddening is that there isn't a universal approach to this. Some papers should be read from start to finish, with each section being essential for the next, whereas others can and should be read according to whatever section the reader is most interested in (otherwise the reader will bore themselves unnecessarily). But even those of the most modular structure will often reference parts of other sections, so the reader has to move through the text like the world's worst Choose Your Own Adventure book. To say nothing of the notorious paper chase, when you just want a single parameter but end up following a whole chain of citations through a dozen different papers that ends up with an unpublished proceedings, or worse, cites back to the original paper in a closed loop...

Now of course, it just isn't sensible to insist on a linear narrative structure all the time. Sometimes that just wouldn't be appropriate. The problem is that while some papers do have entirely self-contained sub-sections, others don't. This means the only way to be certain you're not missing some vital caveat is to read the whole damn thing. And I know from direct experience that referees often prefer papers to avoid repetition (i.e. implicitly insisting on a linear structure even when this isn't a good idea, never mind that repetition is a very useful tool for increasing understanding), making it a risky business indeed to take any statement in a paper out of context.

The point is that there's no agreed-upon format. This makes reading a paper far more of a challenge than it has any real need to be. And that goes a long way to explaining why papers are so incredibly dull - even the basic structural format is ill-defined. It's like trying to build a house without having decided if you want to end up with a three-bedroom semi or an igloo.


The second major problem I'd highlight is the supposed need to prioritise clarity to an absurd and counter-productive extreme. Any kind of flair or rhetoric tends to be exorcised as though it were an evil spirit haunting the text with its terrible screams of Trying To Make Things Lively For The Reader. Again the inhomogeneity of standards here is extremely frustrating : a few authors seem to get away with colourful remarks and even jokes (!) but most are absolutely reduced to the barest facts. And reducing things to the extent that they're unreadably dull would seem to defeat the whole purpose of publishing something that's intended to be read !

Similarly, when I've tried to introduce some pedagogical remarks for the less uber-specialist reader, on occasion I've been told to remove them because they're well-known. It's very hard to gauge what is indeed generally well-known and what's just well-known to the reviewer - a problem inherent with using highly expert referees. The deeper you are in a field, the harder it is for you to judge what's confusing to someone less entrenched. And again, there's no agreed-upon standard as to who the primary target audience should be, or what level of knowledge they're expected to have.


There isn't really any one obvious solution to this. Some options are technological, even mundane. A decade or two ago it was standard practise to write all mathematical symbols in this downright cryptic swirly italic font, such that the letter "M" looked like the result of a Victorian robot getting into a fight with an angry snake. Thankfully this has largely died out, although it's still common to use really weird, unpronounceable Greek* letters as variables when perfectly decent English letters would do just fine**. Using a bunch of unreadable squiggles is presumably only intended to make the thing harken to an earlier era of arcane wizardly and occult mysticism, because it certainly doesn't help anyone follow what the equations are supposed to actually mean.

* Of course, if you happen to be Greek, you probably take a different view. That English is the language of academia is historical happenstance, but there's no reason learning Greek letters should be a key part of scientific practise.

** There are some standardised exceptions, of course. I'm referring here to customised variables not likely to be used outside a single paper, which have no need to be obfuscated so.

One possibility might be to alter the physical format of papers, to better reflect the fact that nobody reads the original hardcopy any more; making it standard practise to use internal links to relevant sections, along with some way to quickly return to the original point at which the reader jumped section. Embedded dictionary entries, as can be found on e-book readers, would also be a great benefit, allowing the novice reader to very quickly see a one-paragraph summary of a common term with citations to the major relevant papers. And some facility to instruct the reader on whether a paper was intended to be linear or not would go a long way to making the whole process easier. There is value in insisting the reader experience the complete paper, and there is also value in sacrificing efficiency to ensure a more complete understanding, but there are a lot of cases where this is downright cruelty.

On a purely practical note, we should do away with LaTeX. It's just not a natural way of writing a document and forces authors - especially beginners - to spend more time figuring out how to produce an article than they do on writing comprehensible text. There's no energy left over for anything else.

Thinking further ahead, some AI-based way to use the internal links so that the paper could be customised for the individual reader would be even better. Say, if a paper catalogues a hundred different properties of a sample but you're only interested in two or three, then being able to reduce all the description to only those parameters would make everyone's life a lot more pleasant.

Likewise, having dedicated, specialist journals of scientific methods might help. If an author could simply describe their sample and then say, "we analysed this using the procedure of Horace et al. 1995", where Horace et al. describe nothing but the methodology, that would reduce the paper size and leave the details accessible to the enthusiast. Such a methodology journal would have to be especially rigorous and detailed (almost to the point of describing which buttons to press), ensuring that the reader could reproduce the method exactly, but this would have the benefit of preventing the author from omitting crucial details. There are certain criteria that could easily be standardised and insisting on them should be automatic : integration time, number of particles, etc, which a methodology paper could then state were essential. Hence even a novice reviewer could see at a glance if the authors had included them, since they'd have an easy checklist to follow.

Similarly, where the authors are claiming to have produced some new procedure or recipe, or determined some parameter value, it should be absolutely essential that they state this with the utmost clarity, not bury it deep within the text. It's something of a paradox that insisting on clarity has tended to lead to so much obfuscation; linear narratives, on the other hand, tend to lead more naturally to descriptions of what the reader needs to know. The challenge for the reviewer in that case is to avoid having the wool pulled over their eyes.

Which leads finally to just how damn boring most papers are. Frankly they have all the joie de vivre as a whale carcass. I get that it can be a nerve-wracking experience trying to deliver a lively oral presentation, and similarly writing is a skill like any other, but papers - which the author has ample preparation time in a controlled environment - do not have to be this bad. This is the hardest issue to address, as it's hard to know who to blame. Do authors write dull papers because they can't find a way to say anything interesting* that will get past the reviewers, or simply out of expectation that that's how papers should be written ? Do reviewers want more lively text ? Do they tend to be such experts that they no longer care about writing style ? It's hard to say.

* Or rather, in an interesting way. The content is another topic entirely - here I'm only concerned with style and readability, not scientific merit.

I'm not advocating that papers be full of random jokes about vampire hamsters. I just think the general level of formality is absurdly extreme to the point that clarity is reduced because no-one actually wants to read anything.

One possibility would be to always have two reviewers : one expert (e.g. someone with a decade or more experience in the field), who would comment on nothing but the scientific content, and one novice (e.g. a PhD student), who would comment on nothing but the style. The expert would then not have to worry about petty typesetting and minor spelling mistakes, but they'd also be forbidden from saying much about the clarity and/or brevity of the explanations. That would be the job of the novice, who, conversely, would not get to say much about the content. 

In principle the downside of this is that it increases the workload. But it would have the positive side-benefits of getting beginners in the field involved in the whole process right from the start, in developing the ability to say "I don't understand this" (which many people, myself included, don't like to do, just in case someone has the temerity to try and explain things...), and the work involved in reviewing style is a lot less than that in reviewing content.

An alternative might be to have a structured review form that clearly sets out what the reviewer should do, asking them to think whether terminology really would be clear to a beginner. Whilst a few papers may need to be written with the expectation that only other tenured professors are capable of understanding them, this should be far the exception rather than the norm. Having an objective checklist to check understandability wouldn't be as effective as using someone who actually lacks knowledge, but it's better than letting an ultra-specialist run wild and free decide what's comprehensible for themselves. Getting people to stop and think, "Is this jargon ? Could this be misinterpreted ? Could I use something simpler without sacrificing rigour ?" would at least help.

Rant over. Science is hard, but that's no reason a simple sentence has to be constructed with such malevolent exactitude as to cause unwitting novice observers, bent on seeking a greater understanding of the particular contents of the implicitly aforementioned scholarly work through a dedicated period of detailed scrutiny and examination, a series of progressively increasing difficulties in their apprehension of the implicitly aforementioned scholarly work that can, in the most serious instances, lead to headaches and other common mental conditions that severely restrict their intended goal of comprehending the piece before them, ultimately leading to disillusionment and dissatisfaction with the entire academic edifice.

See what I mean ?

Why scientific papers are growing increasingly inscrutable

A group of researchers in Australia, including a marine ecologist and a statistician, looked at almost 25 million scientific articles between 1950 and 2019, searching for trends in acronym use. They published their analysis in eLife.

Monday, 31 August 2020

Mysterious molecular mysteries

Ultra Diffuse Galaxies are big flooffy things without many stars. But they do have some. So at some point they must have had some level of star formation activity. Maybe they made them all in one go, or maybe they made them very slowly, but they must have done something, at some point, somehow.

What's especially weird is that some of them have quite a lot of atomic gas - comparable to Milky Way-sized galaxies, which in general form stars at a quite respectable rate. But very little is known about the molecular gas, and while recently the mood is swinging away from "molecular gas is the be-all and end-all of star formation", it's still undeniably an important part of the process. So these authors use the IRAM 30m dish to look for CO emission (everyone's favourite tracer of H2) in six lucky UDGs, all rich in HI gas. Which, you may remember, is a project I've done myself, but not got around to publishing yet.

This is just a letter, so they concentrate heavily on their main result : a very tentative detection in one galaxy of their sample. It's difficult to tell exactly how tentative it is, since the part of the spectrum they show is so small that there's almost no baseline visible for comparison. They say it's 4 sigma, but it would be a lot more interesting with more of the spectrum visible.

To be fair, they don't ever claim this as more than a tentative possibility. They note that the HI doesn't overlap the velocity range of this possible CO emission, but it would have been easy to actually show this directly (I'd have made the figure myself but they don't plot the absolute velocities on the x-axis, so it's above the effort/reward threshold for me). It's true that this doesn't rule out the CO emission, but one would think you could say something more interesting about the kinematics than simply commenting on the validity of the detection. If these galaxies have low star formation thanks to a very extended, low-density gas disc, then why should the kinematics of the CO be different from the HI ?

The other five galaxies have no CO detected at all, though they don't give any tables and figures. They say from the upper limits that this means they have low H2/HI ratios, so "the inefficient star formation in such galaxies is mainly due to the low efficiency of forming molecules from atomic gas". Well, yes, but what's keeping that molecular star formation low ? There's just as much atomic gas as in galaxies which are perfectly happy to form stars, so why don't these ones ? For now, the middling molecular mass measurement makes mysterious mysteries more mysterious. Maybe.

CO observations toward HI-rich Ultra Diffuse Galaxies

We present CO observations toward a sample of six HI-rich Ultra-diffuse galaxies (UDGs) as well as one UDG (VLSB-A) in the Virgo Cluster with the IRAM 30-m telescope. CO 1-0 is marginally detected at 4sigma level in AGC122966, as the first detection of CO emission in UDGs.

Tuesday, 25 August 2020

A Bridge Quite Far

Specifically, a bridge between two galaxy clusters that spans a whopping 2 Mpc. They'd seen some earlier hints of this in previous data, but now have higher sensitivity from multiple telescopes to try and confirm it. And it's rock solid at 144 MHz and (in my opinion - they're more cautious) 53 MHz, but tentative at best at 383 MHz and not visible at all at 1.5 GHz.

Such bridges are rare in the extreme - this is one of only two such features known, making them hard to understand. You can't, for example, say what types of system most commonly host them, so it's difficult to comment on their origins. Even the emission mechanism is uncertain, since it's only clearly detected at one frequency. They say :
According to Brunetti & Vazza (2020), radio bridges may originate from second-order Fermi acceleration of electrons interacting with turbulent motions triggered by the complex dynamics in the overdense region between premerging clusters.
Basically it's from very low energy, low density, ionised gas. Interestingly there's a correlation with the much hotter X-ray emission, pointing to a connection between thermal and non-thermal sources, but that's about all they can say from this.

The larger question of whether this gas is primordial, i.e. we're seeing the cosmic web itself, or non-primordial gas from interactions, isn't addressed directly. And fair enough really - when you've discovered a bridge of gas 2 Mpc in length, you're absolutely entitled to write a short letter than says little more besides, "we found a bridge of gas 2 Mpc in length". Interestingly though, both cases of giant radio bridges (or in one slightly amusing typo, radio brides) are found in massive, pre-merging clusters that themselves consist of interacting sub-structures. So I guess both options are open, though presumably the sheer length of the thing would more naturally suggest a primordial structure. But until more of these are found, there's little else to be said apart from, "hey, that's neat !".

A giant radio bridge connecting two clusters in Abell 1758

Collisions between galaxy clusters dissipate enormous amounts of energy in the intra-cluster medium (ICM) through turbulence and shocks. In the process, Mpc-scale diffuse synchrotron emission in form of radio halos and relics can form. However, little is known about the very early phase of the collision.

Thursday, 23 July 2020

The Virgo Cluster is feeling left out

Ultra diffuse galaxies were once thought to be oddly rare in the Virgo cluster. This was very disappointing since all the other clusters seem to be full of the damn things. What's wrong with poor old Virgo ? Is it too violent for them to survive ? Unlikely, since it's got hundreds of more generic low surface brightness galaxies. Is Virgo the extragalactic equivalent of the angry neighbour no-one wants to meet because they don't know about deodorant ?

Probably not. As previously suggested, it could all be due to the way the data was processed. Over such a large area as Virgo, few are brave enough to attempt a search by eye, and automatic algorithms are very sensitive to their input parameters. Typically they return initial catalogues of the order of millions of objects, which have to be carefully culled down to a few hundred or thousand objects that can be examined the old-fashioned way. And even slight differences in the way the data was processed, which might not make any difference at all to the brighter galaxies, can cause big changes in how many UDG candidates pop out.

This paper searches the previously UDG-free data from the Next Generation Virgo Survey (the most sensitive large-scale optical survey of the cluster to date) and produces a catalogue of up to 70 objects. Oddly, they don't mention any special data processing, so why previous searches of this same data set have come up empty (if I recall correctly, only three UDGs were hitherto known in Virgo) is anyone's guess.

The broad definition of a UDG, which became widely accepted very quickly, is that it's large and faint. These two parameters are relatively easy to measure (but see this) but the authors decide they need a better definition since there isn't a good physical motivation for the conventional criteria. So, annoyingly, they come up with a more physical but far more complex selection criterion that a UDG is anything which deviates by more than two sigma from no less than three non-linear scaling relations. Yikes.

This isn't as mad as it sounds. As clear from their plot, the conventional definition would miss quite a lot of really interesting objects - specifically the largest ones. But it's awkward, and it doesn't help that their plot has an extremely strange vertical stretch. I don't see this catching on, and for smaller objects it doesn't appear to make much difference anyway.

One point that's probably worth mentioning, though, is that choosing objects which are in the tail-end of a distribution does not necessarily mean that those objects are simply more extreme versions of the general population : that would be a statistical bias. It's possible to populate parameter space with objects that form by very different mechanisms, so that the distribution is continuous isn't evidence in itself that the objects are all part of the same population.

Most of the rest of the paper is a tedious slog-through all the possible analysis using rather small number statistics. Great for the enthusiast, no doubt, but not exactly thrilling bedtime reading. Also their thumbnail images (figure 6), while very nice, look extremely red, which seems just a bit odd.

They find two quite interesting things. First, the core of the cluster has more UDGs than elsewhere, in contradiction to other surveys which find that UDGs are not clustered. How this compares with the overall galaxy density they don't say, so I wonder how big a deal this really is. And is it telling us something about the nature of the UDGs or their environment ? The statistics are just too small to say anything. Second, all the globular clusters of the UDGs are redder than globular clusters around non-UDGs of the same stellar mass. Why that should be is anyone's guess.

Overall, they conclude as others have that UDGs likely form by a variety of mechanisms. They seem to occupy the tails of various distributions, suggesting they're normal but extremely faint galaxies. There really doesn't seem to be anything much at all to distinguish them from other galaxies except that they're faint for their size. Potentially this is extremely interesting, but without proper dynamical measurements the key feature - how much dark matter they have - will have to wait. Some of them are definitely tidally interacting, so it's possible that this is part of their formation mechanism in at least some cases, but it seems they don't think this is likely to explain the majority.

In short, Virgo does have UDGs after all, but how they form remains pretty much as mysterious as ever.

The Next Generation Virgo Cluster Survey (NGVS). XXX. Ultra-Diffuse Galaxies and their Globular Cluster Systems

We present a study of ultra-diffuse galaxies (UDGs) in the Virgo Cluster based on deep imaging from the Next Generation Virgo Cluster Survey (NGVS). Applying a new definition for the UDG class based on galaxy scaling relations, we define samples of 44 and 26 UDGs using expansive and restrictive selection criteria, respectively.

Monday, 20 July 2020

A jellyfish running out of gas

 A "jellyfish" galaxy is one which has long tails of ionised gas. The term is a bit liberal, but mainly of them have very thin, tendril-like structures, sometimes much larger than the main stellar disc, so they do often look quite a lot like jellyfish.

But ionised gas is rubbish and everyone hates it, so what about the much more sexy neutral gas ? This nice little paper presents some VLA observations of a jellyfish galaxy in a nearby-ish cluster, which they then compare with an arbitrary other galaxy in some other cluster. To be honest, I didn't find that aspect of the paper at all useful. It's a bit like picking one jellyfish out of the Atlantic ocean and comparing it with another picked at random out of the Pacific : you can't really learn anything by such a comparison. It's not the author's fault, mind - it's hard to get neutral gas observations at this distance, so the current sample size is very small.

The other bit I don't like is the claim that this jellyfish has only a short (40 kpc) tail of HI, compared to the ionised gas which extends ~100 kpc. Given the poor sensitivity of the VLA in C-configuration, I don't think this means anything much except that the density of the gas at the larger distances must be lower.

What is quite interesting is that the star formation activity seems to be somewhat enhanced compared to non-jellyfish galaxies. This particular object is almost certainly the result of ram pressure stripping as the galaxy ploughs through the hot, thin intracluster medium. While this ultimately drains all the galaxy's gas and prevents star formation, in the initial stages it can compress the gas and so give the galaxy a final burst before its untimely death.

But that's not terribly surprising. The most interesting thing to me is that while the HI doesn't seem to be at all spatially displaced from its parent galaxy, or marginally so (its "tail" being a slightly lopsided and ragged edge on one side), its kinematics are quite different. The systemic velocity of the gas and stars differs by ~100-200 km/s. This isn't seen in similar cases in the  much closer Virgo cluster : stripping galaxies there have clear tails but with gas discs that are relatively unperturbed. Perhaps this is a case of particularly strong stripping, or maybe most of the motion is along the line of sight. It'd be nice to get much deeper observations of objects like this, but that will probably have to wait for future instruments.


EDIT : Hot on the heels of this gassy-tailed jellyfish comes another paper about a similar object with higher-resolution data. This is an especially fun object : it has strong stellar tails indicating a tidal interaction with a nearby galaxy, with most of its gas being displaced in both space (only slightly, but very clearly) and velocity (by ~340 km/s). Since the gas morphology does not well-match the stellar morphology, the authors attribute this to ram pressure occuring along the line of sight. Unlike the other object, there's no evidence that this has increased the star formation rate, even though the surface density of the gas is higher outside the stellar disc. They say it might be stretched, so its volume density remains low despite the higher surface density. A very nice object indeed !

GASP XXVI. HI Gas in Jellyfish Galaxies: The case of JO201 and JO206

We present HI observations of the jellyfish galaxy, JO201. This massive galaxy (M$_{\ast} = 3.5 \times 10^{10}$ M$_\odot$) is falling along the line-of-sight towards the centre of a rich cluster (M$_{200} \sim 1.6 \times 10^{15}$ M$_\odot$, $σ_{cl} \sim 982$ km/s) at a high velocity $\geq$3363 km/s.

ChatGPT Is A Competent Source Extractor

This builds on my post from two years ago, ChatGPT Is Not A Source Extractor . My, how things change. Last time I wrote : They're still ...