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.

Wibbly-wobbly gassy-wassy

A nice, extremely careful paper on measuring galaxy gas asymmetries in spectral line profiles. The advantage of this is that it's much easier to get a single-dish measurement of the gas, which is usually much too low a resolution to examine the structures. All you get is a line profile, which shows how the gas brightness varies as a function of velocity.

Resolved observations show that most disc galaxies have flat-ish rotation curves, meaning that most of the gas is rotating at a single velocity. With one side coming towards and the other away from us, this generates the characteristic double-horn (a.k.a. "Batman") profile shape. But if there's more gas on one side than another, for whatever reason, or if the rotation is screwed up somehow, then Batman's horns go all wonky.

Measuring the wonkiness is straightforward enough : determine the central velocity and then measure the total flux on either side of this. By and large, the results tend to be quite subtle and it's by no means clear what the strongest driver of asymmetry really is. Here the authors do a tremendously careful job to measure everything as carefully as they can, carefully controlling for all the different possible errors in a really careful way. Carefully. A few times I even wanted to say, "Okay, okay, I believe you already !" but this is far better than the opposite case, if not as exciting.

They find that some of the asymmetries are just the result of the noise. In mock observations, they show how the distribution of the asymmetry parameter neatly broadens at lower signal-to-noise levels. It's not that they get systematically more asymmetrical, just that the measured asymmetry range increases. But, above a certain S/N threshold, asymmetry measures can be considered reliable. So to properly measure asymmetry, you need to measure it in a population of galaxies, not just in individuals, and you need a good comparison sample. Which they have, on account of being so bloody careful.

Their main result is that galaxies above this threshold tend to be less gas rich than galaxies of comparable S/N ratios with lower asymmetry levels. So asymmetry is an indication not just of gas displacement, but of actual gas loss. They also show that satellite galaxies tend to be more asymmetrical than their larger companions. Environment, it seems, drives gas loss more than other factors.

There's nothing much unexpected about that though. In fact it seems somewhat disappointing that all this work only results in a new way to show something we already knew about. More interestingly, they also show that there are significant asymmetries in isolated galaxies, which can't be explained by interactions with other galaxies. This could be a signature of ongoing gas accretion, but more work is needed to compare with simulations.

All in all, it's a really nice piece of work that explains everything very thoroughly without being outrageously dull. I'm a bit sorry for them that they didn't find anything more unexpected, but it presents a nice tool to use on further samples.

xGASS: Robust quantification of asymmetries in global HI spectra and their relationship to environmental processes

We present an analysis of asymmetries in global HI spectra from the extended GALEX Arecibo SDSS Survey (xGASS), a stellar mass-selected and gas fraction-limited survey which is representative of the HI properties of galaxies in the local Universe.

The early bird gets the one ring to rule them all

Was the early Universe much different from the modern one ? The answer is definitely yes : there are far more quasars, more merging galaxies, higher levels of star formation, and galaxies tended to look a lot messier. But there are some anomalies.


This first paper notes the discovery of a mostly-normal disc galaxy formed just 1.5 Gyr after the Big Bang. Using high resolution observations of ionised carbon with the ALMA radio telescope, the authors were even able to measure its rotation curve... and it looks perfectly normal. Overall, it looks like a galaxy that's a bit bigger than the Milky Way that's somehow travelled back in time to the early universe. The only major difference seems to be that it's forming stars an order of magnitude faster than contemporary galaxies, which is normal for galaxies in this era.

Interestingly, the molecular mass of the galaxy is very similar to its dynamical mass. Considering that its rotation curve is nicely flat, one wonders about the dark matter content - which, oddly, is barely mentioned. Especially since there was much-popularised result a few years ago showing that galaxies in the early Universe show declining rotation curves, which this one clearly doesn't, but they don't cite that. Though, that result probed the rotation out to much greater distances, with this new result only being able to examine the inner regions of the galaxy's disc. Still, it's a bit weird that it isn't even mentioned.


If boring normal galaxies that travel through time aren't exciting enough, how about one with a ring ? This second paper describes a ring galaxy found just a bit later at 3 Gyr after the Big Bang. It's a very small ring compared to modern galaxies, in the bottom 10% of the distribution. Either they spotted it just 40 Myr after it formed, if it was produced in a collision like many contemporary rings, or it was formed by a different mechanism. While most modern ring galaxies show higher star formation rates than their counterparts, this one has about the same activity as typical non-ring galaxies of its era.

The galaxy also has a large, extended stellar disc outside the ring. If this is a collisional object, they say, then the timescales for the formation of both structures (which I assume they get from the kinematics) are mutually exclusive : one must be more than 80 Myr and the other less than 50 Myr. They say this could be explained if the ring is actually the second formed from the collision, with the disc being the remnant of the first ring which has now been smoothed out (I didn't know multiple rings were much of a thing but apparently this is possible).


The strange thing about both of these galaxies is how they were able to form so quickly after the Big Bang. Back then the Universe was much smaller, so mergers were much more common and melodramatic, so even forming a nice stable disc galaxy ought to be a problem. And then to get one to form a ring as well implies things have really settled down quite quickly. One the other hand, rings in general should be more common due to all the merger, but the second paper says their estimates are that rings were no more common back then than they are now. Apparently the higher merger rate is neatly balanced out by the fraction of disc galaxies available for collision.

I'm pretty sure by time-travelling galaxy theory is the most sensible explanation for this, and I expect to be quoted on it. If I don't see headlines like, "MILKY WAY IN DANGER OF TRAVELLING THROUGH TIME, SAYS SCIENTIST", I'm going to be disappointed.

Time flies when you're commissioning a telescope

FAST, the Five hundred metre Aperture Spherical Telescope, a.k.a. the Chinese Arecibo, has already delivered a bunch of totally uninteresting pulsar results. At least I presume they're uninteresting, on the grounds that they're... well, pulsars. Because if it's one thing the world needs, it's pulsar-based racism. ALL STELLAR REMNANTS MATTER !

(I continue my ongoing quest to provoke outrage on Twitter without actually being on Twitter. One of these days I'll succeed.)

Anyway, this paper finally uses FAST to do something useful and look at HI in galaxies like a respectable radio telescope. Annoyingly, they cite just about every HI survey of any importance apart from AGES. This annoys me. Especially as they focus on the importance of sensitivity and looking to higher redshifts, both of which AGES does pretty well (though to be fair we haven't published any of the higher redshift detections yet).

It's a bit of an odd paper. Normally in a first-results paper there's tonnes of stuff about the instrumental capabilities and the technical specifications, but this is almost entirely absent. Basically they looked at four galaxies doing a totally standard observing mode and detected three of them. One of them is nicely consistent with a previous ALFALFA HI measurement, another shows a similar HI and CO profile, while the third shows a bit of a difference. Since the CO and HI line widths are similar, they infer that both components probe the flat part of the rotation curve. This is entirely reasonable but there's just not much more you can do with a sample of three. They also estimate the dynamical masses, although quite honestly I have absolutely no idea why.

There are two other oddities. One is the comment that you can use gravitational magnification to boost sensitivity, which is true but they then list surveys as examples which, as far as I know, do not make use of this. The other is where all their time went. They were allocated a total of 10 hours, or  or an average of 2.5 hours of observing time per source. The galaxy previously detected by ALFALFA required 48s of integration time in the earlier survey, while FAST used a 5 minute scan. They say the sensitivity level would have been about the same, correcting for the difference in observing time. All well and good, but what did they do with the rest of the observing time ?

In practise 5 minutes on-source typically means 15 minutes of actual observing time - you also need 5 minutes off-source to calibrate and a generous 5 minutes for slewing and whatnot. So a typical complete scan is 0.25 hours. Since they were given a total of 10 hours, so that amounts to 40 scans, or 10 scans per source. But they say they used 3-8 scans per source. If there were three scans for one source and eight for the others, then that's only 27 scans out of a possible 40 ! That's several hours of time unaccounted for. Do they have extremely slow slew times ? Does the data reduction take a long time ?

I dunno. It's all just a bit strange. And given that they go for sensitivity, I would have expected some discussion on what their results indicate for future surveys, but there isn't any - they discuss science instead, which there's bugger all you can do with four galaxies. Surely it would have been better to accept a modest drop in sensitivity and go for 40 targets instead of four !

Anyway, good news that FAST is finally starting to do proper science. Even with all the oddities and strange fixation on pulsars, I'm sure it will be a valuable addition to the arsenal of telescopes pointed at the HI sky.

The atomic gas of star-forming galaxies at z$\sim$0.05 as revealed by the Five-hundred-meter Aperture Spherical Radio Telescope

We report new HI observations of four z$\sim$0.05 star-forming galaxies undertaken during the commissioning phase of the Five-hundred-meter Aperture Spherical Radio Telescope (FAST). FAST is the largest single-dish telescope with a 500 meter aperture and a 19-Beam receiver.

These dynamic dimensions are too dynamic

Do Ultra Diffuse Galaxies rotate too slowly or is it all just a measurement error ? Last time I said, "I think this paper reasonably settles any concerns about the inclination angle measurements, though I won't say it's unquestionable."

This paper swings me back the other way. Their sample looks much more likely to be consistent with miniscule measurement errors that shift the velocity widths into an apparent realm of weirdy weirdness.

For those who haven't a clue what's going on, there were claims that certain galaxies are rotating much, much more slowly than expected given their mass. There are a variety of galaxies that do this. Most are so-called Ultra Diffuse Galaxies, meaning that their stars are unusually spread out, but there are a few brighter objects too. This would be extremely strange because the rotation-mass relation is normally quite tight. These oddballs are way off, in some cases as though they had no dark matter at all. And many of them are too isolated to explain by interactions with any other galaxies.

This latest paper is pilot for a big follow-up study of a sample of UDGs, getting gas measurements with the Green Bank Telescope to estimate their rotation. The GBT doesn't have the spatial resolution to get proper rotation curves - for galaxies this far away, it can only do line widths. So they use the optical images to estimate the inclination angle of the discs, and use that to correct the line widths into true rotation speed. (Gas measurements tell us about motion along the line of sight, so if we're viewing a galaxy face-one, it's line width is zero. If it's edge-on, we measure its rotation directly and no correction is needed.)

They observed 70 galaxies and detected 18. Half of these were confirmed to be UDGs while the other half were found to be foreground dwarfs, i.e. not as extended as previously thought. Interestingly, there's basically no morphological difference at all between a distant UDG and a much closer dwarf. The old "these cows are far away" problem is a tricky one indeed when it comes to galaxies.

Most of the rest of the results are not at all surprising, e.g. the gas rich UDGs tend to be bluer and more irregular than ones without detected gas. They have some low levels of star formation detectable via UV emission, but there's nothing much unexpected about that.

The dynamics are the interesting bit. Seven of their nine UDGs lie off the normal baryonic Tully-Fisher relation while the other two are well within the scatter of typical galaxies. For the outliers, they calculate how badly they'd have to have got the inclination angle wrong in order to bring them back to the standard TFR, and it's pretty substantial, ranging from 20 to 42 degrees. Large errors certainly aren't impossible, given that these are all very faint objects and the gas disc might be oriented differently to the observer than the stellar disc. But it sounds at first glance just too big to explain all of them.

Helpfully they also show what these inclination angles would look like in comparison to the ellipses they fitted to the actual data. What's really surprising is that these "corrected" ellipses are often very close indeed to the measured fit, sometimes almost perfectly overlapping despite an error of 20 degrees or more. I even had to manually plot the circles myself to convince myself that this is correct. It is. Should I ever give another course, I'll be sure to add a plot of what inclination angles look like in practise. Here's a simple version that uses thin discs - the thick disc formula is more complicated and I can't be bothered to show that right now.

Circles inclined from 0 (outer) to 80 degrees in steps of 10 degrees.

When you're dealing with faint fuzzy blobs, I can easily believe that errors of tens of degrees are possible. The authors stop short of saying that the previous results suffered from this problem, as they did have gas maps as well as the optical, but it's hard to see the obvious implication. So maybe UDGs don't have anything much unusual about their dynamics at all, which re-opens the whole controversy about galaxies devoid of dark matter. We'll see.

Systematically Measuring Ultra Diffuse Galaxies in HI: Results from the Pilot Survey

We present neutral hydrogen (HI) observations using the Robert C. Byrd Green Bank Telescope (GBT) of 70 optically-detected UDG candidates in the Coma region from the Systematically Measuring Ultra-Diffuse Galaxies survey (SMUDGes). We detect HI in 18 targets, confirming 9 to be gas-rich UDGs and the remainder to be foreground dwarfs.

Ghostly gas or just a ghost ?

Once upon a time, there was a lovely little gas cloud that turned into a star. That much we know. What we know a lot less about is exactly how gas clouds assemble into nice happy star-forming galaxies.

We know for certain that galaxies can lose gas through mutal interactions - we can directly see spectacular tails and streams that closely match predictions as to what such features should look like. But how gas gets into galaxies is much more controversial. Several streams have been proposed as signatures of accretion over the years, but it's never been very clear to me why any of them are more likely accretion features rather than tidal tails. If accretion happens, why don't we see it everywhere ?

This paper describes some exceptionally deep observations of two nearby edge-on galaxies, reaching density levels about ten times (or more) lower than more typical values. They do this is the old-fashioned way, by sheer observing time. That means they're limited to a few different "pointings" per galaxy - they don't produce any shiny maps, just spectra.

What they find, though, is in my opinion extremely suspicious. They detect neutral gas well above and below the plane of the discs, but it very closely matches the velocity profile of the disc - to within 10% or so of the velocity width. That just doesn't make any sense to me. Whatever the extraplanar gas is doing, there's no obvious reason why it should match the velocity width of the disc : tidal tails or infalling clouds should be all over the place. In some cases, they even see hints of a double-horn structure, a classic signature of a rotating disc. But you just shouldn't see that outside the disc itself : it couldn't be stable.

To be fair, this isn't seen everywhere. Some emission closer to the disc is a markedly different shape, but still with a very clear cutoff at the same velocity width of the disc. Going further out, in one case the emission appears to drop and then get stronger again. That sounds a lot like a sidelobe detection to me - detecting the galaxy's disc again because the sensitivity profile of the dish varies non-linearly with distance from the target.

EDIT : Although they don't explicitly discuss sidelobes, they do something much better and show a figure of the beam strength. While they claim that the emission they detect is well above the beam strength, I find it even more suspicious that their detected emission peaks at locations very close indeed to the sidelobes.

Perhaps I'm wrong. At least a few of the authors are way more experienced than me, but they only mention sidelobes once in passing (to say that they're weak). They discuss several possible origins for the gas, but rarely address that suspicious similarity in velocity widths except to note that it's "close to the velocity range expected for random motions in the halos of the galaxies". I don't find that terribly likely, so I'm far from convinced. I should get back to my cloud-mapping project...

Detection of the diffuse HI emission in the Circumgalactic Medium of NGC 891 and NGC 4565

We present detections of 21-cm emission from neutral hydrogen (HI) in the circumgalactic medium (CGM) of the local edge-on galaxies NGC 891 and NGC 4565 using the Robert C. Byrd Green Bank Telescope (GBT). With our 5$σ$ sensitivity of $8.2 \times 10^{16}$ cm$^{-2}$ calculated over a 20 km s$^{-1}$ channel, we achieve $>5σ$ detections out to $90-120$ kpc along the minor axes.

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

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

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

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

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

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

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

* Sadly not in these exact words.

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

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

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

INTERMISSION

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

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

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

Got that  ? Good.

WE NOW RETURN TO OUR FEATURE PRESENTATION

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

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

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

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


2. Dark matter is self-interacting

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

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


3. We're interpreting the velocities incorrectly

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

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


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

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

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

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

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

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



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

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

Baryonic clues to the puzzling diversity of dwarf galaxy rotation curves

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

Six strangely slowly spinning spacey starry systems

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

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

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

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

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

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

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

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

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

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

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

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

Bringing wide open spaces indoors

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

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

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

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

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

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

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

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

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

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

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

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

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