ChatPDF Versus... Me !

The latest offering from the wacky world of chatbots is something that claims to be actually practical.

Now, I love ChatGPT and simply cannot get my head around claims it's not useful. I'm fairly sure that this is just my ultra-cynical social media feed, but I will, however, cheerfully concede that ChatGPT's factual accuracy is less than impressive, as its its moral judgement. This is something that claims Poland is a landlocked country, won't output a funny version of the climax of Lord of the Rings ("because the events are meant to be taken seriously"), but also has no qualms about writing a bunny-based version of the Holocaust.

Go figure.

Anyway, if you approach ChatGPT as some kind of improved search engine then you're idiot plain and simple. That's just not what it's good for, any more than expecting a paintbrush and an easel are going to revolutionise the world of journalism. It doesn't make any sense.

But ChatPDF ought to be different. This is specifically designed to read and interpret the contents of PDF documents, including figures. It can summarise the main points and answer questions about what's in it. Now this is potentially a very useful tool indeed, as all too often, reading papers is a horrible chore. Sometimes I just need to find a specific value or equation and trawling though the whole thing isn't fun, with the Ctrl+F function being of limited help, especially with mathematical symbols. So I would hope (not expect !) that this would have at least a bit more rigour to it.

For reference, you can give a link to a PDF to ChatGPT itself, but it's shite. I sent it this one (selected at random), which is a paper entitled, "On the Reionization-Era Globular Cluster in Low-Mass Galaxy Eridanus II". It complained to me several times that the link doesn't work (it does, and arXiv's downtime is negligible, so that's just erroneous). When I told it I could see the paper in my browser, it came up with this :

I apologize for any confusion. It is possible that there was a temporary issue with the link earlier. I have tried accessing the link again, and this time it appears to be working properly.

The PDF you provided is titled “Exploring the Potential of Deep Learning for Automatic Music Genre Classification.” The paper discusses the use of deep learning techniques for automatic music genre classification. The authors conduct experiments on a large dataset of music recordings and evaluate the performance of various deep learning models. They compare their results to those obtained by other classification algorithms and find that the deep learning models outperform these other methods. The authors conclude that deep learning shows great potential for improving the accuracy and efficiency of music genre classification.

Which is pure garbage. Perhaps worse, when I gave it the correct name of the paper, it gave me a very plausible-sounding summary. Asking it where it got a value of the radius from, it confidently declared it was from a section that didn’t exist. Further corrections just led to a whack-a-mole of the same thing. This is dire, because it's all too easy to fall for its surprisingly coherent drivel.

But testing a random paper isn't a good idea since I've no idea what's in it. What would be a much better way to test this is... this blog ! More than 200 papers summarised by a source I can't have any higher level of confidence in, because it's me.

For this test I'm going to take the five most recent papers on this blog and use the specialised ChatPDF. Let's see if this does better than the ChatGPT app and how it compares to doing to hard work of actually reading the paper.

1) Smooth HI Low Column Density Outskirts in Nearby Galaxies

This one I blogged here with the paper itself here. It's about stacking the radial HI profiles of galaxies to see if the column density has an expected break (based on photoionisation models) or if it actually continues to lower values. The answer is the latter.

At first ChatPDF decided to speak to me in French. No idea why and it doesn't do it for other papers (the authors aren't French and they don't work in French institutes). So I told it to speak English instead it obliged. I asked for an explanation but all I could get was an apology. Weird.

Anyway, its summary of the paper was decent but not any better than the abstract. When I asked it for the main conclusion :

The main conclusion of the paper is that the outer regions of nearby galaxies contain smooth, low-density gas that is likely to be an important source of fuel for future episodes of star formation. The researchers found that this gas is distributed in a more uniform way than previously thought, which has implications for our understanding of how galaxies form and evolve over time. Overall, the paper provides new insights into the complex interplay between galaxies and their surrounding environments.

Initially I thought this was pretty good but on reflection it's actually very poor. Whether the gas could sustain future star formation is irrelevant : kudos for making the connection, but the authors don't so this is not warranted (also the total mass of the gas is unlikely to be significant). "More uniform" is arguable. They do use the word "smooth", but in context this is referring the 1D azimuthally-averaged profiles, i.e. they don't have any distinct break. That the average density of the gas decreases smoothly in profile doesn't mean the local density of the gas isn't highly variable. And the last sentence and a half are very much in the style of ChatGPT in that they contain no actual information but essentially give an opinion about the quality of the paper. They're okay for a general audience who need to know the broader relevance but of no use at all for a specialist who already understands this.

So far, so mediocre. Next I tried asking a more specific question about the lowest density value obtained. Initially it said this wasn't given, but then I corrected myself to column density. It said :

The paper states that the column densities of the gas in the outer regions of galaxies are low, dropping below 10^19 cm^-2. The researchers found that the gas is distributed smoothly and does not exhibit a sharp break in column density, which is contrary to what some photoionization models predict. This suggests that ionization by extragalactic photons may not be the limiting factor for the extent of the HI disk, and that instead, the outskirts of the disk may correspond to a transition to a low column density gas accreted from the cosmic web at later evolutionary stages of disk formation.

Now this is 100% correct and cannot really be improved upon. It gave a similarly impressive summary of the methodology, referencing the page numbers and giving the right quantitative values. On pressing for an exact value of the lowest column density measured, it reported that the authors don't give an exact value, which indeed they don't. So I tried :

Could you estimate the lowest value reached using their figure 2 ?

Based on Figure 2 of the paper, it appears that the lowest column density reached is around 5 x 10^18 cm^-2. However, this is just an estimate based on the graph and not a precise measurement.

Which is seriously impressive stuff. It would have been better if it had volunteered this itself, but that it can do this at all is amazing.

Except... it's a fluke. Repeating this in a later session it at first insisted the value was stated in the file itself, 10^19, which is confusing a limit with a measured value. Asking it to use the figures instead didn't work. Asking it for figure 2 specifically reverted to 10^19, which is just not right at all.

Finally I asked it for a public outreach summary. The result was pretty poor, no better than I could obtain from rewording the abstract, and when pressed, it insisted that the density values obtained were much higher than in ordinary air : this is wrong but about 25 orders of magnitude.

Oh dear. This isn't looking good at all.

2) Discovery of an isolated dark dwarf galaxy in the nearby universe

Blog post here, paper here. A much shorter paper that might be easier to handle. This one's about the discovery of an optically dark HI cloud that looks like it's rotating like an ordinary galaxy. The key point is this is isolated, so unlike other such dark galaxy candidates, it's very hard to attribute it to being the result of any other formation mechanism : a primordial origin seems the most likely.

The summary of this one was okay, but when I asked it for the main conclusion, it got hung up on the fact that the cloud is isolated. This is important, but it stated things in an almost tautologous way, as though isolated clouds should be expected to have nearby companion galaxies, which is obviously nonsense (otherwise they wouldn't be isolated). However, in a reverse of the first paper, when I asked it again in a new session, it gave a much better answer giving a direct quote for the paper that was absolutely relevant.

More disappointingly, it initially couldn't give the distance to the cloud, saying that the authors didn't state this. At first I thought this was correct and they just hadn't mentioned it, so I asked for the distance based on its systemic velocity. Now it gave the correct value. But later I found that they do actually state this value directly, so this is no more impressive than doing a Ctrl+F for "distance". On the other hand, it correctly reported that the rotation curve can't be guaranteed to be flat, which would be the key signature of a dark matter-dominated system.

So far, so very ChatGPT-like. Occasional flashes of genuine brilliance wrapped in large doses of mediocrity and sprinkled with total rubbish. Let's try another one.

3) Young, Blue, and Isolated Stellar Systems in the Virgo Cluster. II. A New Class of Stellar System

Blog here, paper here. This is a long one about unusual stellar systems in the Virgo cluster, which the authors think are most likely produced by star formation in gas removed from galaxies by ram pressure stripping.

The summary of the paper wasn't great, saying that :

The authors suggest that these systems may represent an early stage in the formation of larger galaxies, or they may be remnants of disrupted galaxies that were once part of a larger group or cluster. 

Which is certainly not the main point at all. When I asked for the most likely origin, it did very much better :

The authors suggest that the most likely origin of these young, blue, and isolated stellar systems in the Virgo Cluster is ram pressure stripping. Ram pressure stripping occurs when gas is removed from a galaxy as it moves through the intracluster medium at high speeds. This process can lead to the formation of new stars in the stripped gas, which may then form into compact, low-mass systems like those observed in this study. The authors propose that these systems formed from ram pressure stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs. However, they note that tidal stripping cannot be conclusively excluded as a formation mechanism for these objects.

Spot-on perfect. Going back to its other claims, it told me that the possibility of being an early stage of galaxy formation was mentioned on page 2, giving a quote. But this was just flat-out wrong as this quote just doesn't appear anywhere in the paper at all. The same was true about the idea of being disrupted remnants, giving a quote and page reference that was a barefaced lie. Telling it it's made mistakes does have it correct itself, but this really shouldn't be necessary.

 Dear oh dear oh dear.

4) The Turn-Down of the Baryonic Tully-Fisher Relation and Changing Baryon Fractions at Low Galaxy Masses

Blog here, paper here. This is an especially complicated and technical one. Low mass galaxies appear to rotate more slowly than the BTFR predicts, but when you account for various corrections, they actually rotate more quickly than the linear version found for higher masses and this can be shown to be consistent with the dark matter paradigm. ChatPDF's summary is quite correct :

The main conclusion of the paper is that the baryonic Tully-Fisher relation (BTFR) turns down at low galaxy masses, which indicates a decrease in the efficiency of galaxy formation. This turn-down is likely due to a combination of factors, including feedback from star formation and accretion onto dark matter halos. The authors suggest that further study of this phenomenon could help to resolve discrepancies between observed and predicted galaxy mass functions and improve our understanding of galaxy formation physics.

I asked it to describe this in terms of galaxy rotation speeds and was even more impressed :

The turn-down in the baryonic Tully-Fisher relation (BTFR) at low galaxy masses refers to a decrease in the efficiency of galaxy formation, which is reflected in the relationship between a galaxy's baryonic mass and its maximum rotational velocity. Specifically, at low galaxy masses, the observed rotational velocities are lower than what would be expected based on extrapolation from higher mass galaxies or from galaxy formation models. This suggests that there is a decrease in the efficiency of converting baryons into stars and/or a decrease in the amount of dark matter relative to baryons at low masses.

Asking it for the observational comparison with the BTFR from high mass galaxies also gave a perfect comparison. I asked it for the figure illustrating this and it correctly picked figure 5. Asking it if this could be reconciled with the CDM paradigm was more disappointing, as its answer amounted to a cautious "maybe" rather than describing the author's claims. Pointing to figure 9, where the authors do demonstrate how this reconciliation may be possible, it insisted that they didn't. 

This is disappointing. Asking in a somewhat more technical matter gave an answer that was both better and worse :

Describe this in terms of rotation speeds, comparing with the predictions of a linear BTFR.

The right panel of Figure 9 shows that the observed rotational velocities of galaxies with baryonic masses less than 10^8 solar masses are higher than what would be expected based on a linear extrapolation of the baryonic Tully-Fisher relation (BTFR) from more massive galaxies. This indicates a turn-down in the BTFR at low galaxy masses, which suggests a decrease in the efficiency of converting baryons into stars and/or a decrease in the amount of dark matter relative to baryons at low masses. The green line in the figure represents the best-fitting linear relation between maximum rotational velocity (Vmax) and baryonic mass for galaxies with masses greater than 10^8 solar masses. The galaxies with lower masses that lie to the right of this line have higher Vmax values than what would be expected based on this linear BTFR. This discrepancy between observed and predicted rotation speeds at low galaxy masses is a challenge for the standard cold dark matter (CDM) model and may require modifications or additional physics to explain.

The problem is they say even in the abstract that this is consistent with CDM ! Once again I was given an invented quote. Now, this claim is reasonable but misleading. It's reasonable that we might not fully understand the physics of galaxy formation, especially at low masses, but the point of the paper is to demonstrate that this change of slope in the BTFR is consistent with CDM predictions. Saying it requires additional physics gives a completely different impression of the claim being made.

5) Atomic Gas Dominates the Baryonic Mass of Star-forming Galaxies at z ≈ 1.3

Blog here, paper here. This much shorter papers stacks lots of HI spectra of galaxies to get a detection of atomic gas at z=1.4, comparing the result with the molecular gas. They find that at redshifts where star formation peaks, atomic gas was the dominant component, which is surprising because it's thought that molecular gas is what's mainly involved in star formation.

ChatPDF's summary does not begin well :

We found that the baryonic composition of star-forming galaxies at z≠1 is dramatically different from that of galaxies at z≈0. 

At redshifts not equal to one, eh ? Okay then. It also garbled the symbols in the rest of its responses. However, its statements were generally on point. It completely "got" the central point that the greater atomic fraction during the peak of star formation is surprising, e.g. :

Our study did not directly investigate which type of gas, molecular or atomic, is more important for star formation. However, we did find that the average atomic gas mass of star-forming galaxies with 〈Må〉≈10^10 Me is comparable to the average stellar mass at z≈1 and is significantly larger than both the average stellar mass and the average molecular gas mass at z≈1.3. This suggests that atomic gas may play an important role in star formation in galaxies at high redshifts. 

That being said, it is generally believed that molecular gas is the primary fuel for star formation in galaxies (e.g., Kennicutt & Evans 2012). This is because molecular gas has a higher density and lower temperature than atomic gas, making it more conducive to forming stars. However, atomic gas can also be converted into molecular gas through various physical processes such as cooling and compression (e.g., Krumholz et al. 2009), so it is possible that both types of gas are important for star formation in different ways and under different conditions.

This is pretty perfect, references (which I'm not going to check) and symbols notwithstanding. I asked it for how many redshifts they considered and it correctly differentiated between the two new data points from the study plus the comparison sample at redshift zero. For this paper at least, the only fault seems to be the peculiar symbols. Its factual accuracy was in this case fine.

Conclusions

I'm afraid this one can't be said to be anything beyond the usual "impressive tech demo" stage. It is categorically not ready for actual use and anyone paying the subscription fee is at this stage a complete fool. 

To give credit where credit is due, it does often produce remarkably good summaries that are more accessible than reading the abstracts. It can extract complex variables, even ones which are stated directly in the text. It seems to do better when you ask it very specific questions, but it's capable of handing complicated technical descriptions and distilling them down to their most relevant points in even more generalised ways.

The problem is that accuracy and usefulness do not scale linearly with each other. If it produces accurate statements 70, 80, even 90% of the time, it's only useful 0% of the time. Why ? Because that failure rate is such that its claims always have to be checked, and one would be better off just reading the paper. You have no idea if it's just making stuff up or missing a vital point. Worse, it's dangerously coherent. If you're not already an expert in the field, it produces statements which sound fully convincing but are in fact just plain wrong. I'm glad it references the parts of the text it's getting its information from, but it frequently just invents entire quotes, and that's unacceptable. 

Garbage in garbage out ? In this case it's sometimes "gold in, garbage out", which is worse. It has the frustrating tendency to veer wildly between a superb, concise description and stuff which is pure fabrication, or underperforms compared to just doing a regular Ctrl+F.

That said, thresholds are important. Were it to reach, say, 95%, 99% reliability, then usefulness might well rise markedly and steeply above 0%. It doesn't need to be 100%, because reading papers doesn't produce a 100% accuracy either. It just needs to do a lot better than its current level. I'll be keeping my eye on this one, but, dear reader, it looks like this blog will fulfil a valuable purpose for the foreseeable future.

The gas that just won't die

This is a paper I really thought I'd blogged ages ago, but after searching high and low I think I might have dreamed it. Still, better late than never.

During my PhD it seemed to be common knowledge that atomic hydrogen couldn't exist below a certain density because it would all just be ionised by the cosmic UV background. Where exactly this assumption/calculation came from I don't know, but according to today's paper there's probably no need to worry about this because this threshold doesn't exist.

On the opposite end of the spectrum, it's more well-established that HI densities rarely exceed a certain threshold, about 10 solar masses per square parsec (10²¹ atoms cm^-2). This seems to be because at this point the gas is so dense that it's pretty much inevitable that it starts to form molecules, and H₂ doesn't radiate in the same way that HI does. Gas in general doesn't really have an upper density limit, it's just that its atomic and molecular states require different detection techniques.

There's not much difficulty in establishing this upper limit for the HI. But the lower limit is much more challenging, because getting observations sensitive down to the predicted threshold for ionisation (10¹⁹ atoms cm^-2) is technically difficult : you need a big telescope and lots of observing time. It's also well below the threshold for star formation, and since most people would say that any gas which isn't forming stars is boring, there hasn't been quite so much pressure to study it. Not that people wouldn't like to, it's just too difficult for a relatively small reward.

Two different kinds of telescopes are available for HI, and they haven't helped matters. Single dishes can be incredibly sensitive but have low resolution. This means they can detect very small masses of gas but they can't accurately estimate how large an area it spans - it's hard for them to distinguish genuinely big, fluffy clouds from a series of small dense clouds all close together. To a single dish, a very low mass but very dense cloud would be "smoothed out", and this beam smearing would make it appear as though it was much less dense than it actually was. Interferometers, on the other hand, are much less sensitive but can have far better resolution : they can accurately measure the size of any detected gas clouds, but they have trouble detecting masses as low as those that single dishes can find.

The upshot is that interferometers might not be able to see very diffuse gas because they're just not sensitive to it, while single dishes smooth everything out and make it appear more diffuse than it really is. Hence the lower limit on gas density remains difficult to establish.

Today's paper attempts to overcome this. They use interferometry data, but instead of trying to detect the faintest gas directly, they use a variant of our old friend stacking. In this case they make radial profiles of the gas in a number of different galaxies, essentially averaging the gas over many different annuli. This part's easy, but the novel approach they use is to extrapolate the rotation curve so they can average over approximately the correct velocity range in each bin. This is necessarily a bit crude, because rotation curves aren't always smooth, but it does seem to work : they show convincingly how this recovers signal well beyond the nominal edge of the gas disc.

The bottom line is that most of these improved radial profiles don't show any evidence of a break. If the HI was lop-sided then these circular averages might wash out a break if it existed, but it doesn't appear that that's the case. The only examples where there do seem to be breaks are galaxies which have unusual structures like rings, and aren't representative cases.

I can't say I'm surprised to find there's no break. Of the little data we do have with single dishes where the gas is well resolved (because the galaxy is particularly close), there's no such break visible : the profile continues right down to our sensitivity limit (~10¹⁷ atoms cm^-2). Now for low-density structures outside of galaxies, where the gas has likely been recently removed from its parent, this might not be a problem at all, since it might take some time for it to become ionised after removal. But it's very hard to believe this is the case for the gas discs of galaxies themselves.

It's still an open question as to whether the gas is really at these very low measured densities or actually is in the form of smaller, denser clouds that are smoothed out, but to me that would seem like a very odd coincidence : surely we wouldn't expect such a neat, smooth profile in that case. Again, possible for the displaced gas, but it seems somehow unlikely for the galaxy discs. 

What are the implications of these ? Well, it's not that ionisation in general doesn't happen : gas tails in Virgo are awfully hard to explain without this, and in some cases having even deeper HI observations doesn't get you any new information. There's probably not much point in going all that much deeper than ~10¹⁷ atoms cm^-2, though I'd certainly like to see someone try. 

But with interferometers generally giving out at much higher densities (typically ~10¹⁹ atoms cm^-2 , though some of the new ones are pushing 10¹⁸ ), there's a lot of scope for more sensitive upcoming instruments to tell us an awful lot about the outermost regions of galaxies. Those regions can tell us both about how galaxies assemble (e.g. as the authors suggest, if it's not ionisation then maybe it's the assembly process that sets the lower density limit) and interact (e.g. by finding elongated gas structures invisible at optical wavelengths).

Presumably it's also bad news for the photoionization models that predicted the density cutoff, but since I never paid them too much attention, I'll worry about them another time.

Black holes behaving badly ?

Recently there's been a fascinating suggestion that the source of dark energy is nothing more exotic than the boring old black holes we're all familiar with from innumerable sci-fi movies.

I'm not going to do a blow-by-blow breakdown of this, but for a decent summary, see this press release. Very briefly, we've known for a century that the Universe is expanding. On large scales all galaxies are moving away from all other galaxies, though on smaller scales, like that of groups and clusters, gravity dominates and the expansion is negligible. But for a few decades we've known that the acceleration is, contrary to expectations, accelerating. This is "dark energy", the need for some source of impetus beyond the initial Big Bang that can sustain and increase the rate of expansion. Many other interpretations of the data have been attempted but none have stuck; the acceleration appears to be real.

The suggestion that black holes are the source of this goes completely against my intuition. How does a small, ultra-dense source of inescapable gravity somehow cause acceleration of the largest structures in the Universe ?

In this post I'm not going to delve into the observational details and reasoning behind this conclusion as I've not read the main paper about that aspect, though I will link to others who have. Instead, I just want to grapple with this idea that black holes can do the exact opposite of what we've all been taught to expect them to do.

Now understand that I do this as a layman. Relativistic physics is thought to play little or no role in galaxy evolution, any any understanding of general relativity I might have once had is long since gone. So what I'm looking for is an intuitive framework to understand this, not a mathematical one.

To cut to the chase, I don't have one. But maybe it's interesting to look at all the same.

Let's start with the press release :

If mass growth of black holes only occurred through accretion or merger, then the masses of these black holes would not be expected to change much at all. However if black holes gain mass by coupling to the expanding universe, then these passively evolving elliptical galaxies might reveal this phenomenon.

“Here’s a toy analogy. You can think of a coupled black hole like a rubber band, being stretched along with the universe as it expands,” said Croker. “As it stretches, its energy increases. Einstein’s E = mc² tells you that mass and energy are proportional, so the black hole mass increases, too.”

Right, okay, with you so far. An expanding universe can make black holes grow in mass, because handwaviumrelativitytheoryhandwavium is all very complicated. I’m willing to accept that the expansion of the universe can do weird things on small scales, especially, as Kevin Croker says in an earlier interview (content starts at 18 minutes), with only noticeable effects on “relativistic” materials - black holes and neutron stars, basically. Not much at all would happen to other objects. 

Croker also says that the properties of dark energy could be mimicked by a distribution of objects that gets more dispersed but more massive, so as to keep its density constant. That’s a bit trickier, but it makes sense. Observations inferring the acceleration show it's consistent with a constant energy density, and black holes necessarily become more dispersed due to expansion (like everything else), but, unlike everything else, it seems they've been growing in mass too*. So I can believe that their varying mass-energy density would cancel out and remain constant overall.

* This is a key part of their argument from the view of a necessary (but not sufficient) correlation. If dark energy has a constant energy density then its source should as well, which is apparently what black holes do, whereas normal matter just decreases in density over time. This is not direct evidence for a causal connection but it's at least an important consistency which other sources don't provide.

Where I come really unstuck is this :

Croker added, “This measurement, explaining why the universe is accelerating now, gives a beautiful glimpse into the real strength of Einstein’s gravity. A chorus of tiny voices spread throughout the universe can work together to steer the entire cosmos. How cool is that?”

I could buy the claim that it’s not that dark energy is making black holes grow, but actually the claim is the other way around : black holes provide dark energy. But if anything I would expect the opposite. With the galaxy/BH distribution being approximately uniform on the largest scales, if each one gets more massive, I would expect more gravity and therefore a deceleration.

Initial attempts to find a good explanation of this (I would think very obvious) objection weren't very productive. Even Ethan Siegel wasn't much help; his skeptical write-up is needless to say very good, but doesn't address this fundamental point. So I emailed the first author of the latest papers, Duncan Farrah, and he gave a nice response, but still without much about the bit that most interests me. For those who want them, technical (read : incomprehensible) papers about this "cosmological coupling" can be found here, here and here. 

But of course, a mathematical explanation isn't what I want. Farrah did also link to other papers though; being purely observational the main paper about inferring black hole growth rates doesn't deal with the main issue at all, but the second one (which is a letter, so nice and short), does. Well, a bit. 

So far as I can tell - and there’s a lot here I don’t understand at all - the crux is the nature of the black holes. In this model, they are a totally different beast to the classical event horizon shrouding an infinitely dense singularity. They now have neither (or at least they certainly don't have a singularity), but are composed of vacuum energy. Apparently, theoretical experiments have tried to remodel black holes in such a way before, but failed. The impression I get from the paper (I don’t want to give the idea that this is definitely what they state) is that cosmological coupling helps resolve this issue, making these mathematical inconveniences go away whilst giving results consistent with observations. 

In particular, they seem concerned with the fact that classical solutions of black holes are incompatible with cosmological observations when extended to infinity. Now formally the gravitational field of any object extends indefinitely, propagating at the speed of light. So my very tentative guess is that’s where the solution lies. Being composed in their model of vacuum energy, although they presumably have incredibly dense interiors of ultra-strong gravitational strength, at infinity they now do something much more funky which in some way provides a negative pressure. And while each individual black hole is still in some sense point-like, collectively, since they arise from the end products of stellar evolution, they’re essentially everywhere. So the negative pressure, expanding at the speed of light, could presumably fill the universe.

When I went into work and discussed the topic with a somewhat more knowledgeable friend, his interpretation was more-or-less identical to mine. Hooray ! But he also raised the good point that at large distances the gravitational field of any object tends to be the same, so it's strange to propose that only black holes are contributing to the negative pressure, especially as the mass of black holes is very much smaller than the mass of normal matter.

Then I went on a video binge hoping that some professional outreacher would have helped clarify things. And I think perhaps they have, though only partially.

First, Sabine Hossenfelder has a short section in this video about the idea. Like just about everyone else, she's skeptical of the observational evidence, but does at least touch on the main issue in her own contrarian way : first, she says that black holes don't contribute enough mass-energy to be important (as already mentioned), and second she says that we shouldn't call it dark energy at all, but rather the "curvature of empty space". In her view, it's not a problem that needs investigating. Space just does this, there's no inconsistency with anything else so it's a "bad problem" to work on (for a more detailed video and discussion about this, see this thread).

I have problems with this. Okay, the observed mass of black holes on small scales is tiny, but does this tell us anything at all about their proposed negative pressure on large scales ? Not necessarily. That point earlier that this coupling might only affect relativistic materials is, I think, probably crucial.

As to the "curvature of empty space", it smacks of just trying to define the problem away rather than solve it. While she says that, "When I say “not a good problem” I do not mean “I don’t want to see an answer”, I mean “not a promising route to progress.”", I am honestly not entirely sure I believe her. A lot of it feels very much like brushing what everyone else thinks of as whacking great problems under a convenient carpet : sure, it could be the case that on large scales things just expand more and more rapidly, but this is a damned odd presumption. When I see something accelerating without an obvious cause, I want to explain it.

The analogy to the case of dark matter seems to be pretty direct here. Sure, we could have found flat rotation curves and declared, "oh well, I guess that's just what gravity does on large scales", but this would have been an awfully strange thing to do. There is an inconsistency here, a massive conflict with everyday notions about how gravity/acceleration work. We should be willing and able to relinquish these, but not at the drop of a hat. Presuming "this just happens" is very strange and very dangerous avenue and doesn't feel at all like a "promising route to progress" to be, but the opposite.

YouTube's recommendation algorithm must have improved of late because it came up with a couple more decent videos. First, Dr Becky has a great discussion on the observational problems of the study, not dismissing the idea but rather preferring that the study adds more evidence for our lack of understanding of black hole growth and galaxy evolution. She also notes that the black holes growing in mass is somehow related to the conservation of energy, that for the black holes to increase the Universe has to give something up. Intriguing, but it's not at all obvious to me how the acceleration increase would be countered by more massive black holes.

Finally there's this excellent video from Universe Today's Fraser Cain, interviewing co-author Chris Pearson. If you only watch one of the videos linked here, choose this one. Again there are some hints of some sort of conservation being at work, that the "tension" in the acceleration is analogous to that of an elastic band. But more interesting is Person's explicit statement that according to general relativity, the negative pressure arising from the black holes would be averaged everywhere : we would not, he states categorically, expect to see inhomogeneous expansion as a result of this model. Black holes are truly coupled with the fabric of spacetime; I almost get the impression that they are a nonlocal phenomena. The solution, he says, is in the mathematics but is completely non-Newtonian. At least this is honest enough to bluntly state (in effect), "yes, the negative pressure is not intuitive, but it's frickin' hard to explain how this works without complicated maths".

And I'm fine(ish) with this. I'd far rather have this than not acknowledging the (apparent) problem at all. If it's going to take some time to translate it to something comprehensible, so be it. Note that this is in stark contrast to a paper which appeared today claiming simply that black holes do not create negative pressure, end-of. It's for this reason, though, that we do need that intuitive, non-mathematical framework to consider; the mathematics may be more powerful, but hardly anyone else will be persuaded if you throw some incompressible gobbledegook at them.

Now as per my own remit, I've not gone into the observational evidence for the theory. Other links herein do that very well, and Pearson does a good job of summing up why they might be wrong as well as the reasons they think they might be right. 

It goes without saying that most papers attempting a revolution is cosmology or any other field are simply wrong. It's right and proper that radical claims are intensely scrutinised, but it's also right and proper that people are making such radical claims in the first place (regardless of what Hossenfelder thinks). "Publish or perish" does not yet dominate as an absolute incentive to mediocrity. So while I'd bet that in a year or two this claim will have faded into nothing, I'd hope that it'd do something more like dark energy and just keep growing. I get the distinct impression that this means reinterpreting not just black holes, but cosmology itself. And that would be pretty neat - at least, if outreach improves to the point where I can understand it.

A dark galaxy candidate that's not messing about

Of all the candidate dark galaxies that have come and gone over the years, this new one quite likely deserves to jump straight to the top of the tree.

Apparently there's a dedicated filler project at FAST to search for dark galaxies, though the authors don't say very much about it. It's found "some" candidates, but this one is so good that I guess the others just aren't worth mentioning yet.

What makes it so compelling ? It's isolated. Like, very, very isolated. They don't actually emphasise this much, but a cursory NED search (and examination of WWT images) shows there's bugger all nearby. The object is about 29 Mpc away and there's no other galaxies at all within a 500 kpc search radius, at least with known optical redshift measurements. And that's about as isolated as it's ever going get. And the optical images, as a sanity check, don't show any obvious galaxies anywhere nearby. That immediately makes the popular "debris" explanation incredibly hard to sustain in this case.

The object itself is a hydrogen cloud of about 80 million times the mass of the Sun, a few times larger than the ones I've spent an inordinate amount of time investigating in Virgo. Its spectrum appears to have a hint of a double-horn profile that's characteristic of rotating discs, which is backed up by the position-velocity diagram which looks quite a lot like the standard flat rotation curves which are associated with normal, dark matter-dominated galaxies. Optically, it appears to be completely dark : not just dim, but nothing there at all, whereas typical galaxies with this much hydrogen ought to be readily detectable at optical wavelengths.

This means it's ticking all the boxes : it's detected in radio but not other wavelengths, it's isolated, it's got a flat rotation curve. Numerically its estimated dynamical mass is a few tens of times greater than its hydrogen mass so it would have to be strongly dominated by dark matter. And estimates of the gas density put it well below the threshold above which star formation tends to occur.

In terms of caveats I'm struggling to find any. It can't just be hidden behind a nearby dust cloud because the reddening in this region is negligible. It fits the baryonic Tully-Fisher relation between mass and velocity width for much brighter galaxies. In all honesty it's pretty darn close to the Platonic ideal of a dark galaxy candidate.

Is there anything at all to spoil the excitement ? Not really. I think it's virtually certain that whatever this turns out to be, it'll be something interesting. My only slight concern is that the double-horn shape at detection of a flat rotation curve are both marginal, but that it shows both makes this more convincing. And this marginal nature isn't unexpected for a small galaxy like this one anyway : for low mass galaxies, the gas tends to only just reach the flat parts of the curve.

To me the isolation is the crucial feature here. Higher resolution gas observations ought to be able to pretty decisively determine if it's really rotating or not, and it should be easy enough to get some really deep optical data to make sure there's no low surface brightness component hiding nearby. Since the object is so small and lonely, there's no need for massive levels of surrounding data for this : just point the telescopes and go.

The only thing I can think of that would change the picture is if there were more hydrogen clouds nearby, which would point towards some much larger scale feature. But that is hugely unlikely, because if the gas came from galaxies then at least few ought to be known about, and if it's a large-scale gas feature then there's no way anyone wouldn't report this. I'm scraping the bottom of the barrel pretty hard in an effort to find cautionary notes to end on here.

In terms of excitement levels I... have to rate this one 10/10. I have to. To be blunt, in terms of research directly relevant to me, it's potentially the most exciting thing I've heard about in my whole career. There have been dark galaxy candidates aplenty over the years but none of them come close to matching this one : every single one has particular extenuating circumstances whereas this one has none of that.

The only thing that's keeping me together is the pressing need to debug code and a very very strong habituation to avoid getting excited until everyone else does, in case there's some rudimentary error in the whole thing that I've missed, or the whole thing turns out to be bunk for some reason. That's been known to happen to similar discoveries before, so fingers crosses that this is one that's genuine.

This is exactly what I expected

Today's post is not a paper but a poster (doesn't format well on mobiles so please use an actual PC). I wouldn't normally do anything this preliminary, but we've been waiting long enough for this that I'll make an exception. Keep in mind that content that appears in a poster can change drastically by the time of formal publication, so everything I'm about to say is subject to revision.

To give some necessary context : Robert Minchin and I had this running joke in Arecibo that when set he'd automated a task, he really meant he'd delegated it to the postdoc, i.e., me. Well, it took several years and Robert moved institutions twice, but I finally have my revenge. Mwhahahah !

What's this all about then ? Well, way back in 2012/2013, we published a couple of papers which, among other things, noted the discovery of eight optically dark hydrogen (atomic HI) clouds in the Virgo cluster. These were particularly strange in that they have velocity widths normally associated with rotation-dominated galaxies, but no optical counterparts. They were also pretty isolated, with no obvious nearby parent galaxies to explain them.

What followed for the next decade has been a series of papers looking at the various possible explanations. In the "Flying Snakes" paper, we catalogued other, similar objects, did some simple analytical calculations, and also ran a bunch of simulations to see if we they were more likely to be optically dark galaxies or tidal debris. This was followed up with more, similar but more realistic simulations a little later. And then we tried even more advanced simulations looking at more novel suggestion that the objects were held together by the pressure of the intracluster medium.

Let's cut to the chase. Tidal debris doesn't work for these objects - they are much too far from their parent galaxies and should have dispersed already (clouds with smaller line widths, however, are no problem at all for the tidal debris scenario). And the confining pressure of the ICM just isn't significant.

This leaves the most dramatic explanation : that these objects are indeed rotation dominated, and thus require a substantial amount of dark matter to prevent them from exploding. They are in effect dark galaxies.

... except, this explanation has problems too. We don't really find objects like these except in clusters, and the ram pressure they experience there ought to destroy them almost instantly. So the dark galaxy hypothesis remains at most the best of a bad bunch.

What we've yearned for is data with better spatial resolution. With Arecibo they're just point sources. If we could see their morphology, we could tell whether they had the all-important ordered motions that would truly indicate rotation, while more haphazard motions would mean some other, probably less interesting explanation.

At last, we have it. We put in the proposal to the VLA and got the data back in 2017 (!), but then muggins here left it lying on a shelf ever since. Why ? Basically, stuff kept coming up. Reducing the data is damned hard if you don't know what you're doing and there's always lower-hanging fruit to pick. I mean, sure, I could spend months doing this incredibly difficult thing to get this probably very interesting result, or I could do this easier thing with a guaranteed moderately interesting result instead.

Anyway, Robert has reduced the data and made a shiny poster for the AAS meeting. What did we find ?

Of our eight targets, six were observed. Not sure what happened to the other two but never mind. Of those, three were detected. The non-detection of three is not at all surprising, as the VLA can be hundreds of times less sensitive to low-density gas than Arecibo. So it goes.

Of those three which were detected... (drumroll please)...

Two are galaxies. One is optically dark.

Ho-hum. Exactly as I expected, we found results which have nothing much to do with what were expecting but were interesting all the same - which is exactly why detailed observing proposals are a waste of everyone's time.

One of these, AGESVC1 231, appears to be associated with a very small, compact blue blob. Actually we spotted this blob from the word go, but we can only now say that the HI is associated with it thanks to the higher resolution of the VLA data. Without this, the HI map doesn't really coincide well with the object at all, it's so compact it could be anything. Also the high velocity width would be completely atypical for an object this small, so originally, there was no good reason to think the HI and the blue blob were related. But the match with the VLA data is so good it can't possibly be coincidence.

The other thing the VLA data shows is that the blob appears to be associated with the brightest, narrowest peak of the HI emission, with the high velocity width component being considerably displaced - i.e. a tail. Because of its lower sensitivity, the VLA only detects the narrow, densest component, with the wider, more diffuse component only seen with Arecibo. 

Tentatively, this looks like a dwarf galaxy experiencing ram pressure stripping. The blob is extremely blue (the bluest object in our original sample), which might be because ram pressure can initially compress the gas and enhance star formation before it removes and disperses it. So the object is indeed a galaxy, but whether it has any bearing on the notion of "dark galaxies" is harder to say. It's gas rich but not enormously so. More interesting from this one would be to ascertain the age of the stellar population : given that it's so blue, perhaps it only started forming stars very recently. 

What we're probably seeing here is the narrow velocity width component giving a decent representation of the object's true dynamics. The high width component, which is what marked it out as interesting and made us think the blue bob was unrelated, is likely deceptive : this might exist only because ram pressure is disturbing the gas, not because of rotation. So most likely the dark matter content is nowhere near what we initially guessed, though that needs to be more properly quantified.

The second object, AGESVC1 274, also has an optical counterpart. If you look at Robert's poster I think you'll see why we missed this one : it's truly pathetically faint, and you'd never claim it was associated with the HI based on the lower-resolution Arecibo data. However, we also had more sensitive optical data where this is more clearly visible, so I'm a little surprised I missed this. I'll have to go back and check to see why this one didn't get mentioned.

This object is one of two of our sample which have very low velocity widths. In this case, unlike 231, the VLA and Arecibo spectra are in excellent agreement. In some ways it appears to be a typical though extremely gas rich dwarf galaxy, which puts me in mind of other strange stellar systems. I also wonder if it might classify as an Ultra Diffuse Galaxy. But it really is very gas rich indeed, and certainly has a very low surface brightness. It's a galaxy for sure, but whether this is just an extreme example of the general population or is actually an outlier from the general trends is not yet clear.

The third object, AGESVC1 258, is the most interesting. It's only marginally detected in the VLA, but what we see is a clearly elongated structure. It has no apparent optical emission at all (though maybe I do see a pathetic little blob in the densest part of the gas ?). Now this elongated structure could indicate a disc, but it doesn't show any obvious velocity gradient that would be the smoking-gun signature of rotation. On the other hand, as with 231 unfortunately the VLA is missing a lot of flux compared to the Arecibo data, so we might just not be able to detect the rotation because the thing's so faint. In addition, the density of the detected gas is well below the typical threshold for star formation.

Again tentatively, this object might still be consistent with the dark galaxy hypothesis, but it could also be some really weird debris. If it's debris then it's still a mystery how it survived.

So as usual, this data answers some questions but raises whole new ones. Why are some optically bright but some are dark ? Do they have a common formation mechanism or we looking at completely different objects ? If they are debris then how do they survive and reach such great distances from their parent without dispersing despite their high velocity widths ? How much - if any - dark matter do they have ?

For me this is pretty exciting stuff. It's not a huge amount of new data, but it's more than enough to provoke a whole new inquiry. Watch this space.

When is a galaxy not a galaxy ?

Today's paper is a long one, so I'm going to heavily condense it down to the main result. I should say in advance that, although I think there are a couple of points (literally, just two) which are just gut-wrenchingly wrong, it's an excellent, detailed yet very readable work which is interesting from start to finish.

First, you might remember a couple of oddballs found in the Virgo cluster. The first is SECCO 1, a collection of very young, blue stars without any older component that's apparently just wandering around the cluster nowhere near any plausible parent galaxy. It's got gas, and simulations show it can survive moving slowly over such long distances, but why it only has a young stellar population is a mystery. What started this recent episode of star formation, given that there's bugger all around it ?

The second object is more controversial. It appears to be a case of gas stripping from a ultra diffuse (faint, spread out) galaxy with star formation occurring in the stripped gas. That would be the first time we've seen such a galaxy caught in the act of gas loss, and relatively few of them appear to even have any gas, so this would be a pretty neat result. And it would make sense : lots of UDGs known in clusters but virtually all are gasless, whereas those with gas are a bit more common in other, less dense environments.

But then, another team (actually the same as today's) came along and said that the UDG was actually not in the cluster at all, and that the star forming stuff was embedded in a stream of hydrogen coming from another galaxy.

I'm actually a bit skeptical about this second result. We've seen galaxy distance estimates be revised and revised again, and revised some more... so I wouldn't automatically trust any one claim. And the "stream" of hydrogen looks... well, maybe. It's not definitely wrong, but it's not definitely not wrong either. It looks quite a lot like an artifact of smoothing the data, to me.

Still, there are weird bits of star formation happening in the Virgo cluster. Today's paper is an attempt to tackle such objects a bit more systematically. The authors decided to do a search of the entire Next Generation Virgo Survey data and get a more complete catalogue of such objects, rather than stumbling on them at random as in all previous cases.

They found... a total of six candidates, including those two previous cases. And one of those they rule out, as it's most likely just a bunch of distant background galaxies. So three genuine new objects, bring the total to a staggering five.

Yes, that was slightly sarcastic, but never mind.

As usual with such small number studies, it's unclear if these objects really are all that similar to each other. They're not found in any particular location in the cluster. It's not really known if they have a distinct gas content - two are detected and have similar amounts of neutral hydrogen (a few tens of millions of solar masses), but of the other three, their redshifts are so low that any gas would be hard to distinguish from the much brighter gas in our own Galaxy. Only one is at all a significant non-detection, but if its gas was even just slightly less than the others, it would still be extremely gas rich compared to typical galaxies.

But in other ways they are actually remarkably similar. They're all very blue patches of starlight with no obvious underlying old stellar component - everything seems to have happened in the last hundred million years or even less. Their morphologies also look similar, and their total stellar masses are all a few tens of thousands of solar masses. And they have similar chemical compositions, indicating they weren't formed from primordial gas, but likely from material that was pre-enriched inside another galaxy. Star formation rates are also similar, and if they're not found in the same particular region of the cluster, they do seem to have similar levels of isolation - which is pretty unusual by itself for objects in a cluster.

So, how do you form such an object ? There are various possibilities. None are especially compelling, although one does seem more promising than the rest.

The mechanism I would be naturally drawn towards is that the objects might be very faint galaxies, with their stars bound in a dark matter halo just like much brighter ones. This, however, is the first point where I think the authors commit and aaaaargh noo this is wrong level of mistake. It doesn't help that they cite me in order to point out that there are "few convincing" results to date : this is technically correct, but very much missing the point. Worse is the claim that because the gas is not primordial, the objects themselves cannot be primordial and therefore they cannot be galaxies.

I don't think this is fair. If you want to decide if something is a galaxy, you need to look at its dynamics to infer its dark matter content. Whether or not it's composed of primordial material is secondary, especially given that nobody know much about how chemical enrichment works in objects like these. Two points in particular are that they can't rule out an old stellar population, and they say the stars detected are so blue they're hard to fit with a model even assuming very young ages. So there might be something unknown at work here. As it stands, this is confusing two quite separate issues.

Much more convincing is the data on dynamics which does exist, little though it is, doesn't suggest any hint of the high rotation speeds that would indicate the presence of dark matter. If anything it seems that their velocities are extremely low. Ironically, given the size of the objects, this implies they're not gravitationally self-bound and so are likely transient : they exist for just a short time and then disperse.

But how can this be ? How they reach a high distance from their parent galaxy, suddenly form stars, and then bugger off into void ?

There are two possibilities. One is that they could be tidal dwarf galaxies, formed from material stripped off in gravitational encounters between two or more galaxies. But this has a lot of problems. Tidal dwarfs need to be quite massive to survive the gravitational field from their parent galaxies (otherwise they just disintegrate immediately), and these guys are just too little. They could be from smaller parents, but then it's hard to eject material with the necessary velocity to get them to such isolation. And there's a relationship between mass and metallicity, which these objects would violate : their chemistry suggests bigger parents, not smaller.

A more promising alternative is that these are a new class of stellar system : ram pressure dwarfs. Note the lack of the word "galaxy" here, commendably emphatic in its absence*. We know ram pressure in clusters can push out gas at basically any velocity – this is dependent on the mass of the cluster itself, not that of the galaxy. We also know that star formation can happen in this stripped gas, with star formation initially boosted by the compression, though declining afterwards in the parent as its gas supply is rapidly exhausted.

*Claiming a new type of galaxy is tantamount to claiming you've discovered a new planet in the Solar System, so kudos for them for not doing this.

So this sounds great. This mechanism would allow the big, metal–rich parents to eject gas at high enough velocities to produce these slightly star-forming blobs that appear to be very isolated. Hurrah !

But even this is not without issue. One is the second oh god no why did you say that moment. They note that the objects are only found in clusters as evidence for their ram pressure origin. Well, yes, ram pressure is a distinctly cluster–based process... but they only looked for these objects in clusters ! Who knows if they're found in groups or not - they're not at all easy to spot, and no-one's been looking.

Certainly it's fair to say that they must be produced by some mechanism that does operate in clusters, however. The other issues here might be more difficult : the objects don't seem to be found near to objects with known gas tails where we know stripping is still ongoing, and they estimate that such objects could remain detectable for about half a billion years – a not insignificant amount of time.

There's a couple of possible future options for really nailing down what's going on. First, their search was "visual", meaning they took the data and looked at it. That's fine (although I would have like a lot more details), but doesn't tell us anything about their expected completeness, which could surely be quantified, and search techniques refined. Second, getting molecular gas measurements with ALMA or other instruments would help understand if star formation really is occurring the way they think it is.

So in the end this feels like an incremental result. And it is. But it's important to realise the amount of work that goes into some of these incremental results, which is... not small. A better way to think about it would be that this one interesting result has spawned the potential for yet more interesting results, in a never–ending cataclysmic chain reaction of interest that will eventually consume us all. Or something.

Wibbly-wobbly curvey-wurvey

Fresh from having mastered the art of fitting the baryonic Tully-Fisher relation the hard way, I thought I'd resume a long hiatus from reading papers with a look at this one.

The BTFR is an empirical relation between the rotation speed of a galaxy and its baryonic mass (stars and gas). In logarithmic units, this is a nice straight line with (arguably) no intrinsic scatter. This is rather surprising since even very simple calculations show that star formation and environmental effects ought to disturb the gas and stars - galaxies with low surface brightness (brightness per unit area) ought not to follow the same relation as high surface brightness galaxies, but they do anyway.

It's been argued that this is evidence for alternative theories of gravity. If it's just mass that dictates rotation speed, then it doesn't matter if the mass consists of stars or gas : everything should follow the same relation. Under the standard paradigm of galaxies being dark matter-dominated, this would involve a conspiracy between different components that's comparatively hard to justify.

But while the BTFR has been shown to hold true over a wide range of rotation speeds and masses, for the very largest galaxies this is known not to be the case. Establishing whether this is also the case for the smallest galaxies is harder. Not only are they rare, but to get accurate measurements of the rotation speed requires high resolution observations - and resolving the smallest galaxies of all is naturally more difficult. And since you want the systems to be stable, they need to be isolated. Ideally, you want them to be gas-dominated as well, as it's easier to measure the mass of gas than it is for stars.

So getting a statistically significant sample to probe this end of the BTFR is difficult. That's where this paper comes in. At long last, the authors have obtained just such a sample, of 25 objects. Enough, at least, to say whether galaxies of the lowest mass typically follow the same BTFR as for normal galaxies.

The answer ? They don't. They rotate significantly more slowly than the relation found for more massive galaxies.

The really tricky part comes in demonstrating whether this is consistent with standard model expectations or not. The problem is you can only measure how fast a galaxy is rotating out as far as you can observe its material, whereas the dark matter halo ought to extend much further. To correct for this, they compare with analytic predictions for different dark matter halo density profiles. With enough clever tweaking, it's possible to get a result that agrees really well with numerical simulations.

You may well read a note of skepticism here. Thing is, since we don't know the true distribution of dark matter, there's quite a lot of freedom to choose which one to use to get a result which agrees with simulations. And the corrections are so extreme that these galaxies then have a faster rotation than the BTFR for more massive galaxies predicts. But to be fair, they state their results quite cautiously, emphasising the need for more data. So I have no problem with saying this is an intriguing result that needs to be reported.

That said, I'm more persuaded of the claim that the galaxies are more likely to have a constant density inner "core" than an extremely high-density spike (a.k.a. a "cusp"). Trying to get a self-consistent cusped profile that agrees with observations just doesn't seem to work very well, a well-known problem that might be solved because of all the gas and stars sloshing around in the middle.

The provisional, take-home message, then, is that the BTFR is really more the result of the complicated physics of star formation and galaxy formation than it is something as fundamental as gravity. Naively, while you can reproduce the BTFR for ordinary galaxies using nothing more than dynamics, it looks likely that this is really just an elaborate selection effect : once we go outside the standard range of galaxies we've been accustomed to observing, we don't see the nice neat linear relation any more.

Where does this leave Ultra Diffuse Galaxies, ones of extremely low surface brightness that look to be rotating so slowly they may have no dark matter at all ? The authors say, "dunno". My bet remains that we've simply estimated their velocities incorrectly, but if there is something funky going on with how star formation works at these mass scales, that'd be much more interesting.

The wrong sort of gassy

Back in June last year, a nice little paper showed that it was possible to use stacking to measure the atomic gas content of very distant galaxies indeed. Normally, directly measuring the atomic hydrogen content of galaxies becomes excessively difficult beyond the relatively local Universe. Beyond a couple of hundred megaparasecs it becomes really quite challenging; to reach a gigaparsec is just about possible, but at the extreme limits of technical capabilities. The 21cm emission from hydrogen is just too bloody faint.

So the paper used a standard technique of "stacking" the detections : observing lots and lots of galaxies and averaging all their signals together. In this way, you get the sort-of-but-not really equivalent to spending an insane amount of observing time on an individual galaxy (see link for subtleties). The previous paper didn't do much but establish the main result, which is fair enough, but in this new paper they've gone quite a bit further.

Using new, better data for over 11,000 galaxies, they can now recover a signal from a redshift z=1.4, equivalent to a lookback time of 9 billion years. This is well in the range where we'd expect to see evolution in the typical gas content. Last time they established that, as expected, earlier galaxies have a higher gas fraction than their nearby contemporaries, but now they do two additional things. First, they can split their sample to check the gas content at z=0.74-1.25 and for z=1.25-1.45. Adding a separate sample of very nearby objects means they can plot the evolution of the gas content with no less than three, count 'em, three data points - enough, some would say, to see a trend. Second, they also estimate the molecular gas content, so they can compare how the atomic and molecular fractions evolve.

I'm going to gloss over some pretty hefty caveats. First, not only is stacking itself a subtle and oft-misunderstood procedure, but it's also subject to a host of biases and selection effects. However, their detections are convincing, so I note that only for caution, not as criticism. Potentially more problematically, their estimate of the molecular gas comes not from direct measurement but relationships between star formation rate and molecular gas content established elsewhere. As they note, this too may be biased.

However, let's give them the benefit of the doubt and take their results at face value. It seems that the atomic fraction has been monotonically decreasing over time, dropping rapidly from z=1.4 to 1.0, but then more slowly to the present-day z=0.0 (with the reminder that this is based on three data points). The molecular gas fraction is more complicated : it's lowest at z=0.0, peaks at z=1.0, then decreases a bit at z=1.4.

What's going on here ? Well, the prevailing view is that atomic gas the reservoir of fuel for star formation, but it's molecular gas which is actually where star formation happens. If atomic gas is what's in the tank, then molecular gas is what's in the engine. So we might naively expect the gas to be predominantly molecular during the peak of star formation activity. The problem is that's not what we see here : star formation peaks when the gas is dominated by atomic gas instead.

That's a bit strange, but I wouldn't be overly-concerned that everything is broken just yet. Conversion of molecular gas to stars involves a great deal of highly complex feedback, where the hot young stars and supernovae inject energy back into the gas and helping to slow further conversion into molecular gas and thus stars. It looks like the early rapid drop of atomic gas is balanced by the corresponding, combined increase in molecular gas and stars, so that fits the basic narrative. The hard bit is explaining why star formation activity was higher at earlier times, when there apparently wasn't all that much molecular gas around at all.

There's been some hints that the basic scenario is too simple, that actually the atomic gas itself can be directly involved in star formation. That could well be the case. No galaxy, so far as I'm aware, in the nearby Universe, shows atomic gas fractions as high as the ones at high redshift described here - so things might just work differently. Alternatively, it could be that the molecular content has been underestimated, or that the statistical effects of all this stacking are washing out a plethora of important details. Unfortunately, we're probably not going to be able to remedy this without direct detections of atomic hydrogen as z=1, which isn't going to happen anytime soon... though it's likely to be years (rather than decades) away. So, we'll see.

Nothing to see here

The last few years have seen a lot of hoo-hah about galaxies apparently lacking dark matter, but this may at last be drawing to a rather boring conclusion.

First, we had a whole kerfuffle about two dwarf galaxies, NGC 1052-DF2 and DF4. These seemed to have such a low velocity dispersion that they wouldn't require any additional matter apart from their stars to hold them together. There was some dispute about the magnitude of the velocity dispersion (in my opinion not a very credible one) which went away quite quickly because yes, their velocities really were very low. A much more protracted and complex dispute arose about their distance, which was only settled quite recently and found that yes, they are far away (and lacking dark matter) after all.

Those first discoveries were pure starballs. Then, we've also had the saga of galaxies with gas, for which we can much more easily examine the rotation velocity. They were also so far away that there was no prospect of a distance controversy. But just the other day, despite many arguments in favour of these being a population of weird objects, simulations showed that such objects can't be stable. It's far more likely that there's been a slight measurement error in their inclination angle. I wouldn't personally call that one settled just yet, but it's definitely going in that direction.

There was also bad news for those of us hoping the original objects would turn out to be something truly weird : simulations found a plausible way of producing objects like this through unusual but entirely plausible encounters, specifically requiring direct collisions between galaxies, which are relatively rare. Which is great news for the boring old standard model of cosmology.

Today's paper feels like another nail in the coffin for anyone hoping these objects would turn out to be truly strange. Not the final nail by any means, and it's still just about possible they could burst back out like an enraged vampire, but it's definitely making it harder for any would-be children of the night to go on a killing spree.

It's a nice little paper but I do have to wonder why on earth it made it into Nature. Noting the simulations showing that collisions are a viable explanation, they go out and have a look at the context of DF2 and DF4 on a larger scale. They show they're in a neat line of maybe 11 objects in total, stretching over 2 Mpc. This is nicely consistent with the collisional origin hypothesis.

And honestly... that's it. Yes, it's a nice discovery, but they don't go on - as I assumed they would - to do numerical simulations to back this up. They don't have any new observations of these additional galaxies to see if any of them have anomalously low velocity dispersions. They just report the basic discovery along with their (undeniably) very interesting hypothesis. I mean, I'm glad they reported this, but it feels much more worthy of a letter to MNRAS than to Nature.

As I said though, this is not the final nail. More observations of those new galaxies are essential to determine their distances and velocities, otherwise this is just a chance alignment. More simulations to examine exactly what we expect in a collision similar in quantitative detail (major kudos to the authors for proposing this, which largely offsets publishing it in the wrong journal) would also help, a lot. But it's awfully tempting to declare the mystery... well solved is too strong a word, but shall we say, addressed then ? Yeah, that'll do. Mystery addressed !

There is also perhaps some prospect of a weird afterlife for these oddballs. If they do form through good old-fashioned collisions, they're like teeny-weeny versions of the famous Bullet Cluster. This has caused a lot of controversy because Standard Model enthusiasts have pointed and laughed at MOND believers by saying, "Hahah ! Look, we found a case where dark matter can be separated from normal matter, and you can't do that just by modifying gravity !". To which MOND devotees responded by saying, "Yes it can, you don't understand our theory at all, and look the collision velocity you need is just too dang high." But then the Standard Modellers retorted with, "Yeah, but you don't have a unique relativistic solution, and anyway, the collision velocity is way lower than we though, so pfffffp !".

Or something like that anyway. I tend to heavily favour the Standard Model, but if we had more cases to compare, especially on such dramatically smaller scales like these galaxies, this might give us a much more rigorous comparison. The thing about MOND though it's that it gives such similar results to the dark matter paradigm that one wonders what the point of it is. So, we'll see.

Curiously cloudy

Today's paper is one that fell through the cracks of time from March 2021 all the way to the present day. Meaning, I should have read it a year ago, but didn't.

Returning to my favourite topic of optically dark gas clouds, this is an early science project of one of the SKA pathfinder telescopes. Naturally they chose a nearby galaxy group as a target. It's not an especially interesting group as far as I can tell, but looking at any galaxy group with the higher sensitivity and resolution of a new instrument is always a good idea.

What they found is... not much. None of the galaxies seem to be doing much of anything. Except, there are these two quite hefty HI clouds with no optical counterparts floating around and no obvious reason how they got there. Hooray !

Unfortunately though, God appears to have been very cruel by aligning both of these with optically bright sources. One is right behind a bright foreground star, which must be within our own galaxy and cannot possibly have anything to do with the cloud. The other is in front of a more distant galaxy, likely too distant to have any relation either. But of course this doesn't mean that the clouds don't have their own optical emission and it isn't simply being wiped out by the brighter intervening sources. They try to model and remove these, and don't find anything obvious, but I'd be very cautious about drawing any strong conclusions about this. There's just too much other optical crap in the way to be sure they've removed it all.

There's still a lot the data can say though. One of the clouds looks quite a lot like a standard rotating disc. The other is an elongated blob, but here the velocity gradient is across the shortest axis which is frankly just confusing - a spinning cylinder ? Doesn't make a lot of sense to me.

Combined with their size of a few kpc, the velocity gradients aren't very high. In fact they're low enough that the objects could be gravitationally self-bound with their HI alone. And their HI mass is pretty substantial, comparable to dwarf galaxies. So are these the long-sought "dark galaxies" of popular radio astronomy myth and legend ?

Maybe ! But no explanation is very satisfying, which is why these are really neat objects.

First, the standard go-to explanation for all HI clouds is that they must be tidal debris. This is a perfectly sensible default option. The problem in these cases is that there aren't many galaxies around, and the only others with HI are many hundreds of kpc away and don't show any signs of disturbances. Where are the long HI tails ? You'd expect to see these even if much smaller amounts of gas were removed, but that we see substantial amounts of gas in just these two clouds implies a factor of several more must have been removed - tidal encounters invariably produce streams, not just isolated clouds.

But given that the clouds appear to be self-bound, this may not be such a problem. Perhaps the encounter just took place a long time ago, so that most of the rest of the gas has dispersed. Without some dedicated numerical simulations, and even more sensitive observations to look for streams, we can't really rule out this scenario yet.

On the other hand, if they were dark galaxies, their self-bound nature is very surprising. The clouds we found in Virgo had such high line widths that they directly imply the presence of a dark matter halo to keep them stable, just like for normal galaxies. These objects don't fit that. The authors search existing numerical simulations for analogues, and they do find a few, but those tend to have higher line widths.

I would also note (though the authors don't plot this themselves) that these clouds have such narrow line widths and such high amounts of gas that they deviate from the standard Tully-Fisher relation for normal galaxies. Actually this looks very similar to those controversial galaxies without dark matter, which have been quite a challenge to explain. So it does seem a bit strange to invoke a dark galaxy for objects where there doesn't seem to be much indication of a dark halo. On the other hand, they do fit the standard mass-size relation for normal dark matter-dominated galaxies. 

What's going on ? I don't know, but this opens up quite a bit of parameter space for HI clouds. The Virgo clouds I've spent so much time on really can't be self-bound by their HI because of their high line widths : they'd have to be so compact that they'd have densities so high we'd expect explosive levels of star formation. These clouds, though they have ten times as much gas, are in a regime of size and velocity where they could indeed sit around quietly doing nothing. That they're so massive is really interesting : why haven't we found more objects like this already ? They were in fact already detected with much less sensitive instruments. That kindof implies a special explanation, rather than them being part of a larger population.

My preference would be old tidal debris. But I can't see any good reason why such clouds are found only in this group and not elsewhere, so this is more instinctive than rational. Those bloody intervening stars and galaxies make things especially tricky... yet even if they do have optical counterparts, that wouldn't necessarily make the situation any clearer : objects which are self-bound by ordinary visible matter are just strange, regardless of whether they're purely gas or purely stars. This is definitely a case where the rallying cry of "we need more data !" is absolutely legitimate.