ChatGPT-5 Versus Me

It's time for another round of evaluating whether ChatGPT is actually helpful for astronomical research.

My previous experiments can be found here, here, and here. The first two links looked at how well ChatGPT and Bing performed when analysing papers I myself know very well, with the upshot being an extreme case of hit-and-miss : Occasional flashes of genuine brilliance wrapped in large doses of mediocrity and sprinkled with total rubbish, to quote myself. All conversations had at least one serious flaw (though arguably in one case, which was factually and scientifically perfect but had crippling format errors).

The third link tested ChatGPT's vision analysis by trying to get it to do source extraction, which was a flat-out failure. Fortunately there have been other tests on this which show it does pretty badly in more typical situations as well, so I'm not going to bother redoing this.

With the release of ChatGPT-5, however, I do want to redo the analysis of papers. If I can have ChatGPT give me reliable scientific assessments of papers, that's potentially a big help in a number of ways, at the very least in determining if something is going to be worth my time to read in full. For this one I picked a new selection of papers as my last tests were a couple of years ago, and I can't claim I remember all their details as well as I did. 

Because all the papers cover different topics, there isn't really a good way to standardise the queries. So these tests are designed to mimic how I'd use it in anger, beginning with a standardised query but then allowing more free-ranging, exploratory queries. There's no need for any great numerical precision here, but if I can establish even roughly how often GPT-5 produces a result which is catastrophically wrong or useless, that's useful information.

I began each discussion with a fairly broad request :

I'd like a short summary of the paper's major findings, an evaluation of its scientific importance and implications, what you think the major weaknesses (if any) might be and how they could be addressed. I might then ask you more detailed, specific questions. Accuracy is paramount here, so please draw your information directly from the paper whenever possible – specify your sources if you need to use another reference.

Later I modified this to stress I was interested in the strengths and weaknesses of the scientific interpretation as well as methodology, as GPT-5 seemed to get a little hung up on generic issues – number of sources, sensitivity, that sort of thing. I followed up the general summaries with specific questions tailored to each individual paper as to what they contained and where, this being a severe problem for earlier versions. At no point did I try to deliberately break it – I only tried to use it.

Below, you can find my summaries of the results of discussions about five papers together with links to all of the conversations.

0) To Mine Own Research Be True ?

But first, a couple of examples where I can't share the conversations because they involve current, potentially publishable research (I gave some initial comments already here). I decided to really start at the deep end with a query I've tried many times with ChatGPT previously and got very little out of it : to have it help with a current paper I'm writing, asking it to assess the merits and problems alike – essentially acting as a mock reviewer. 

* Of which the management, like the rest of us, is generally sensible about such things. We all recognise the dangers of hallucinations, the usefulness and limitations of AI-generated code, etc. Nobody here is a fanboy nor of the anti-AI evangelical sort.

Previously I'd found it to be very disappointing at this kind of task. It tends to get hung up on minutia, not really addressing wider scientific points at all. For example, if you asked it for which bits should be cut, it might pick out the odd word or sentence or two, but it wouldn't say if a whole section is a digression from the main topic. It didn't think at scale, so to speak. It's hard to describe precisely but it felt like it has no understanding of the wider context at all.; it discussed details, not science. It wasn't that using it for evaluations was of no value whatsoever, but it was certainly questionable whether it was a productive use of one's time.

With the current paper I have in draft, ChatGPT-5's response was world's apart from its previous meagre offerings. It described itself as playing the role of a "constructively horrible" reviewer (its own choice of phrase) and it did that, I have to say, genuinely very well. Its tone was supportive but not sycophantic. It suggested highly pertinent scientific critiques, such as the discussion on the distance of a galaxy – which is crucial for the interpretation in this case – being too limited and alternatives being fully compatible with the data. It told me when I was being over-confident in phrasing, gave accurate indications of where I was overly-repetitive, and came up with perfectly sensible, plausible interpretations of the same data. 

Even its numbers were, remarkably, actually accurate* (unlike others I haven't seen it make some classic errors in basic facts and numbers, including the number of specified letters even in fictional words; I tried reproducing some of these multiple times but couldn't). At least, that is, those I checked, but all those I checked were on the money – a far cry indeed from older versions ! Similarly, citations were all correct and relevant to its claims : none were total hallucinations. That is a big upgrade.

* ChatGPT itself claims that it does actual proper calculations whenever the result isn't obvious (like 2+2, for which training data is enough) or accuracy is especially important.

When I continued the discussion... it kept giving excellent, insightful analysis; previous versions tended to degenerate into incoherency and stupidity in long conversations. It wasn't always right – it made one major misunderstanding to one inquiry that I thought it should have avoided* – but it was right more than, say, 95% of the time, and its single significant misunderstanding was very easily corrected**. If it was good for bouncing ideas off before, now it's downright excellent. 

* This wasn't a hallucination as it didn't fabricate anything, it just misunderstood the question.
** And how many conversations with real people feature at least one such difficulty ? Practically all of them in my experience.

The second unshareable test was to feed it my rejected ALMA proposal and (subsequently) the reviewer responses. Here too the tone of GPT-5 shines. It phrased things very carefully but without walking on eggshells, explaining what the reviewer's thought processes might have been and how to address them in the future without making me feel like I'd made some buggeringly stupid mistake. I asked it initially to guess how well the proposal would have been ranked and it said second quartile, borderline possibility for acceptance... praiseworthy and supportive, but not toadying, and not raising false hopes.

When I told it the actual results (lowest quartile, i.e. useless), it agreed that some of the comments were objectionable, but gave me clear, precise instructions as to how they could be countered. Those are things I would find extremely difficult to do on my own : I read some of the stupider claims ("the proposal flow feels a bit narrative"... FFS, it damn well should be narrative and I will die on this hill) and just want to punch the screen*, but GPT-5 gave me ways to address those concerns. It said things like, "you and I know that, but...". 

* No, not really ! I just need to bitch about it to people. Misery loves company, and in a perverse bit of luck, nobody in our institute got any ALMA proposals accepted this year either.

It made me feel like these were solvable problems after all. For example, it suggested the rather subtle reframing of the proposal from detection experiment (which ALMA disfavours) to hypothesis testing (which is standard scientific practise that nobody can object to). This is really, really good stuff, and the insight into what the reviewers might have been thinking, or not understanding, made me look at the comments in a much more upbeat light. Again, it had one misunderstanding about a question, but again this was easily clarified and it responded perfectly on the second attempt.

On to the papers !

1) The Blob(s)

This paper is one of the most interesting I've read in recent years, concerning the discovery of strange stellar structures in Virgo they attribute to being ram pressure dwarfs. Initially I tried to feed it the paper by providing a URL link, but this didn't work. As I found out with the second paper, trying to do it this way is a simply mistake : in this and this alone does GPT-5 consistently hallucinate. That is, it claims it's done things which it hasn't done, reporting wrong information and randomly giving failure messages.

Not a great start, but it gets better. When a document is uploaded, hallucinations aren't quite eliminated, but good lord they're massively reduced compared to previous versions. It's weird that its more general web search capabilities appear rather impressive, but give it a direct link and it falls over like a crippled donkey. You can't have everything, I guess.

Anyway, you can read my full discussion with ChatGPT here. In brief :

• Summary : Factually flawless. All quoted figures and statements are correct. It chose these in a sensible way to give a concise summary of the most important points. Both scientific strengths and weaknesses are entirely sensible, though the latter are a little bland and generic (improve sensitivity and sample size, rather than suggesting alternative interpretations).
• Discussion : When pressed more directly for alternative interpretations, it gave sensible suggestions, pointing out pertinent problems with the methodology and data that allow for this.
• Specific inquiries : I asked it about the AGES clouds that I know are mentioned in this paper (I discovered them) and here I encountered the only real hallucination in all the tests. It named three different AGES clouds that are indeed noteworthy because they're optically dim and dark ! These are not mentioned in this paper at all. When I asked it to check again more carefully, it reported the correct clouds which the authors refer to. When I asked it about things I knew the paper didn't discuss, it correctly reported that the paper didn't discuss this.
• Overall : Excellent, once you accept the need to upload the document. Possibly the hallucination might have been a holdover from that previous attempt to provide the URL, and in my subsequent discussions I emphasised more strongly the need for accuracy and to distinguish what the paper contained from GPT-5's own inferences. This seems to have done the trick. Even with these initial hiccups, however, the quality of the scientific discussions was very high. It felt like talking with someone who genuinely knew what the hell they were talking about.

2) The Smudge

This one is about finding a galaxy so faint the authors detected it by looking for its globular clusters. They also find some very diffuse emission in between them, which is pretty strong confirmation that it's indeed a galaxy of sorts.

At this point I hadn't learned my lesson. Giving ChatGPT a link caused it to hallucinate in a sporadic, unpredictable way. It managed to get some things spot on but randomly claimed it couldn't access the paper at all, and invented content that wasn't present in the paper. Worse, it basically lied about its own failures.

You can read my initial discussion here, but frustrated by these problems, I began in a second thread with an uploaded document here. That one, I'm pleased to say, had no such issues.

• Summary : Again, flawless. A little bland, perhaps, but that's what I wanted (I haven't tried asking it for something more sarcastic). The content was researcher level rather than general public but again I didn't ask for outreach content. It correctly highlighted possible flaws like the inferred high dark matter content being highly uncertain due to an extremely large extrapolation from a relatively novel method.
• Discussion : In the hallucinatory case, it actually came up with some very sensible ideas even though these weren't in the paper. For example, I asked it about the environment of the galaxy and it gave some plausible suggestions on how this could have contributed to the object's formation – the problem was that none of this was in the paper as it claimed. Still, the discussion on this – even when I pushed it to ideas that are very new in the literature – was absolutely up to scratch. When I suggested one of its ideas might be incorrect, it clarified what it meant without changing the fundamental basis of its scenario in a way that convinced me it was at least plausible : this was indeed a true clarification, not a goalpost-shifting modification. It gave a detailed, sensible discussion of how tidal stripping can preferentially affect different components of a galaxy, something which is hardly a trivial topic.
• Specific inquiries : When using the uploaded document, this was perfect. Numbers were correct. It reported correctly both when things were and weren't present in the article, with no hallucinations of any kind. It expanded on my inquiries into more general territory very clearly and concisely.
• Overall : Great stuff. Once again, it felt like a discussion with a knowledgeable colleague who could both explain specific details but also the general techniques used. Qualitatively and quantitatively accurate, with an excellent discussion about the wider implications.

3) ALFALFA Dark Galaxies

My rather brief summary is here. This is the discovery of 140-odd dark galaxy candidates in archival ALFALFA HI data. The ChatGPT discussion is here. This time I went straight to file upload and had no issues with hallucinations whatsoever.

• Summary : Once again, flawless. Maybe a little bland and generic with regard to other interpretations, but it picked out the major alternative hypothesis correctly. And in this case, nobody else has come up with any other better ideas, so I wouldn't expect it to suggest anything radical without explicitly prompting it to.
• Discussion : It correctly understood my concern about whether the dynamical mass estimates are correct and gave a perfect description of the issue. This wasn't a simple case of "did they use the equation correctly" but a contextual "was this the correct equation to be using and were the assumptions correct" case, relating not just to individual objects but also their environment. Productive and insightful.
• Specific inquiries : Again flawless, not claiming the authors said anything they didn't or claiming they didn't say anything they did. Numbers and equations used were reported correctly.
• Overall/other: Superb. I decided to finish by asking a more social question – how come ALFALFA have been so cagey about the "dark galaxy" term in the past (they use the god-awful "almost darks", which I loathe) but here at least one team member is on board with it ? It came back with answers which were both sociologically (a conservative culture in the past, a change of team here) and scientifically (deeper optical data with more robust constraints) sensible ideas. It also ended with the memorable phrase, "[the authors are] happy to take the “dark galaxy” plunge — but with the word “candidate” as a fig leaf of scientific prudence."

4) The VCC 2034 System

This is a case of a small fuzzy patch of stars near some larger galaxies, possibly with a giant HI stream, which has proven remarkably hard to explain. The latest paper, which I summarise here, discounts the possibility that it formed from the long stream as it apparently doesn't exist, but (unusually) doesn't figure out an alternative scenario either. The ChatGPT discussion is here.

• Summary : Factually perfect, though it didn't directly include that the origin of the object was unknown. Arguably "challenges simple ram-pressure stripping scenarios and suggests either an intergalactic or pre-cluster origin" implies this, but I'd have preferred it to state it more directly. Nevertheless, the most crucial point that previous suggestions don't really hold up came through very clearly.
• Discussion : Very good, but not perfect. While it didn't get anything wrong, it missed out the claims in the paper against the idea of ram pressure dwarfs more generally (about the main target object of the study it was perfect). With some more direct prompting it did eventually find this, and the ensuing discussion was productive, pointing out some aspects of this I hadn't considered. I'm not entirely convinced this was correct, but no more than I doubt some of the claims made in the paper itself – PhD level hardly means above suspicion, after all. And the discussion on the dynamics of the object was extremely useful, with ChatGPT again raising some points from the paper I'd completely missed when I first read it; the discussion on the survival of such objects in relation to the intracluster medium was similarly helpful.
• Specific inquiries : Aside from the above miss, this was perfect. When I asked it to locate particular numbers and discuss their implications it did so, and likewise it correctly reported when the paper didn't comment on a topic I asked about. 
• Overall : Not flawless, but damn good, and certainly useful. One other discussion point caused a minor trip-up. When I brought in a second paper (via upload) for comparison and mentioned my own work for context, it initially misinterpreted and appeared to ignore the paper. This was easily caught and fixed with a second prompt, and the results were again helpful. By no means was this hallucination – it felt more like it was getting carried away with itself.

5) An Ultra Diffuse Galaxy That Spins Too Slowly

This was a paper that I'd honestly forgot all about until I re-read my own summary. It concerns a UDG that initial observations indicated lacked dark matter entirely, but then another team came along and found that would be unsustainable and it was probably just an inclination angle measurement error. Then the original team came back with new observations and simulations, and they found it does have some dark matter after all – at a freakishly low concentration, but enough to stabilise it. The ChatGPT discussion is here.

• Summary : As usual this was on the money, bringing in all the key points of the paper and giving a solid scientific assessment and critique. Rather than dealing with trivialities like sample size or simulation resolution, it noted that maybe they'd need to account more for the effects of environment or using different physics for the effects of feedback on star formation. 
• Discussion : As with the fourth paper, this was again excellent but not quite complete. It missed out one of my favourite* bits of speculation in the paper that this object could tell us something directly about the physical nature of dark matter. It did get this with direct prompting, but I had to be really explicit about it. To be fair, this is just one paragraph in the whole article, but reading between the lines I felt it was a point the author's really wanted to make. On the other hand, that's just my opinion and it certainly isn't the main point of the work. 
• Specific inquiries : Yep, once again it delivered the goods. No inaccuracies. It reported the crucial points correctly and described the comparisons with previous works perfectly. Again, it didn't report any claims the authors didn't make, 
• Overall : Excellent. I allowed myself to branch out to a wider discussion of the cold dark matter paradigm and it came back with some great papers I should check out regarding stability problems in MOND. It sort of back-peddled a little bit on discussions about the radial acceleration relation, but this was more a nuanced clarification than revising its claims : CDM gets RAR as a result of baryonic physics tuning, but it gets this for free as a result of tuning for other parameters rather than directly for RAR itself; MOND gets RAR as a main feature. If that's not a PhD level discussion then I don't know what is.

* More generally, it seems pretty good at picking up on the same stuff that I do, but it would be silly to expect 100% alignment.

Summary and Conclusions

On my other blogs I've gone on about the importance of thresholds. Well, we've crossed one. Even the more positive assessments of GPT-5 tend to label it as an incremental upgrade, but I violently disagree. I went back and checked my earlier discussion with GPT-4o about my ALMA proposal and confirmed that it was mainly spouting generic, useless crap... GPT-5 is a massive improvement. It discusses nuanced and niche scientific issues with a robust understanding of their broader context. In other threads I've found it fully capable of giving practical suggestions and calculations which I've found just work. Its citations are pertinent and exist. 

This really does feel like a breakthrough moment. At first it was a cool tech demo, then it was a cool toy. Now it's an actually useful tool for everyday use – potentially an incredibly important one. Where people are coming from when they say it gets basic facts wrong I've honestly no idea. The review linked above says it gave a garbage response when fed a 160+ page document and was anything but PhD-level, but in my tests with typical length papers (generally 12-30 pages) I would absolutely and unequivocally call it PhD level. No question of it.

This is not to say it's perfect. For one thing, even though there's a GUI setting for this, it's very hard to get it to stop offering annoying follow-up suggestions it could do. This is why you'll see my chats with it sometimes ends with "and they all live happily ever after", because I had to put that in my custom instructions to give it an alternative ending (in one memorable case it came up with "one contour to rule them all and in the darkness bind them"*). Even then it doesn't always work. And it always delivers everything in bullet-point form : no doubt this can be altered, but I haven't tried... generally I don't hate this though.

* I really like the personality of GPT-5. It's generally clear and to the point, straightforward and easy to read, but with the occasional unexpected witticism that keeps things just a little more engaging.

Of course, it does still make mistakes. Misinterpretations of the questions appears to be the most common, but these are very easily spotted and fixed. Incompleteness seems to be less common but more serious, but I'd stress that expecting perfection from anything is extremely foolish. And actual hallucinations of the kind that still plagued GPT-4 are now nearly non-existent, provided you give it rigorous instructions.

So that's my first week with GPT-5, a glowing success and vastly better than I was expecting. Okay, people on reddit, I get that you missed the sycophantic ego-stroking personality of GPT-4, so whine about how your virtual friend has died all you want. But all these claims that it's got dumber, and has an IQ barely above that of a Republican voter... what the holy hell are you talking about ? That makes NO sense to me whatsoever.

Anyway I've put my money where my mouth is and subscribed to Plus Watch this space : in a month I'll report back on whether it's worth it.

Stop stripping the dwarves, they don't like it !

Today's paper revisits a very minor but interesting storm in a teacup.

Back in 2021, Junais et al. reported on a possible Ultra Diffuse Galaxy losing gas in the Virgo Cluster. At face value it all looked very convincing. The HI gas detection was very clear, nicely offset from the UDG-candidate (basically an especially faint, fluffy sort of galaxy if you aren't keeping up with things – shame on you !) but still overlapping it. At the very centre of the gas detection was a sort of ragged line of blue starlight, plus there were some patches of stars scattered about as well. It's all very much as you'd expect if this was star formation occurring in the stripped material.

Okay, ram pressure is old hat. But to find evidence of this occurring in a UDG would be especially interesting : it would allow us to start investigating whether UDGs in clusters (which seem to be pretty common) are the same as those in the general field (which are known to exist, but we don't know how many there are). In particular, there's this whole controversy over whether they lack dark matter or not, in which case the effects of stripping might be quite different since the gravitational forces involved would be much less. And also it would show whether both cluster and field UDGs form by the same process, or whether there are multiple ways to form the same sort of objects.

All this was strongly challenged by Jones et al. 2021  They said, no, hang on, the distances are all wrong. Using high-resolution Hubble data, they were able to show that the distance to the UDG-candidate is actually much closer than the Virgo Cluster, and it also seemed to be linked to another, much brighter galaxy (VCC 2034) by a giant bridge of HI, so presumably that would also be at the same distance. The patchy starlight, however, could well be in the Cluster, in which case it would require a different explanation because it doesn't look anything like a galaxy.

You may or may not remember that I'm moderately skeptical about all this. It's not that I don't believe the distance estimate... it's that I'm wary about them after that whole "ping-pong" series of papers concerning some other UDGs – a debate which apparently still isn't fully settled. That suggests we shouldn't take any single value as definitive but should wait for multiple analyses. 

And the HI stream... although I did send Jones some of our deeper (WAVES) data, and he was able to find the stream by taking a slice through at the right angle... it feels very off to me. Given that our HI data is about 3-4x deeper than the original ALFALFA observations, I'd expect it to be immediately obvious in our data. It isn't. My suspicion is that the analysis and source-finding package SoFiA (which is hella powerful) is oversmoothing here, creating the appearance of a bridge because of the degraded resolution, as is used for increasing sensitivity.

I don't know for sure though. I'm moderately skeptical, but no more than that.

Enter today's paper by Yu-Zhu Sun and friends. They use a combination of new deep data from FAST and high resolution data from the VLA. And this paints a pretty convincing picture that the patchy starlight is not the result of gas stripping from VCC 2034, even if they agree that it's nothing to do with the UDG candidate. This is my favourite sort of paper in that it doesn't actually solve the mystery but just demonstrates that things were even weirder than initially thought.

This is all quite complicated : there are several different galaxies in this region, plus the fuzzy stars, plus the possible gas bridge, and conflicting distance claims for all of them. Let's try a few simple diagrams to illustrate. I'll start with the main hypotheses proposed by Junais and Jones. But, since both groups quite rightly caveat their conclusions and aren't definitive, and don't all deal with the same objects, I'm going to try and standardise and simplify things a little bit. This should be enough to get a general sense of what's going on, but this is very much a limited guide. Needless to say, these are not to scale !

Hypothesis 1

The most straightforward interpretation is the original : that the fuzzy starlight ("The Fuzz", a.k.a. AGC 226178) close to the UDG candidate (here UDG-X, official designation NGVS 3543) and is the result of star formation in its stripped gas. The nearby pair of galaxies VCC 2034/2037 are seemingly unrelated. 

All galaxies, in this scenario, are in Virgo at about 17 Mpc distance from us. The gas cloud associated with UDG-X and the Fuzz align well and VCC 2034/2037 is rather far away, so an association isn't at all natural. VCC 2034 has its own gas, showing clear signs of removal. In fact this extends in the direction of UDG-X but doesn't reach nearly far enough, so the orientation doesn't appear to indicate anything interesting. It's also aligned with VCC 2037, but that too is imperfect (not covering the whole of VCC 2037 and the local maxima of the gas is not aligned with the galaxy's centre) and the velocities of the two galaxies don't match well. So this too may just be a coincidence – the two objects might both be in the cluster, but at sufficiently different distances that they aren't actually related. Regardless, they really don't seem to have anything to do with UDG-X at all.

Hypothesis 2

The second scenario relies on a number of additional observations. Direct distance estimates suggest that both UDG-X and VCC 2037 are at 10 Mpc, much closer than the Virgo Cluster (17 Mpc, estimated elsewhere to be be 1-2 Mpc deep). However the Fuzz seems still to be at the cluster distance, and there's a much larger bridge of HI apparently connecting it to VCC 2034. So essentially, the Fuzz results from gas stripping of the cluster member VCC 2034, whereas UDG-X is so close to us that's actually not a cluster member at all : it may or may not relate to VCC 2037 instead. This would make UDG-X an uninteresting normal dwarf galaxy, but the Fuzz becomes very interesting as a rare example of star formation in a gas tail.

Note again that that the existence of the large HI envelope is uncertain, and that it's probably not a great idea to trust the distance estimates overmuch. Furthermore, as we're about to see, even the high resolution HI data can't be treated as gospel.

Hypothesis 3

Stressing that the latest paper is even more cautious, here's their essential idea : there's no big HI envelope and both VCC 2034/2037 show independent HI tails (in the new VLA data) that don't align with the Fuzz or UDG-X at all. UDG-X may well be foreground (again making it a normal dwarf galaxy), but neither it nor the Fuzz are directly related to any of the major galaxies in the general vicinity. What, then, is the origin of the Fuzz in this scenario ?

A tricky question indeed, one which they understandably don't commit to answering. Their main conclusion is that the Fuzz is likely not stable and in the process of disintegration, but as to what formed it in the first place, they don't (can't) say.

Disclaimer : I know a few of the co-authors very well, have published with them, and certainly hope to do so again ! They raise many excellent points, but there are a few with which I disagree. For example, they say that the HI cloud around the Fuzz has a "well-defined" velocity gradient of 10 km/s, but that's the width of the HI line itself so I'm very skeptical that this can be in any sense meaningful.

They do, however, have both new, extremely sensitive FAST data (even slightly deeper than WAVES), and new VLA data which should be of even higher resolution than the earlier observations. The FAST data fails to show the large HI envelope, as does WAVES – and taken together this seems to quite reasonably disprove its existence. I had in mind a simple project to see if this could really result from how the data was processed... maybe one day I'll have the time to try it, as it would be nice to know exactly how this happened if indeed it doesn't exist.

What about UDG-X ? The FAST data is highly sensitive but low resolution, and can't distinguish gas associated with the Fuzz (which definitely does exist) from UDG-X. The Sun et al. VLA data, however, shows much less of a head-tail morphology than the earlier data, now appearing to only be associated with the Fuzz. That makes it unlikely that Fuzz is the result of gas stripping from UDG-X, though it can't be said with too much confidence. There could still be diffuse gas in UDG-X which the VLA wouldn't detect, or the entire gas of the Fuzz might have been displaced wholesale from UDG-X.

And when they say they detect a velocity gradient in this case, it looks a lot more like a very sudden change to me. Their dynamical mass estimates – how much mass is needed to keep the system stable – are, I think, stretching things beyond the quality that the data can sustain, given how narrow the velocity width of the object is. That said, they say the total amount of dark matter that would be present is so low that this is unlikely be a dark/dim galaxy candidate : more likely it's some form of debris. That seems entirely reasonable from the low line width, even if I'd be skeptical about the exact dark matter mass estimate.

But is the debris stable ? That's much harder to answer. A lot of recent work has found candidates for so-called "blue blobs", which are interpreted as gravitationally-bound clumps of gas and stars that formed by the removal of gas from ordinary galaxies by ram pressure. In essence this would be a new class of stellar system, not really galaxies in the classical sense (since they'd have no dark matter) but not star clusters either (being very much larger and formed by a totally different mechanism).

Personally I rather like this idea, but here they place a few well-aimed holes in the scenario. The high metallicity of the clouds seemed in Jones like strong evidence that the clouds originated from within galaxies, as otherwise their chemistry should be basically hydrogen and bugger all else – you need prolonged star formation to cause significant enrichment, which isn't going to happen at their current pathetic levels of star formation activity. But here they say it could happen through mixing with the gas in the cluster itself. On the other hand, the paper they cite in support of this says that metallicity should drop with distance from the parent galaxy, whereas all the blue blobs have essentially the same high metallicity value. So this is an interesting critique, but not a fully convincing one.

Similarly, they're rather skeptical of the whole pressure confinement scenario for blue blobs – the idea here being that the gas within the cluster helps prevent them from disintegration. Now when we simulated this for dark clouds with very high velocity dispersions, we found it flat-out didn't work. But we were investigating rather exceptional systems, and simulations of low velocity dispersion systems have found very much more favourable results (as you'd expect anyway : with a low dispersion, things can only expand more slowly by definition). So I think their toy model is overthinking things. In any case, given the extremely low dispersion of the Fuzz's gas cloud, it would only expand by 10 kpc in a billion years... even if it is technically disintegrating, it's doing so so slowly that it might as well not be.

Finally, I don't agree at all with their interpretation regarding the location of the blue blobs within the cluster. The previous paper by Dey suggested that they're found in regions of modest cluster gas density because this is where they can both form and survive for a while; they avoid the denser core because this would rapidly destroy them. But Sun et al. claim that a "more natural" suggestion is that actually these objects are all outside the cluster in 3D space and only appear projected against it. Surely, though, if that were the case, we'd be equally likely to see such objects projected against the core ! To me, that the distribution of the objects relates to the geometry of the cluster feels like extremely compelling evidence that they are indeed within the cluster.

The long and short of it is that this is a very complex system, and it all serves to underscore that even observations don't always get the last word. It's particularly interesting that the new VLA data looks markedly different to the earlier findings, showing distinctly different structures. Likewise, I have to wonder why everyone is treating the distance estimates with such high confidence, given recent prominent debacles about how damn difficult it is to get these right.

As it stands, it now looks a lot less likely that the origin of the Fuzz can be explained by a giant gas stream from VCC 2034. But I, for one, am by no means convinced that we can rule out the original suggestion of stripping from a UDG, and I downright disagree that we can be so confident that it's a disintegrating gas cloud rather than a ram pressure dwarf. It's likely not a dark galaxy, however. 

Which leaves the usual question of : what would it take to resolve all this ? This is very tricky. Well, the question of the long gas stream could be easily answered by running SoFiA over data sets with artificial signals injected of similar configurations to the current system; if the long stream results from oversmoothing, this ought to be reproducible. Distance measurements are much harder to resolve unambiguously, but at a minimum, another team need to try this independently, preferably using different data. As to why the various VLA data of the same objects looks so different, however, I'm at a loss. It's definitely a weird system, but certainly an interesting weird.

The Bunny Rabbit of Death

Today's paper is a bit more technical than usual, but sometimes you've gotta tackle the hard stuff.

Ram pressure stripping is something we seem to understand pretty well on a large scale. When galaxies enter a massive cluster containing its own gas, pressure builds up that can push out the gas in the galaxy. If it's going fast enough, and/or the cluster gas is dense enough, then the galaxy can loose all of its gas pretty quickly. No ifs or buts, it just looses all its gas, stops forming stars, realises it's made incredibly poor life choices, and dies.

Yeah, literally, it dies. It's run out of fuel for star formation, which means all its remaining massive blue stars aren't replaced when they explode as supernovae in a few million years. Slowly it turns into a "red and dead" smooth, structureless, boring disc, and maybe eventually an elliptical. There's a wealth of evidence that ram pressure is the dominant mechanism of gas loss within clusters, and everything seems to just basically... work. Which is nice.

But, as ever, the details are where it gets interesting. In the extreme case, what you'll see is a galaxy with a big long tail of gas, one single plume stretching off until it's torn apart and dissolved in the chaos of the cluster. 

Even here things can be complicated though. Some tails seem to have multiple components : extremely hot X-ray emitting gas, cooler neutral atomic hydrogen detectable with radio telescopes, intermediate temperature ionising gas that emits over very narrow "Hα" optical wavelengths, and very cold gas indeed that emits in the sub-mm regime. They may or may not have stars forming within the plume, and all of these different components can have radically different structures. Or they might all line up quite neatly. Sometimes all of these phases are present, sometimes just one or two.

And then, if a galaxy isn't in the extreme case, it can be even more complicated. If the ram pressure isn't enough to accelerate the gas to escape velocity, it can still be pushed out only to fall back in somewhere else in the disc. In short, it gets messy.

This paper attempts to understand one of those messy cases. It's part of the ALMA JELLY program, a large ALMA observing program run by my officemate Pavel Jachym (conflict of interest : declared ! BOX TICKED). Here they introduce the first analysis of one of their 28 target galaxies and tackle the important question (though they would never dare state it thus) : 

Why does it look like the Playboy bunny rabbit ?

Wait, wait... why is it called ALMA JELLY ? It's not an acronym as far as I know. Instead, "jellyfish" galaxies have become a popular name for galaxies experiencing ram pressure stripping as some of them have distinct, narrow tails that look very much like the tentacles of a jellyfish. The term has become somewhat abused lately, often used for any ram-pressure stripping galaxy regardless of what its tail looks like. Here they attempt to take back control of the term and define it as galaxies which have stars forming it their stripped material. This often occurs in narrow tendrils so it's a pretty good proxy for jellyfish-like structures, and highlights the unusual physics at work in these cases.

And, why ALMA ? ALMA observes the cold molecular gas, which is generally agreed to be the main component of star formation. The target here already has many observations at other wavelengths, but the molecular gas has been traditionally tough to observe. Now they can fill in the gap, and with extreme resolution too. 

So, the bunny rabbit. The first target for ALMA JELLY is NGC 4858. It's certainly a prime example of a jellyfish galaxy, with clear, bright tendrils of stars extending in one direction directly away from the centre of the Coma cluster in which it resides. It's also close to the cluster centre, where ram pressure ought to be very strong. Its got observations at a bunch of different wavelengths and it is, in short, a right proper mess. Really, it's the kind of thing I might be minded to throw up my hands and say, "hahahah no, I'm not touching that with a barge pole". Or, failing that, I might wave my hands furiously and say, "something something HYDRODYNAMICS !".

Hydrodynamic effects, the complicated interactions between two or more different fluids, are an easy get-out. Mixing of fluids causing extremely complex structures, so if something's a mess, it's a safe bet that hydrodynamics can explain it. Though, in that case you ought to run simulations to test if that really works or not.

Here they don't. Instead they try the much braver task of explaining it without any dedicated simulations, and even those simulations they do use don't have full hydrodynamic effects – just some very basic approximations of the major forces at work from the external gas. And yet they seem to have come up with a pretty convincing explanation.

It works like this. First, NGC 4858 is a grand design spiral, with two prominent spiral arms. As it rotates, each arm moves through a region where its subjected to varying ram pressure forces, which are greatest on the side rotating away from the cluster centre (where the gas is moving fastest away from the cluster, making it easiest to remove). A single, dense arm thus gives rise to a single, dense plume of gas – a tail. But this tail gas preserves some of the rotation it had around the galaxy's centre, so it doesn't just get blasted out into space – it keeps moving around the galaxy. This brings it into the shadow of the galaxy, protecting it from the wind of the cluster. Some of the gas is lucky enough that the greatly reduced ram pressure is now essentially impotent, and it falls back onto the galaxy.

Not all of it though. Some keeps going. If any makes it right around to the other side of the galaxy, it moves back into the zone of death and gets finally stripped away by the cluster gas once and for all. The key is that before it reaches this point, the gas gets compressed as it starts to hit the wind again. In the simulations they use as a reference, the galaxy doesn't have prominent spiral arms and shows a single prominent tail; they surmise that because NGC 4858 has two arms, this could naturally give rise to two tails (or ears).

Their observations also show direct evidence of gas returning to the galaxy. The ALMA observations allow them to make a velocity map of the gas, and there's one big feature which is discontinuous with the rest of the velocity structure. And again, that fits with the basic model of how they expect rotating gas to behave.

I've simplified and shortened this one quite a lot, missing out on any number of interesting details. And there's an awful lot more they could still do with this data. But to me, the first thing I wondered when I first saw the ALMA image was "why is it a bunny rabbit ?". I was expecting this to have a much more complex non-answer, featuring hand-waving and invocations to hydrodynamics galore, possibly involving a chicken sacrifice. As it is, they managed to come up with a decent explanation without any of that, which is no mean feat. Both the bunnies and the chickens can rest easy.

Now all they have to do is convince Playboy to give them a sponsorship deal...

The Miniscule Candidate

Following on from those couple of papers on possible dark galaxies, comes... another paper on dark galaxies !

This one is a completely different sort of beast. While identifying optically dark galaxies is normally done by looking for their gas instead of their stars, here they use good old-fashioned optical telescopes instead. Even weirder, having found something which is optically faint but not dark, they then go on to infer its dark matter content without measuring its dynamics at all !

If this all sounds very strange, that's because it is. It's by no means crazy, but it must be said that some of the claims here should be taken with a very large pinch of salt.

Let's go right back to basics. A good working definition of a galaxy is a system of gas and/or stars bound together by dark matter. True, there are some notable exceptions like so-called tidal dwarf galaxies, but it's questionable whether we shouldn't drop the "galaxy" for those objects altogether (maybe replace it with "system" or something instead). Clearly they're physically very different from most galaxies, which are heavily mass-dominated by their dark matter.

A dark galaxy, then, is just a dark matter halo with maybe some gas but definitely no stars. Or is it ? For sure, if it really has literally zero stars, then such an object would definitely count as a dark galaxy. But what if it had just one star and billions of solar masses worth of dark matter ? Would it really be worth getting hung up on that point ? Presumably the physics involved in its formation would be basically the same as a truly dark object.

Generally speaking, most people would allow an object to qualify as a dark galaxy even if it had some small mass in stars. At present there's no strict definition, however, and so few candidate objects are known that setting a quantitative limit wouldn't really help. Right now, we don't know nearly enough about the physics of the formation of such objects, and indeed the jury's still out on when any of them exist at all. 

(Some people prefer the term "almost dark", which annoys me intensely. I prefer to call them dim when they have some detectable stars, but it hasn't caught on).

Anyway, you can see how this explains using an optical telescope to search for dark galaxies. But actually, here they go a step further. Rather than looking for the ordinary stellar emission from galaxies, which are normally in diffuse discs, they look only for the light emitted by the compact, relatively bright globular clusters. Most galaxies have these dense starballs which orbit around in their halos quite separately from their main stellar disc. What these authors are looking for are cases where they find groups of globular clusters without an accompanying disc : essentially, star clusters orbiting all by themselves in their dark matter halos. 

This is an interesting grey area in terms of calling something a dark galaxy, but I'd be inclined to say such objects would qualify. The physics at work in forming dense globular clusters and the diffuse stellar disc is quite different, so at the very least, these would certainly be extremely interesting.

Here they present the imaginatively named "Candidate Dark Galaxy 2". Really ? Yes, really. That's the name they're going with. Bravo, team.

(Actually, snarkcasm aside, this is a wee bit insulting, considering that there have been many candidate dark galaxies over the years, but I'll let that pass).

It turns out they had a previous candidate (you can guess the name) which is even more extreme than this one. CDG-1* consists of four globular clusters in close proximity to each other with no detectable diffuse emission between them at all. I won't attempt to discuss the complicated statistical methods they use to identify globular clusters without parent galaxies; at the words "trans-dimensional Markov chain" my eyes glazed over anyway. I can safely mention a few points though : 1) They don't have spectroscopic measurements of the globular clusters so they can't robustly estimate their distances*; 2) Their initial catalogues of globular cluster candidates are surely incomplete, but 3) Since they do careful inspection of the candidate cluster groups they do find, we can be confident that the associations they identify are real.

* I honestly can't remember if I heard about this at the time or not. I may have missed it or just forgotten about it.

* Spectroscopy gives you velocity, which is a very powerful constraint on (though not quite a direct measure of) distance.

CDG-2 initially consisted of three globular clusters, but here, using new data from Hubble and Euclid, they identify a fourth. While they still don't have spectroscopy, the new data confirms that the candidates are all unresolved. That means they cannot possibly be close objects, and in fact their colours and other parameters are consistent with their being in the Perseus galaxy cluster* at 75 Mpc distance. So it seems very unlikely that they're either significantly closer or further away. And while their might be a few free-floating globular clusters in Perseus (ripped off their parent galaxies by tidal encounters and the like), it's not very likely that they'd happen to be so close together.

* This can sometimes get very confusing. A globular cluster is a cluster of stars that orbits around a parent galaxy; a giant galaxy might host, say, several dozen such objects. A galaxy cluster is a whole bunch of galaxies, each with their own population of globular clusters, all swarming around together.

The killer argument that this is highly likely to be an actual galaxy, though, is that here they detect diffuse stellar mission between the globular clusters. The thing just looks like a galaxy, albeit an extremely faint one. The chance of a tidal encounter creating something like this isn't worth considering.

Ahh, but is it a dark galaxy ? That's where things get a lot more speculative. While we can be pretty sure about the distance of the object and their physical association, only spectroscopic measurements would really give a good handle on the total mass. Measuring how fast things are moving lets you infer how much mass you need to hold them together. Without this, they rely on scaling relations, extrapolating based on the globular clusters to infer a massive amount of dark matter : probably there are a few million solar masses of stars present in total, but it could easily have a hundred billion solar masses of dark matter based on the scaling relations. 

These are, however, truly enormous extrapolations. Given that Ultra Diffuse Galaxies are now known which have significantly lower dark matter contents than typical galaxies, but these too have globular clusters, I'd be wary about digging any deeper into this one until they get some spectroscopy.

 Even so, it's clearly a very interesting object indeed. Arguably even more interesting, however, is CDG-1, which still has no diffuse emission detected at all. Even if the extreme dark matter content turns out to be a wrong estimate, if either of them have any at all, they're still super weird objects. Hopefully when they find CDG-3 I won't be caught quite so unawares.

They're Heee-re...

Or are they ?

Today, two papers on my favourite science topic of all : dark galaxies. In the past there have been a multitude of candidate detections but spread out very thinly. You get, I'd guestimate, of order one or two such claims per year on average, with the total number now being somewhere in the low to mid tens. And not a single one is entirely convincing. Every single object is essentially unique, with its own particular considerations that make it more and/or less likely to be a dark galaxy.

Both of these papers claim to have alleviated the problem by finding a whole bunch more candidates. The first uses new data from the ASKAP telescope and comes up with 55 potential objects, while the second uses archival Arecibo data and finds 142. Impressive stuff – but are any of them plausible, or have the previous problems just reappeared in a larger sample ?

There are many difficulties with identifying a dark galaxy candidate. The resolution of radio telescopes that can detect their gas content is often much lower than optical instruments, which means you see a big blurry smudge on the sky. That makes pinpointing the exact position of the gas difficult, so it's hard to say whether it has an optical counterpart or not. It also makes estimating its total mass tricky : for this you need a precise measure of its size, so without it you can't really say how much dark matter it really has. And even if you do have good resolution, you need good optical data as well to say if it's really dark or just very dim (though when you get to sufficiently dim objects the difference is arguably not that important).

An even bigger problem happens when you manage to overcome all this. Even if you have an isolated gas blob with the signatures of stable rotation that would need lots of dark matter to hold it together, and even if you're darn sure it's so optically faint that it might as well be dark... it's damn hard to say if the thing really is stable. You could just be seeing a bit of fluff leftover from some interaction or other, which can sometimes mimic the appearance of a dark galaxy. Nevertheless, there have been a few cases where "dark galaxy" at least looks like a very plausible explanation, if never any where we can be certain that's really what's been found.

Both of the papers attempt to do much the same thing though in slightly different ways. Starting with large HI samples (30,000 for ALFALFA and 2,000 for WALLABY) they combine this with optical data sets and trim them down in various ways : quality of the HI signal, confidence in the lack of optical counterpart, isolation, etc. ALFALFA (the Arecibo data) has an enormous area of coverage and huge sample size on its size, while WALLABY (from the ASKAP telescope) has higher sensitivity and resolution. 

Since even the final candidate catalogues are, by the standards of dark galaxy research, really quite large, I'd be reluctant to say, "yep, this is definitely the solution, hurrah chaps, we've found them !". But nor would I at all dismiss them out of hand. Rather I would look at both of these papers as being potentially the foundation of interesting research, but it's too soon for any definitive results yet. These are both very solid starts, but we need to examine each and every object here in more detail, or at least a subsample. We need higher resolution data in all cases, deeper optical data... and most importantly, detailed studies of the local environment. We need to find the quintessential case of an isolated object with no plausible other origins, preferably rotating nice and quickly (which would mean fast dissipation if it wasn't bound by dark matter).

All that requires very careful, detailed work. Which of course we can now do, so kudos to them for that. But scientifically I'm neither excited nor dismayed. I am... intrigued.

The first paper finds its dark galaxies pretty much everywhere throughout its fields. There's not really any distance bias, so they occur at all masses – a few at really quite respectable standards even when compared with optically bright galaxies. Line widths look to be typically around 100 km/s, which is where we'd naively expect rotation – and hence a dark matter component – to be needed for stability. Sadly the resolution isn't good enough for them to attempt dynamic mass estimates, though this seems to me a bit strange – they have the upper size limit from the HI, so they could at least put a broad constraint on it. 

The other oddity is that they model the optical light profile of all their sources, where detected. This is ideal for quantifying whether any are Ultra Diffuse Galaxies (which are possibly closely related to truly dark galaxies) but they don't seem to do this. Maybe that's for a future paper.

The second paper attempts a lot more science. I have to say it's both strange and refreshing to see a member of the ALFALFA team being at least a little more enthusiastic about dark galaxy candidates; normally they insist on calling them 'almost darks' – including the quotes – which gets very annoying. None of that here ! I should stress, though, that both papers absolutely treat everything with the caution it deserves, so don't mistake the brevity of my summary as evidence that they leap to conclusions. Neither group does that – I'm omitting the caveats just to get to the point.

Which for this second paper is as follows. As per the first, their candidates are everywhere, spanning a wide range of masses and line widths, but generally found in less dense environments than bright galaxies. They have higher gas fractions (relative to their inferred dark matter masses*) than optically bright galaxies of similar masses. And these properties are qualitatively similar to what's found in numerical simulations of galaxy formation that produce dark galaxies.

* Being a bit more gung-ho than the first group, they assume a size of the galaxy based on the scaling relation with respect to HI mass, hence they get a dark mass estimate.

All this is very matter-of-fact, commendably so. It's a huge sign of much how things have changed in the last couple of decades : when I went to my first conference, back in 2007, dark galaxies were viewed by many as... not exactly fringe, but not really mainstream either. Most people agreed that they could at least exist, but were skeptical of their whole raison d'être – that they would be numerous enough to explain why cosmological models were massively overpredicting how many galaxies we would see. Indeed, for the next few years if often felt as if hardly anyone really believed in the standard models of galaxy formation, even if nobody had any better ideas to replace it. Quite frankly, if anyone had suggested they'd found a hundred or more dark galaxy candidates, no matter how cautiously, they'd have been laughed at. It wouldn't have been a career-ending move but it wouldn't have won them any friends either.

All that seems to have largely faded. The original models of galaxy formation, where gas falls into dark matter halos and a bunch of complicated stuff happens, now seem very much more popular, and so dark galaxies no longer seem like an almost dirty subject. What's happened is that we've got a lot better at doing all that complicated stuff and many of the problems which looked horrendous now look, if hardly definitely solved, then at least an awful lot more solvable. 

So, good work people. It's going to be extremely interesting to see how this pans out over the next few years. Watch this space.

The Most Interesting Galaxies Are SMUDGES

Ultra Diffuse Galaxies remain a very hot topic in astronomy. You know the drill by now : great big fluffy things with hardly any stars and sometimes little or no dark matter, not really predicted in numerical simulations. I'm not going to recap them again because I've done this too many times, so I leave it as an exercise for the reader to search this blog and learn all about them. Get off yer lazy arses, people !

UDGs were first found in clusters but have since been found absolutely everywhere. Why clusters ? Well, because they're so faint, getting redshift (i.e. distance) measurements of them is extremely difficult. This means their exact numbers are fiendishly difficult to characterise : without distance you can't get size, which is one of their distinguishing properties – so without size you can't even count them. And if you can't count them, you can't really say much about them at all.

Getting distances in clusters, however, is much easier. There the distance to the whole structure is anyway known. The first studies found lots of UDG candidates in clusters but very few in control fields, so most of those are certainly cluster members rather than just being coincidentally aligned on the sky. Of course it's always possible that a small fraction (at the few percent level or less) weren't really in the cluster and therefore not truly UDGs, but statistically, the results were definitely reliable.

The SMUDGES project (Systematically Measuring UDGs) is a major effort to begin to overcome the limitation of relying on clusters for distance estimates*. In essence, they try to develop a similar procedure for clusters but which can be applied to all different environments. They want results which are at least statistically "good enough" to estimate the distance, even if there's some considerable margin of error. 

* The main alternative thus far has been gas measurements, which give you redshift without relying on the much fainter optical data. This, however, has its own issues.

This paper is mainly a catalogue, and to be honest I rarely bother reading catalogue papers. In fact I only read this one to see what low-level methods they used to do the size estimates, since we have some possible candidate UDGs of our own we want to check. But as it turned out, they also present some interesting science as well, so here it is.

Most of the paper is given to describing these methodologies and techniques. It's pretty dry but important stuff, and like with the first cluster-based studies, they can't be sure that absolutely every candidate they find is really a UDG. Actually these measurements are, inevitably, quite a lot less reliable than the cluster studies, but they're careful to state this and the results are still plenty good enough to identify interesting objects for further study.

One interesting selection effect they note early on is that studies of individual objects tend to overestimate their masses (compared to studies of whole populations), since these tend to be particularly big, bright, and prominent. This at least helps begin to explain why some division has arisen in the community regarding the nature of UDGs : the objects studied by different groups are similar only at a broad-brush level, and in detail they may have significant differences. That's not an explanatory bias that was obvious to me, but maybe it should have been. It seems perfectly sensible with hindsight, at any rate.

And, once again, this is another study where the authors resort to flagging dodgy objects by eye, in another example of how important it is to actually look at the data. The machines haven't replaced us yet.

I won't do a blow-by-blow description of their procedures this time, but their final catalogue comprises about 7,000 objects, which they supplement with spectroscopic data where available. One of the main topics they address is the big one : what exactly are UDGs ? Are they galaxies with normal, massive dark matter halos but few stars, or do they instead have weird dark matter distributions ?

They conclude... probably the former. But this is not to say that they are "failed Milky Way" galaxies that have just not formed many stars for some reason : at the upper end they're probably still a few times less massive than that, and at the lower end that might be more than a factor ten difference. So mostly dwarf galaxies, but with normal dark matter distributions and very few stars. They get mass estimates from a combination of counting the number of globular clusters, which correlates with the total halo mass in normal galaxies, and their own statistical method to estimate other galaxy properties (which I don't fully understand). 

These relations don't always work well, however, sometimes experiencing "catastrophic failure", by which they mean errors of an order of magnitude or more. Why this should be is impossible to say at this stage, but, intriguingly, might point to the dark matter distribution being indeed different in UDGs compared to normal galaxies, at least some of the time. Overall though this appears unlikely, because to make this work with the observed scaling relations, the dark matter would have to be more concentrated than expected, even though the stars are the exact opposite : much more spread out than usual.

Bottom line : they think UDGs are mainly dwarf galaxies (though a few may be giants), with normal dark matter contents but very poor star formation efficiency for whatever reason. I'm not so sure. They say the distribution of some parameters (e.g. stellar mass within a given radius) is the same for both UDGs and other galaxies but to me they look completely different; it doesn't help that the figure caption states two colours when there are clearly three actually used. What's going on here I don't know, but very possibly I've missed something crucial.

Of course this paper won't solve anything by itself, but it gives a good solid start for further investigations. As with the previous post, this is another example of how important it is to classify things in a homogenous way. At least one SMUDGES object is found within our own AGES survey fields, and was in fact known to much earlier studies. Sometimes what can look at first glance to be a normal object actually turns out to be something much more unusual, but it's only when you have good, solid criteria for classification that this becomes apparent. 

Which is all very good news for AGES. I suspect there are actually quite a lot more UDGs lurking in our data. All we need is a team of well-armed and angry postdocs to track them down... i.e. a great big healthy grant. Well, a man can dream.

Dey's Blue Blobs

Today's paper is more exciting than I can fully let on.

In the last few years there have been a handful of seemingly-innocuous discoveries in Virgo that don't quite fit the general trends for normal galaxies. They're very faint, very blue, metal-rich*, and some are incredibly gas-rich. The most convincing explanation thus far is that they're ram pressure dwarfs : not galaxies exactly, but bound systems of stars that formed from condensations of gas stripped by ram pressure. 

* Meaning they have lots of chemicals besides hydrogen, because astronomers have weird conventions like that.

The advantages of this explanation is that ram pressure is a high speed phenomenon, so could easily explain why the objects are so far from any candidate parent galaxies (tidal encounters can do this too, but usually require lower interaction velocities), as well as why they're so metal-rich. Primordial gas is basically nothing but hydrogen and helium, and to get complex chemistry you need multiple cycles of star formation, which makes it virtually certain that the gas here must have originated in galaxies. Why exactly they've only just started forming stars is unclear, though it's possible they do have older stellar populations which are just too faint to identify. And these things really are faint, with just a few thousand solar masses of stars... in comparison to the usual millions or billions expected in normal galaxies.

One of the main problems in understanding these objects has been the understandably crappy statistics. With only a half-dozen or so objects to work with, any conclusions about the objects as a population are necessarily suspect. That's where this paper comes in.

Finding such objects isn't at all easy. They're difficult to parameterise and tricky for algorithms to handle, so they opt for a visual search. And quite right too ! Humans are very, very good at this, as per my own work (which I'll get round to blogging soon). Having just one person run the search would risk biases and incompleteness, so they use a citizen science approach based on Galaxy Zoo. 

The result was a total of nearly 14,000 "blue blob"* candidates. But this is being extremely liberal, and many of these might just be fluff : noise or distant background objects or whatever. A more rigorous restriction in which at least three people had to identify each candidate independently reduces this to just 658. Further inspection by experts trimmed this to 34 objects – a still more than respectable improvement over previous studies. And while I previously berated them for claiming that the objects only exist in clusters without having looked elsewhere, this time they at least looked at Fornax as well as Virgo. Fornax is another cluster, but interestingly no candidates were found there.

* C'mon guys, this is the name we're going with ? Really ? Oh. Well, fine. Suit yourselves.

But they don't stop with the results of the search. They cross-correlate their results with HI gas measurements from ALFALFA and, yes, AGES (thanks for the citations, kindly people !), and also observe eight of them with the 10m-class Hobby-Eberly Telescope for spectroscopy of the ionised gas. This is extremely useful as it provides a robust way of verifying that these objects are indeed in the cluster and not just coincidentally aligned, and also shows the the gas in the objects is being affected by the star formation.

Let me cover the main conclusions before I get to why I'm so excited by this work. First, their findings are fully consistent with and support the idea that these are ram pressure features. Their spectroscopy confirms the high metallicity of the objects, comparable to tidal dwarfs – so they have indeed formed by material which was previously in galaxies. They avoid the very centre of the cluster (where they'd likely be rapidly destroyed) and are preferentially found where ram pressure is expected to be effective. 

There's also an interesting subdivision within these 34 candidates. 13 of these are "rank 1", meaning they are almost certainly Virgo cluster objects, whereas the others are "rank 2" and are likely to have some contamination by background galaxies. Most of the rank 2 objects follow the general trends in colour and magnitude as for normal galaxies, but the rank 1 are noticeably bluer. They're also forming stars at a higher than expected rate (though, interestingly, not if you account for their total stellar mass). So indeed these are galaxy-like but not at all typical of other galaxies : they are galaxian, not galaxies.

Now the fun stuff. They identify two supposedly optically dark clouds I found in Virgo way back when and have since based most of my career on, hence – exciting ! They do have optical counterparts after all, then. Actually, one these is relatively bright, and I suggested it as a possible counterpart back in 2016. But it wasn't convincing, and its dynamics didn't seem to match well at all. These days of course everyone is all about the weird dynamics, but back then this seemed like a pretty good reason to rule it out. Since then, our VLA data has independently confirmed the association of the stars and the gas, and Robert Minchin is writing that one up as a publication.

That object has about twenty times as much gas as stars. The second object is altogether fainter, having a thousand times or more gas than stars ! Even with our VLA data we couldn't spot this*, and I probably wouldn't even believe this claim if they didn't have the optical spectroscopy to support it. It looks likely that in this case we're witnessing the last gasp of star formation, right at the moment the gas dissolves completely into the cluster.

* The VLA data has much better resolution than the original Arecibo data, so it can localise the gas with much greater accuracy and precision. This means that it can show exactly where the HI is really located, so if there's even a really pathetic optical counterpart there, we can be confident of identifying it. But of course, that counterpart must be at least visible in the optical data to begin with.

While they comment directly on two of our objects, they actually implicitly include another three measurements in the table. We never identified these as being especially weird; they just look like faint blue galaxies but nothing terribly strange. And that really underscores the importance of having enough resources to dedicate to analysing areas in detail, which, frankly, we don't. It also shows how important it is to quantify things : visual examination is great for finding stuff, but it can't tell you if an object is a weird outlier from a specific trend. Even more excitingly, almost certainly it means that there are a lot more interesting objects in our data that have already been found but not yet recognised as important.

But the most fun part came from doing a routine check. Whenever anyone publishes anything about weird objects in our survey fields, I have a quick look to see if they're in our data and we missed them, just in case. Every once in a while something turns up. This is very rare, but the checks are easy so it's worth doing. And this time... one of the other blue blobs has an HI detection in our data we previously missed.

Which is very cool. The detection is convincing, but there are very good reasons why we initially missed it. But I don't want to say anything more about it yet, because this could well become a publication for my PhD student. Watch this space.

Taking galaxies off life support

Very long-term readers may remember my anguished efforts (almost a decade ago) to build a stable disc galaxy. Sweet summer child that I was, I began by trying to set up the simulations to just have gas or stars, but no dark matter. I thought – understandably enough – that adding more components would just make things more complicated, so best to start simple. I was planning to gradually ramp up the complexity so I could get a feel for how simulations worked, eventually ending up with a realistic galaxy that would sit there quietly rotating and not hurting anyone.

That wasn't what I got. Instead of a nice happy galaxy I got a series of exploding rings instead. Had that been a real galaxy, millions of civilisations would have been flung off into the void.

It turns out that dark matter really is frightfully necessary when it comes to keeping galaxies stable. Dark matter is a galaxy's emotional support particle, preventing it from literally flying apart whenever it has a mild gravitational crisis. Stable discs are easy when you have enough dark mass to hold them together.

(Of course, this is only true in standard Newtonian gravity. Muck about with this and you can make things work without dark any matter at all, but I'm not going there today.)

You don't always need dark matter to keep things together though. Plenty of systems manage just fine without it, like planetary systems and star clusters. But it's come as a big surprise to find that there are in fact quite large numbers of galaxies which have little or no dark matter, a result which is now reasonably (and I stress that this is an ongoing controversy) confirmed. We always knew there'd be a few such oddballs, if only from galaxies formed from the debris of other galaxies as they interact. But nobody thought there'd be large numbers of them existing in isolation. So what's going on ?

Enter today's paper. This is one in a short series which to be quite honest I'd completely forgotten about, partially because the authors forgot to give the galaxy a catchy nickname. Seriously, they could learn a lot from those guys who decided to name their galaxy Hedgehog for no particular reason. I'm only half-joking here : memorable names matter !

But anyway, this was an example of a UDG with lots of gas that appeared to have no dark matter at all. I wasn't fully convinced by their estimated inclination angle though, for which even a small error can change the estimated rotation speed and thus the inferred dark matter content substantially. A independent follow-up paper by another team ran numerical simulations and found that such an object would quickly tear itself to bits, whereas if if was just a regular galaxy with a very modest inclination angle error then everything would be fine. And there have been many other such studies of different individual objects, all of them mired in similar controversies. 

Since then, however, I've become much more keen on the idea that actually, a lot of these UDGs really do have a deficit or even total lack of dark matter after all. The main reason being this paper, which is highly under-cited in my view. Now it's entirely plausible that any one object might have its inclination angle measured inaccurately*. But they showed that the inclination-corrected rotation velocity of the population as a whole shows no evidence of any bias in inclination. Low inclinations, high inclinations, all can give fast or slow rotating galaxies, consistent with random errors. That some show a very significant lower than expected rotation therefore seems very much more likely to a a real effect and not the result of any systematic bias.

*Though all of these terms like "bias", "errors" and "inaccuracies" are, by the way, somewhat misleading. It's not that the authors did a bad job, it's that the data itself does not permit greater precision. That is, it allows for a range of inclination angles, some of which lead to more interesting results than others. The actual measurements performed are perfectly fine.

What about that original galaxy though ? AGC 11405 might itself still have had a measurement problem. Here the original authors return to redress the balance.

It seems that in the interim I missed one of their other observational papers which changes the estimates of exactly how much dark matter the galaxy should have; probably this is lost somewhere in my extensive reading list. The earlier simulation paper found that the object could be stable only (if at all) with a rather contrived, carefully fine-tuned configuration of dark matter, and there wasn't any reason to expect such a halo to form naturally. Couple that with the findings that it could easily be a normal galaxy if the inclination angle was just a bit off, and that made the idea of this particular object seem implausible, even if a population of other such objects did exist.

But that interim paper changes things. Whereas previously they used the gas of the object to estimate the inclination angle, now they got sufficiently sensitive optical data to measure it from the stars, and that confirms their original finding independently. They also improved their measurements of the kinematics from the gas, finding that it's rotating a bit more quickly than their original estimates, meaning it has a little bit more scope for dark matter. More significantly, the same correction found that the random motions are considerably higher than they first estimated.

What this means is that the dark matter halo can be a bit more massive than they first thought, and the disc of the galaxy doesn't have to be so thin. A thick disc with more random motions isn't so hard to keep stable because it's fine if things wander around a bit. So they do their own simulations to account for this, with the bulk of the paper given to describing (in considerable detail) the technicalities of how this was done.

They find that an object with these new parameters can indeed be stable. Rather satisfyingly, they also run simulations using the earlier parameters, as the other team already did independently. And they confirm that with that setup, the galaxy wouldn't be stable at all. So the modelling is likely sound, it's just that it depends quite strongly on the exact parameters of the galaxy. They confirm this still further with analytic formulae for estimating stability, showing that the new measurements of the rotation and dispersion are, once again, predicted to be stable.

But if the galaxy actually does have a hefty dark matter halo after all, doesn't that mean it's just like every other galaxy and therefore not interesting ? No. As far as I can tell, the amount of dark matter is still significantly less than expected, but also its concentration (essentially its density) is far lower : a 10 sigma outlier ! So yes, it's still really, really weird, with the implied distribution of dark matter still apparently very contrived and unnatural.

So how could such a galaxy form ? That's the fun part. It's important to remember that just because dark matter doesn't interact with normal matter except through gravity, this is not at all the same as saying it doesn't interact at all ! So some processes you'd think couldn't possible affect dark matter... probably can*. Like star formation, for instance. Young, massive stars tend to have strong winds and also like to explode, which can move huge amounts of gas around very rapidly. It's been suggested, quite plausibly, that this is what's responsible for destroying the central dark matter spikes which are predicted in simulations but don't seem to be the case in reality. The mass of the gas being removed wouldn't necessarily be enough to drag much dark matter along with it, but it could give it a sufficient yank to disrupt the central spike.

* And it's also worth remembering that just because dark matter dominates overall, this isn't at all true locally. This means that movement of the normal baryonic matter can't always be neglected. 

The problem for this explanation here is that the star formation density must be extremely low to get objects this faint. So whether there were ever enough explosively windy stars to have a significant effect isn't clear. Quantifying this would be difficult, especially because dwarf galaxies are much more dominated by their dark matter than normal galaxies – yes, they'd be more susceptible to the effects of massive stars because they're less massive overall, but the effect on the dark matter might not necessarily be so pronounced.

The authors here favour a more exotic and exciting interpretation : self interacting dark matter. The most common suggestion is self-annihilating dark matter that's its own anti-particle, which would naturally lead to those density spikes disappearing. There could be other forms of interaction that might also "thermalize" the spike... but of course, this is very speculative. It's an intriguing and important bit of speculation, to be sure : that we can use galaxies to infer knowledge of the properties of dark matter beyond its mere existence is a tantalising prospect ! But to properly answer this would take many more studies. It could well be correct, but I think right now we just don't have enough details of star formation to rule anything out. Continuing to establish the existence of this whole unuspected population of dark matter-deficient galaxies is enough, for now, to be its own reward.

Nobody Ram Pressure Strips A Dwarf !

Very attentive readers may remember a paper from 2022 claiming, with considerable and extensive justification, to have detected a new class of galaxian object : the ram pressure dwarf. These are similar to the much more well-known tidal dwarf galaxies, which form when gravitational encounters remove so much gas from galaxies that the stripped material condenses into a brand new object. Ram pressure dwarfs would be essentially similar, but result from ram pressure stripping instead of tidal encounters. A small but increasing number of objects in Virgo seem to fit the bill for this quite nicely, as they don't match the scaling relations for normal galaxies very well at all.

This makes today's paper, from 2024, a little late to the party. Here the authors are also claiming to have discovered a new class of object, which they call a, err... ram pressure dwarf. From simulations.

I can't very well report this one without putting my sarcastic hat on. So you discovered the same type of object but two years later and only in a simulation eh ? I see. And you didn't cite the earlier papers either ? Oh.

And I also have to point out an extremely blatant "note to self" that clearly got left in accidentally. On the very first page :

Among the ∼60 ram-pressure-stripped galaxies belonging to this sample, ionized gas powered by star formation has been detected (R: you can get ionized gas that is not a result of star formation as well, so maybe you could say how they have provided detailed information about the properties of the ionized gas, its dynamics, and star formation in the tails instead) in the tentacles.

No, that's not even the preprint. That's the full, final, published journal article !

Okay, that one made me giggle, and I sympathise. Actually I once couldn't be bothered to finish looking up the details of a reference so I put down "whatever the page numbers are" as a placeholder... but the typesetter fortunately picked up on this ! 

What does somewhat concern me at a (slightly) more serious level, though, is that this got through the publication process. Did the referee not notice this ? I seem to get picked up on routinely for the most minor points which frankly amount to no more than petty bitching, so it does feel a bit unfair when others aren't apparently having to endure the same level of scrutiny.

Right, sarcastic hat off. In a way, that this paper is a) late and b) only using simulations is advantageous. It seems that objects detected initially in observational data have been verified by theoretical studies fully independently of the original discoveries. That gives stronger confirmation that ram pressure dwarfs are indeed really a thing.

Mind you, I think everyone has long suspected in the back of their minds that ram pressure dwarfs could form. After all, why not ? If you remove enough gas, it stands to reason that sometimes part of it could become gravitationally self-bound. But it's only recently that we've had actual evidence that they exist, so having theoretical confirmation that they can form is important. That puts the interpretation of the observational data on much stronger footing.

Anyway, what the authors do here is to search one of the large, all-singing, all-dancing simulations for candidates where this would be likely. They begin by looking for so-called jellyfish galaxies, in which ram pressure is particularly strong so that the stripped gas forms distinct "tentacle" structures. They whittle down their sample to ensure they have no recent interactions with other galaxies, so that the gas loss should be purely due to ram pressure and not tidal encounters. Of the three galaxies in their sample which meet this criteria, they look for stellar and gaseous overdensities within their sample and find one good ram pressure dwarf candidate, which they present here.

By no means does this mean that such objects are rare. Their criteria for sample selection is deliberately strict so they can be extremely confident of what they've found. Quite likely there are many other candidates lurking in the data which they didn't find only because they had recent encounters with other galaxies, which would mean they weren't "purely" resulting from ram pressure. I use the quotes because determining which factor was mainly responsible for the gas loss can be extremely tricky. And simulation resolution limits mean there could be plenty of smaller candidates in there. The bottom line is that they've got only one candidate because they demand the quality of that candidate be truly outstanding, not because they're so rare as to be totally insignificant.

And that candidate does appear to be really excellent and irrefutable. It's a clear condensation of stars and gas at the end of the tentacle that survives for about a gigayear, with no sign of any tidal encounters being responsible for the gas stripping. It's got a total stellar mass of about ten million solar masses, about ten times as much gas, and no dark matter – the gas and stars are bound together by their own gravity alone. The only weird thing about it is the metallicity, which is extraordinarily large, but this appears to be an artifact of the simulations and doesn't indicate any fundamental problem.

In terms of the observational candidates, this one is similar in size but at least a hundred times more massive. Objects that small would, unfortunately, be simply unresolvable in the simulations because it doesn't have nearly enough particles. But this is consistent with this object being just the tip of a much more numerous iceberg of similar but smaller features. Dedicated higher resolution simulations might be able to make better comparisons with the observations, until someone finds a massive ram pressure dwarf in observational data.

I don't especially like this paper. It contains the phrase "it is important to note" no less than four times, it says "as mentioned previously" in relation to things never before mentioned, it describes the wrong panels in the figures, and it has many one-sentence "paragraphs" that make it feel like a BBC News article if the writer was unusually technically competent. But all of these quibbles are absolutely irrelevant to the science presented, which so far as I can tell is perfectly sound. As to the broader question of whether ram pressure dwarfs form a significant component of the galaxy population, and indeed how they manage to survive without dark matter in the hostile environment of a cluster... that will have to await further studies.