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.

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.

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.

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.

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.

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.

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.

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.

A long time to say "I told you so"

Back in 1997, Raul Jimenez predicted that some galaxies could be completely optically dark, or nearly so. Such a "dark galaxy" would be a big disc of hydrogen spinning serenely inside a dark matter halo, but with such a low density that star formation was at worst an occasional irritation, and at best averted completely, rather than the more typical state of galaxies as star formation factories that are good for only one thing. Here, with the assistance of some delightful nominative determinism of Alan Heavens, he revisits his predictions.

I can't claim to fully understand the details of modelling, but the gist of it is simple enough. Atomic hydrogen needs to reach a certain density before its self-gravity reaches the point where it overwhelms any other counteracting forces and collapses into stars. One such counteracting force is the thermal temperature of the gas, generally around 10,000 K, equivalent to random motions of 10 km/s. Another is the spin, which keeps the gas on stable circular orbits, and this can be much larger - up to ~500 km/s in extreme cases. The "spin parameter" is a measure of how much of a role the rotation plays in preventing collapse. It's a bit more complicated than ordinary rotation, but the two are roughly equivalent : the faster the rotation, the harder it will be for gas to collapse and form stars.

Twenty-three years ago, Jimenez predicted the value of the spin parameter above which galaxies ought to remain entirely dark. Back then detecting the gas of such galaxies was a formidable challenge indeed, but two decades of improvements have allowed his predictions to be tested. In particular, the gas-rich "Ultra Diffuse Galaxies" from the ALFALFA survey are natural targets for comparison : they have a few stars, but far less than typical galaxies of comparable size.

Jimenez finds that these UDGs are indeed well-described by the high-spin tail model, where the most extreme spin-dominated galaxies remain optically dark in the conventional cold dark matter scenario. The number of discoveries (22) is in excellent agreement with the prediction (24). They're also in good quantitative agreement for his prediction of just how bright - allowing "dark" to be a synonym for "very dim" - they should be and what colours they should have. He says that ALFALFA has probably detected nearly all the largest dark halos and that only a survey of an even larger volume would detect any more. In contrast, detecting less massive galaxies requires a more sensitive survey.

What does this mean for my beloved clouds in the much deeper AGES survey ? I'm not sure yet, but the spin parameter would be interesting to compare. Those clouds are totally optically dark, but one of the main doubts about their galaxian nature is that we only found them in the Virgo cluster... on the other hand, Virgo is uniquely dense so this would be the best place to detect rare objects.

Even more interesting would be what this means for the very smallest objects, i.e. the missing satellite problem that prompted the whole "dark galaxy" thing in the first place. Jimenez doesn't describe this, or the other predictions of dark galaxies that were in vogue at the time. Still, this is potentially a super-interesting result. My main question is : what about all those other UDGs that don't have gas ? How do they fit into the model, and why don't more of them have gas ? But after twenty years, this is probably interesting enough already.

The distribution of dark galaxies and spin bias

In the light of the discovery of numerous (almost) dark galaxies from the ALFALAFA and LITTLE THINGS surveys, we revisit the predictions of Jimenez et al. 1997, based on the Toomre stability of rapidly-spinning gas disks. We have updated the predictions for $Λ$CDM with parameters given by Planck18, computing the expected number densities of dark objects, and their spin parameter and mass distributions.

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.