Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Sunday, 26 June 2016

An Open Letter To My Local MP

EDIT : It was pointed out that this is a bit long so might not actually get read. A shorter version is below.

Dear Mr Williams,

I am writing to ask you to consider advocating in the House of Commons that Article 50 of the Lisbon Treaty should not be invoked by the current or future Prime Minister, without at least a full public consultation. I am an expatriate scientist currently living in the Czech Republic. Cardiff North was my home for 27 years and it remains my permanent residence. Cardiff University benefits directly and substantially from E.U. funding which may be impossible to replace in the event of Brexit, and scientists such as myself profit enormously from the freedom of movement provided by the E.U. Modern scientific practise would suffer immeasurably without this freedom. Thus, I have a vested personal and professional interest in preventing a Brexit.

It has become abundantly clear that many of the so-called "negative" warnings of the Remain camp were entirely accurate (for example the value of the pound has already dropped dramatically), while the Leave campaign consisted of outright lies (that immigration could be cut and that there would be an extra £350 million per week for the NHS) and vague, undefined promises that we could either find unspecified replacements for the E.U.'s many benefits, or simply renegotiate them from outside the E.U. The Leave camp themselves have admitted these mistakes, which were major parts of their campaign.

An e-petition has been created calling for a change in the rules of the referendum so that a second referendum would be required in the result of a narrow vote. Ironically the instigator of the petition was a Leave voter concerned that the Remain victory would be narrow and indecisive, but nonetheless it has now attracted over 3.3 million signatures at the time of writing. This makes it by far the most successful e-petition of the UK Parliament and represents about 10% of those who voted in the referendum. It is not clear how many of the signatories originally voted Remain and how many voted to leave but have since changed their minds, given the damage visibly being done to the UK following the vote. Therefore, the most sensible course of action is to (at the very least) delay implementation of Article 50 until a proper assessment of the situation can be made, possibly but not necessarily resulting in a second referendum.

It may be fairly argued that asking the public to vote repeatedly on an issue (or Parliament simply ignoring the result) because one side does not like the outcome is undemocratic. However, that is not necessarily the case, as I will try to argue.

First, the referendum was not legally binding and the result was very close. This is a decision of major importance with profound consequences that will last not for a few years as in a general election, but for decades. Membership of the E.U. is not something that can be routinely renegotiated on a whim, so the result ought to be decisive if we are to change course. It was a serious error that this was not included in the terms of the original referendum.

Second, the petition for a second referendum has already attracted more votes than the margin by which Leave won. If nothing else, some attempt should be made to estimate how many of those originally voted for Leave but have now changed their minds.

Third, it is clear that lies formed an important part of the Leave campaign, thus making the choice inherently undemocratic.

Fourth, voting for facts makes no sense. Politics may be a far more slippery arena than science, where opinion is more prevalent than fact, but even so it does not escape factual reality entirely. It is a demonstrable fact that leaving the E.U. is hurting the U.K., and no sane person votes for self-harm. Having been lied to and misinformed by the Leave campaigners, it is not undemocratic in the slightest to ask them to vote again (or even ignore the result) given the facts of the situation. By analogy, the American state of Indiana once almost voted to declare the value of pi to be exactly 3.2 - this would have been objectively wrong, but no-one would have complained about ignoring or repeating the vote in this case.

Fifth, this would not be entirely unprecedented. The Leave campaigners explicitly stated that they would continue to push for a second referendum in a result this close. Even more pertinently, the E.U. itself had Ireland conduct a second referendum on the Lisbon Treaty after Ireland initially rejected it. Considering to hold a second vote in the event that people have changed their minds based on the evidence is not the slightest bit undemocratic. Rather, seeking to avoid the years of turmoil that Brexit would ensure is the only responsible course of action possible in our representative democracy.

With kind regards,

Rhys Taylor


UPDATE : Here's a somewhat shorter version that stands more chance of being read.

Dear Mr Williams,

I am writing to you, my local MP, to consider advocating with the rest of Parliament that Article 50 of the Lisbon Treaty not be invoked at the current time. I am an expat scientist originally from Cardiff currently living in the Czech Republic. Cardiff University benefits significantly from EU funding, and the scientific rewards of freedom of movement cannot be overstated. It is not known if or how these could be replaced in the event that Brexit actually occurs.

Furthermore, it has now become abundantly clear that many of the the so-called "negative" warnings of the Remain camp were entirely accurate (for example the value of the pound has already dropped dramatically), while the Leave campaign consisted of outright lies (that immigration could be cut and that there would be an extra £350 million per week for the NHS) and vague, undefined promises that we could either find unspecified replacements for the E.U.'s many benefits, or simply renegotiate them from outside the EU. The Leave camp themselves have admitted these mistakes, which were major parts of their campaign.

The fact that the damaging effects of Brexit are already being felt should give us pause to reconsider the marginal result of the referendum. An e-petition calling for a change in the rules has currently attracted over 3.5 million votes, far more than the margin by which the referendum was won. The damaging consequences are now objective facts, not predictions. The consequences of Brexit are too serious and too dangerous to allow the result to be determined by a small minority who were repeatedly misinformed throughout the campaign. As in science, so in politics : once new facts are known, it is not the slightest bit undemocratic to reconsider one's position.

Finally, this move to stall Article 50 with the possibility of a second vote is not unprecedented. Nigel Farage himself is on public record as stating that a result this narrow would require a second referendum. The EU even required Ireland to hold a second referendum after the Lisbon Treaty was initially rejected. All things considered, it seems prudent to at least hold a period of public consultation to determine if the mood of the Leave campaigners has shifted.

With kind regards,

Dr Rhys Taylor

Saturday, 25 June 2016

Are There Hats ?

So Brexit is here, and it is easily the most depressing British political event in modern times. Will it bring about the next apocalypse, a plague of locusts and the end of civilisation ? No. But it is a seriously, historically, objectively bad thing. We are all of us going to suffer for this.


Nobody knows how badly the UK economy will suffer, but suffer it most certainly will. The sudden plunge of the pound (predicted by economists) was mostly market shock at us actually doing something so stupid, but the long-term effects are harder to foresee. How we will make up the £3.4 billion science shortfall (and yes, I place that of prime importance*, science is an important driver of social progress) is unknown. The 1707 Act of Union - one of the most successful political unions in world history, which ten years ago looked rock solid - looks unlikely to last much longer. The United Kingdom will dissolve and no longer be Great Britain, there will just be Britain - a wet, gloomy island home to some unimportant, disparate countries clinging to faded visions of past glories. We shunned openness with our neighbours and turned our back on the tolerant, inclusive society we have built over many decades. Worst of all, Nigel Farage - a demonstrably racist, xenophobic, bigoted idiot - is hero of the hour. That alone terrifies me.

* But if you insist that you want more, consider the effects to the financial sector, peace in Northern Ireland, the loss of job opportunities due to restrictions on freedom of movement, and basically all those benefits from joining the E.U. that are now under threat.

None of this is hyperbole or spin, it is simply reality. You might not like it - God knows I find it appalling - but it's true anyway.

Not that Britain hasn't suffered some pretty calamitous events before, only to eventually rise again. During the Viking invasions, England was reduced to merely Wessex before it saved itself from obliteration - only for the entire thing to then be conquered by Canute, and then his empire broke up on his death, temporarily restoring the Anglo-Saxon legacy. By the time of the Norman invasion England was the wealthiest country in Europe, and despite the carnage of Norman conquest, a century later British dominions stretched from the Pyrenees to Scotland. Most of that was lost during the disastrous reign of King John, but two hundred years later and Britain looked to be on the verge of winning the whole of France - only for Henry V to die at a young age. Thus began the chaos of the Wars of the Roses... and that's all centuries before the British Empire even began.

We have a long, rich, and complicated history. We endure. We have, indeed, suffered far worse than leaving the E.U., only to eventually emerge stronger than ever before, and we will likely do so again. A germ of hope can be seen in that the vote was split by age, with the youth strongly preferring to remain (it is no exaggeration to state that this decision has been inflicted on us by racist older people who won't have to live with the consequences of their decision nearly as long as those who didn't want to leave). The problem, of course, is in the "eventually". Things do not look likely to get better anytime soon.

The medieval chronicler Gerald of Wales put it thus :
For a sensible man ought to consider that Fortune’s favour is variable and her wheel is ever turning ... the Prince must take care, and always have imprinted on his mind the fact that although the merciful Creator ... is long-suffering and patient... He is likewise severe in executing punishment and vengeance upon the stubborn and wilful, and usually begins to exact that punishment here on earth.
Or in more secular terms things might not be apocalyptic and they may indeed one day improve, but you can't escape the consequences of your actions. Brexiteers have demanded they suffer no negative consequences for an action which everybody told them would have negative consequences. This is stupidity of the highest order. And it is stupidity, because despite a goddman overwhelmingly strong consensus from the experts that Brexit would be worse than staying in the E.U., we decided to do it anyway.

It wasn't even difficult - not in the slightest degree - to see that Brexit would be disastrous. It was crystal clear and obvious. The whole point of the E.U. is economic integration with its member states, but no-one put forward a remotely sensible plan for dealing with the aftermath of exit. "Oh, but we were a great power before the E.U." doesn't matter, because we are now dependent on the E.U. and they on us. Many things are complicated, but this isn't one of them. We didn't even have a plan to deal with the poll itself : it was non-binding (a much-overlooked fact) and no constraint was placed on how strong the result needed to be for change (a really, really stupid policy given the magnitude of the issue).

My predictions for the vote were wholly wrong. Generally, the only people who ever vote in E.U. elections in Britain are the ones who hate it, hence most of our MEPs are UKIP. Polls at the start of the campaign were well in favour of remain, so I was quite confident that the majority of Brits were not stupid enough to actively choose self-harm when it came to the push. My only worry was that the sheer disinterest in Europe would again act as a filter and only those passionately opposed to it would bother voting. I was wrong. I was quite sure that the large number of undecided voters would act conservatively, as they did at the Scottish referendum, and vote for a known quantity rather than taking a massive, incalculable leap into the dark. I was wrong about that too.

But the hardline element is one thing, and the less vocal majority quite another. No doubt the internet will be awash with many more excellent analyses than mine, but still I would like to offer a few thoughts on how so many people can have effectively decided that the Earth is flat.


Media Bias


First, there is a huge media bias in the UK newspapers. On the right, the Daily Mail, The Sun, the Daily Express, The Times and The Telegraph have a combined circulation of around 4.9 million. On the left, the Daily Mirror and The Guardian have a combined circulation of just 1 million. That only rises to about 1.3 million if you add the neutral The Independent and its shorter version, the i. And while the right-wing papers tend to be overtly political (the Daily Mail especially so, which runs essentially nothing but xenophobic headlines), on the left only The Guardian is really much of a political paper these days, with the Mirror tending to be mainly focused on celebrity boobs in bikinis (oddly, its website is rather better). Accounting for this, it would be entirely fair to say that the right wing political papers are about ten times more numerous than left wing or neutral ones.

Things are, however, considerably better for the left on television, with the ostensibly neutral but actually rather left-wing BBC News being by far and away the nation's most-watched news channel. Additionally, while Sky News are somewhat to the right, they are vastly more moderate than the terrifyingly racist liars at the Daily Mail. So media bias - if you give people constantly the wrong information they can't do anything but form the wrong conclusions, even if they're very intelligent - plays a role, but it can't be the only factor.

On the other hand, even the BBC has to report what politicians say - and large numbers of Tory politicians have been singing the same dreary song about immigration for years. For some reason that I am utterly unable to comprehend, they also gave significant air time to Nigel Farage even when he was a political no-hoper. Initially, Nigel made a lot less noise about foreigners and a lot more about Brussels bureaucracy, which is a lot nicer than talking about people being scared of Romanians. But it was insidious. As support grew, the BBC were then more obliged to report on UKIP, even as what they were saying increasing tended toward far-right propaganda. And that's something that should given anyone pause for thought - OK, people can say things without being locked up, but does that really oblige major networks to report what they said ?*

*And what they said is often also a lie, or a u-turn. Farage has been caught lying about the amount of money Brexit could save for the NHS and, with truly hideous distaste, that Brexit won without a shot being fired, barely a week after pro-E.U. M.P. Jo Cox was shot dead by a man shouting, "put Britain first !".

The upshot is that even the media most biased against xenophobia and the anti-E.U. lobby still had to report this, so people were still exposed to it more often than not. And you reap what you sow. A particularly pernicious fallacy has crept in that's even more damaging - the notion that anyone who disagrees with the right must be one of those on the "loony left", which exposes the great lie of the right that they are more tolerant than the left. They are not. Time and again those on the right call for for discriminatory policies against gays, transexuals, Muslims, and basically anyone who isn't a white Christian male age 25-45. No, not everyone on the right is like this (David Cameron deserves praise for his championship of gay marriage), but enough of them are. The idea that the left is more intolerant (even though it is, of course, very far from perfect) is just so much nonsense.

Unless we actively do something about this, it's only going to get worse. The moderating influence of Scotland will soon be lifted from England, which can only lead to more Tory governments. That can only make it easier to get away with right-wing bias.


Failures of the Left

Secondly, there has been a chronic failure of the left to properly address immigration. This started with Ed Milliband, who instead of championing the virtues of immigration decided to try and play the other side at their own game. This was a disaster, because no-one really believed that Labour either could or would enact a policy that restricted immigration. And I don't want that policy, I think it would be useless and divisive. I want Labour to welcome and stand up for minorities, not restrict their access.

Then again, we could have had the proper, grown-up debate about immigration that Nigel Farage "wanted" if he hadn't behaved like such an absolute tosser. If you want a sensible debate, you keep it entirely about population. That's all. You do not talk about foreign criminals or wealth or culture, because those things inevitably head south very quickly. As with all discrimination, you set one law for everyone and if people can't abide by it then tough on them. If that means their cultural values lead them to being criminalised, then they'd better learn to assimilate.

Corbyn's failure was different. While pro-immigration, he was (for no reason I could ever tell) remarkably lacklustre about the whole campaign. With just days to go he was talking about the removal of roaming charges and rating the E.U. as "7.5/10". OK, I'd probably give the E.U. about the same, but that's a pathetic way to run a campaign ! The value of freedom of movement - everything that Corbyn holds dear - cannot be overstated. As the leader of the opposition he should have been dancing from the rooftops about it, instead he just sort of quietly waved from behind a quite tall fence.


Anti-knowledge

Thirdly, and most importantly, expert concerns were brushed aside in a torrent of hate and fear. "I think the people of this country have had enough of experts" declared Michael Gove. Well, sorry people, but even in politics there are some facts, it's not all opinions. You can have whatever opinion you like on whatever subject you like, but that doesn't make it true. Voting on it doesn't make it true either. And just as in science, thinking you know better than the experts on fiscal or political policy is, well, arrogant and silly. You don't know how to do open heart surgery, why do you think you understand complex economic arrangements and political treaties ?

Inspired by Last Week Tonight in case you have no idea what I'm talking about.
Which is not to say that experts always get it right, because they don't. No-one can, that's impossible. But so many experts in so many different fields were saying that this was a bad idea, the whole notion that it would actually be a good idea ought to have been obviously wrong to everyone.

Democracy is a noble ideal. But it cannot function correctly when its citizens are misinformed and a culture of anti-knowledge exists (after the vote, searches for "What does it mean to leave the E.U. ?" soared, because apparently people hadn't thought to fact-check this beforehand). How can you really be making a free and rational choice if you're going to dismiss people who have spent far more time studying these complex issues than you, and if you weight opinions more heavily than facts ? What's the point of a democracy if people are allowed to pretend the Earth is flat ?

This is why people holding anti-science views makes me very angry, even when those views aren't actually harmful in and of themselves. Believing the Earth was created in six days doesn't make you evil. The problem is that when you allow utter bullshit (ranging from outright lies to ridiculously exaggerated levels of doubt) to replace hard, testable facts, you open the door to letting bullshit win everywhere. Encouraging the use of scientific method is not only about laboratory experiments, it is every bit about the political world as well. As Paul Kriwaczek put it  :
Social, artistic and scientific progress as well as technological advance are most evident where the ruling culture and ideology give men and women permission to play, whether with ideas, beliefs, principles or materials. And where playful science changes people's understanding of the way the physical world works, political change, even revolution, is rarely far behind.
Listening to the experts does not mean that everyone will think the same way or even agree with the experts 100% of the time about everything (the "flaw of averages" means that practically no two people agree on 100% of the issues). It simply means that when there is a consensus as strong that of Brexit, there wouldn't have been a chance in hell that we would have voted for it, any more than if a major cartography company were to decide the Earth was flat all along.


Where do we go next ?

The central issue here has to come down to freedom of speech. The UK has laws against libel - saying falsehoods which are damaging to an individual's reputation - but it's far less clear when it comes to statements which don't affect individuals but are nonetheless objectively wrong. Is it time, perhaps, to consider some sort of regulation about this ? I am not proposing any specific solution, just that the media bias is clear and demonstrable and we have to find away to make things more objective than they are now.

This doesn't mean that I don't want people to publish opinions I disagree with. But, again, not everything is an opinion, and the proliferation of outright lies and bullshitting are leading us down an extremely dangerous road. Can you really have freedom without truth ? Journalists are supposed to be impartial, which is not the same as objective. Currently much of the British press doesn't seem to be even doing that; just about every single paper apart from The Independent feels more like a campaign leaflet, trying to push an agenda rather than reporting the facts and opinions in any sort of proper context.

How we regulate what happens when people clearly and wilfully publish lies, I don't know. Perhaps we should try to deal with this with direct laws, or maybe a subtler approach is needed. We could consider limiting how many media outlets any particular individual or corporation can be associated with. Then there are the even trickier issues of the media reporting the truth systematically without any larger statistical context, e.g. only reporting the crimes committed by immigrants rather than the benefits they bring. At the very least, we need to start thinking about regulating political advertising.

I do not know how we deal with all this, but I am convinced we need to discuss the current state of the media. It is absolutely abysmal, and without some sort of reform our democratic society is going to be in very serious trouble.

The long-term solution to all this is actually relatively straightforward : invest more in education, especially at the primary school level. Teach children statistical methods constantly from a very young age, they're not that hard and they equip them with many other skills essential for rational thinking. Teach them the humanities subjects properly : get them to analyse poetry and search for hidden meanings, help them discover how they're being manipulated. In science lessons, emphasise the experimental and observational approach and always explain how conclusions are reached - never reduce it to base facts. Read them The Apology starting at age seven, because among other things it teaches the worth of thinking, the importance of realising that you might be wrong and the unjustified but lawful killing by the state of a philosopher telling truths that no-one wanted to hear. Apply a universal basic income as soon as possible, so that people actually have time to consider the issues in detail rather than working themselves to death.

But in the short term, I have no idea where we go. We are in ghastly, uncharted waters. I signed the petition calling for a second referendum; at the time of writing it had over 2 million votes. I have no idea if we'll get another chance or even if this wouldn't just make things worse*, but we need to continue the message that this is a bad idea. The issues that people are concerned with won't go away even if we won the second vote - we've got to continue to persuade people to see reason. Some of their concerns are even legitimate, the E.U. is sometimes overbearing, making new members join the Euro is just plain bloody daft, and it desperately needs a clear, consensual goal of what its role in the world should be.

* While updating this article this had reach 2.9 million, which is sufficiently large that I start to wonder if this might do some good after all. While I stop short of committing myself to the need for a second referendum, two things should be noted : 1) Nigel Farage is on record of saying that there should be a second referendum in the event of a narrow win for Remain; 2) The E.U. itself made Ireland have a second referendum on the Lisbon treaty after it voted the "wrong" way. So this outcome, difficult as it would be, would be neither unprecedented nor against the E.U.'s style of democracy. And consider this : if an uninformed map-making institute did vote that the Earth was flat, would it not be perfectly sensible to ask them to vote again after they had correct information ?

Right now I'm angry, depressed, and scared for the future of my country. I don't know how bad things will get, but there is a very real possibility that we will diminish ourselves to a level we have not seen in generations. What scares me the most is the popularity of fascists and populists at a time of economic hardship. We've seen this before, and the prospect that they will continue to gain ground is something I find too awful to contemplate.

Whatever happens, eventually better times will return. But unless there's a political miracle I can't see the next ten or twenty years as being anything other than an unnecessarily bleak and dark chapter in our long history. We are breaking apart when we should be standing together. I hope that somehow, in defiance of all the evidence, I'm wrong about this, but right now I'm seeing precious little cause for optimism about the future.
So long as the Arabs fight tribe against tribe, so long will they be a little people, a silly people, greedy, barbarous, and cruel. - from the film, "Lawrence of Arabia".

Monday, 20 June 2016

Into Darkness (II) : Attack Of The Flying Snakes

Cosmological Models Are Lousy And I Don't Like Them

Last time we looked at the observational evidence for and against dark galaxies. But that post was boring, why did you bother wasting your time reading it ? This one is much better. Here we'll look at the latest theoretical evidence for galaxies which don't do the whole star-spangled thing because it's "too mainstream".

The standard model of cosmology is that most of the mass in the Universe is dark matter. Simulations predict that this can reproduce the very large structures we see - filaments of galaxies and voids where there's not much of anything - extremely well. But on smaller scales, they're about as successful as any character ever played by Sean Bean.

With the notable exception of Sharpe, obviously.
The problem is that the simulations predict about ten times as many galaxies as are actually observed. Now, because dark matter is so dominant - about ten times as much as the visible matter (gas and stars, normally known as baryonic matter) - the simulations only use dark matter. Clever mathematical recipes are used to work out how much visible matter each dark matter "halo" should contain. Since there's so much more dark matter, the baryonic matter shouldn't be able to affect it very much.

On the other hand, the physics of gas and stars is very much more complicated. Stars emit radiation and winds which pushes gas away and heats it up. Occasionally they explode and spew heavy elements back into the interstellar medium, which can help the gas cool and form more stars. Then there are active galactic nuclei caused by supermassive black holes, which can inject even more energy into the gas. Not to mention magnetic fields, different phases of the gas, etc.


So even though the the baryons probably can't affect the dark matter all that much, the amount of baryons each halo contains could be extremely complicated : i.e. we might have got that wrong. Maybe the reason we don't detect all those galaxies is because they've never gained enough gas to start forming (m)any stars.


Dark Galaxies... Or Just Cosmic Fluff ?

As we saw previously, there are some very intriguing candidate objects for these (nearly) starless galaxies. The major objection is that simulations have shown they could just be "tidal debris" - gas that's been ripped out of galaxies during interactions. In that case these gas clouds wouldn't solve one of the most important problems in contemporary cosmology at all : they'd just be uninteresting bits of hydrogen fluff, floating through the Universe feeling a bit foolish and causing no end of red-faced embarrassment for anyone claiming to have found a dark galaxy.


Which of these explanations if correct ? Given that solving a very important problem is generally defined to be a very important thing, remarkably there are only two studies showing how such hydrogen fluff could form. And neither of them do a very good job, either.

First, here's a reminder of the parameters of the clouds that might be dark galaxies :
  • Gas mass of about 30 million times that of the Sun.
  • No more than about 55,000 light years diameter (though they could be smaller).
  • Line width (how fast they appear to be rotating, even if it's not actual rotation) of 100 - 170 km/s.
  • At least 300,000 light years from the nearest other detectable gas.
Both studies I'll describe were prompted by one particular object, VIRGOHI21, which is broadly similar to the AGES clouds. The main difference is that it's part of a much larger hydrogen stream, which these clouds aren't - but see the previous post and also this for details.


Tidal Debris : Attempt No. 1

The first attempt was a simulation by Bekki et al. 2005. This publication was only a letter, which means it's just five pages long so doesn't have much in the way of details.  But what they did was to model two galaxies and bang them together. One galaxy was modelled properly (like this), the other used (essentially) a single particle just to approximate its gravity. Which is reasonable as computation time is a limited resource, but of course it would be better to model both galaxies properly.

The results of Bekki's simulation of two galaxies having an uncomfortably close encounter. Blue shows stars and pink shows gas.
At this point the whole scenario proposed breaks down miserably. They state they tried a "large number" of models, but don't say how many. They show a representative sample of those models which do produce isolated debris, but don't state what fraction of their models produce this debris. So we're told nothing as to how likely is is that tidal encounters can produce these sort of features. They don't quantify what happens to their modelled galaxy, but the figures make it appear to be pretty devastated. And bizarrely, they define "optically dark" to mean "anything that's fainter than the Magellanic clouds"*. The Magellanic Clouds are bright enough to be visible to the naked eye. Ummm.... ?

* They don't phrase it like this, nonetheless, it's what their statement means.


Perhaps worst of all, the debris they produce is huge - at least five times larger and more massive than our observed clouds. In fairness, at the time of that study the observations of VIRGOHI21 only had low spatial resolution, so it could have been much larger. Subsequent observations showed that it's very much smaller. So the Bekki scenario is now decisively ruled out. There are other problems with the paper but they're not worth mentioning.



Tidal Debris : Attempt No. 2

The second main effort was by Duc & Bournaud 2008. This is a much more detailed paper which attempts to reproduce VIRGOHI21 with great precision. By this time the high resolution observations of the object had been released, so they knew the size of the object. Their procedure was similar to that of Bekki - hurling two galaxies past each other, one with gas and one without.

Contrary to popular opinion, the results of Duc and Bekki are actually extremely similar. Which is not all that surprising considering they did essentially the same thing. While they do form a gas blob in the tail that has the right mass and is the right size, its velocity gradient is about three times smaller than VIRGOHI21 or the other clouds. There are some other questionable points too : their progenitor galaxy is incredibly gas rich with a very extended gas disc, meaning it has lots of gas that can be easily stripped, and it's significantly less massive than the real galaxy. It's at the very limit of what's permitted by the observations.

Or in other words they gave it the best chance possible, and still it failed.

Now, since VIRGOHI21 was, at the time, thought to be a really exceptional object, those latter points might be acceptable. A weird galaxy that produces a weird object. OK, fair enough. But now we know there are more such clouds, ones which don't have streams at all. Worse though is the failure of the Duc model to reproduce the velocity gradient - they very sneakily adjust the scales on their figures 2 and 6 (observation and simulation respectively), making it look as though they made something much more similar to the real VIRGOHI21 than they actually did.

The observation (left) and simulation (right) data as they appear in the Duc paper, on separate pages. Let's face it, the sharpness of the "kink" in the stream at VIROGHI21 isn't well-reproduced by this simulation even in this figure.
After correcting the figures to have the same scale, it's clear that the simulated VIRGOHI21 is even worse. There's absolutely nothing in the simulation that anyone in their right mind would mistake for a dark galaxy, which was the whole aim of the paper. Tsk tsk !


On the positive side, the Duc model does also produce a lot of gas north of VIRGOHI21. Very sensitive ALFALFA observations had detected this, which a third, far less well-known model of the system had failed to predict. This approach, by my PhD supervisor, had VIRGOHI21 as dark galaxy that came along and pulled out a long gas tail. This one was never published in a refereed journal, which is why I'm not going to go into any details.

So both major studies have failed to reproduce one of the most well-known dark galaxy candidates. The Duc model got pretty close, but that velocity width is a real show-stopper. You may not think that's such a big deal, but, as I shall show, you'd be wrong. It's a massive problem.

Which is not to say the Duc paper isn't important, because it is. It showed that the large-scale properties of the system could be reproduced in quite a simple way that didn't require a dark galaxy. It's completely understandable that people would assume the sharp kink is a mere detail. Understandable, but wrong.


I'd Like To Hear From Fictional Mathematician Ian "I Nearly Got Eaten By A Tyrannosaurus" Malcom At This Point



Not quite, Ian. Actually the weakness of the previous studies was that the authors never stopped to think if they would. That is, they came up with possible formation scenarios, but they never investigated how likely they were to really occur. Which was reasonable at the time (with just one weird object, it's perfectly fine to invoke a weird explanation), but with more such objects now known that needs to be addressed.

So what we did in the latest paper was to model the entire cluster, using an existing simulation. Not just two galaxies any more, but 400. Of course we couldn't model the gas in each galaxy because that would be far too computationally expensive, but now we could model the gravity far more accurately than the previous studies. Rather than dropping a gas-rich galaxy through the cluster, we dropped a gas stream.

"WHAT ?!?!" I hear you cry. "THAT'S F*£!@ING RIDICULOUS, YOU DOLT !"


I know, I know. If we wanted to do a proper comparison of the previous studies, we should have used a galaxy. Our scenario was even more ambitious though, because there's another mystery in the Virgo cluster : there aren't many long hydrogen streams there. Only four, in fact, plus a few much shorter ones. Simulations predict that such features ought to be very common - but they're actually extremely rare.
All known hydrogen streams in the Virgo cluster. Only the big ones are easily visible and labelled here - there are in fact four large streams in the cluster (HI1225+01 is probably a bit further away) plus another half-dozen or so very short streams.
So our idea was simple : maybe the long gas streams get broken up by interactions with all the galaxies. Maybe tearing up the stream produces fragments like the clouds that we see. Then we'd kill two birds with one stone and everyone would shout, "hurrah !".

Except the birds, obviously.
OK, big problem. The simulations which predict the existence of those streams are about something called ram pressure stripping. Now I have to disappoint you here, because that's nothing to do with forced bestiality at all.


Sorry. Anyway, galaxy clusters also contain hot, diffuse gas of their own. As galaxies move through it they should create a "ram pressure" which is strong enough to strip their own gas into long streams. Our simulations don't include this hot gas, which is not good. We really would like to include it, but it's much harder from a technical standpoint. Better to start simple and build in the more complicated physics gradually. And neither Bekki nor Duc included the hot gas, so there.

video
Simple simulation showing gas being ram-pressure stripped
from a galaxy.

But while in some ways our simulations weren't any better than the previous ones, in one very important respect we made a huge improvement. We didn't just drop one stream into the cluster, oh no. We dropped them in batches of 27, with each one at the corner or midpoint of a cube. Like this :

The grey spheres represent the other galaxies in the cluster. If you really want to you can watch a movie of this here, but it's quite dull, so don't.
We varied the starting mass, temperature, and distance of each batch of streams from the cluster centre. All told we ran something like 200 simulations or more. Then (unlike Duc) we converted the data into what we would detect with Arecibo so we could directly compare the simulations and observations.

To be clear, we didn't drop all 27 in at once. We dropped them in one at a time. The point is not to find out if there's some particular path through the cluster which produces fake dark galaxies, but to see how likely such objects are to occur by chance. And although we used a realistic model for the cluster gravity, we're still missing a lot of important physics - not only the intracluster gas but also heating and cooling, star formation... and of course the galaxy from which the gas stream originated. I suspect that most of these won't change the end result that much, though you'll have to read the paper for more details.

The most satisfying result from the whole shebang was that a lot of the streams look quite remarkably like snakes. The second most satisfying result was that the referee of the paper didn't object to the title.

ATTACK OF THE FLYING SNAKES !
... although I suppose that should really be falling snakes.

Life Finds A Way ?

Despite the many, many limitations, the results were remarkably decisive. The "tidal debris" idea fails miserably!



Yes, really ! We produced features just like those seen in Duc and Bekki easily - long features with shallow velocity gradients occur pretty nearly all the time. But clouds like the ones we observe in the real cluster ?


They just don't happen. Strictly speaking they happen 0.2% of the time, which means that "tidal debris" is a patently ridiculous mechanism to form all the clouds we see in the real cluster*. Don't believe me ? Watch the movies for yourself. Like Bekki, we made synthetic observations so we could accurately measure what we'd actually detect.

* Of course we're not claiming that tidal debris is never a good explanation - it's fine for larger clouds or ones with smaller velocity widths - just that it doesn't work for features like the ones we found.

White shows the particle data, red shows what we'd actually observe with Arecibo, and green shows the very isolated clouds similar to the weird ones we were trying to explain. Too difficult to see ? Have a look here.
There's more. We also tested the alternative idea that the clouds could be dark galaxies by dropping in galaxies containing gas but no stars. And again, you can watch the results for yourself.

See the full set of discs here.
Quite unlike the streams, nothing of any consequence happens to the discs. Even after being harassed by 400 galaxies for 5 billion years, they generally lose no more than half of their gas. To an AGES-class survey, they'd appear to do... well, nothing much. The gas density stays low too, so they wouldn't even form stars.

In fact it's even more of a dramatic win for dark galaxies. Last time we saw how one of the weirdest properties of the real clouds is not that they appear to be rotating rather fast, but that they're rotating faster than we'd expect based on their mass. It turns out that the dark galaxy models can explain this pretty well, whereas the tidal debris scenario just can't. In fact, if you consider clouds which not only have the correct line width but also the correct mass, that 0.2% drops to 0.0% ! Even the bits of tidal debris which do occasionally come close to matching the deviation only do so for about 50 million years, and then they typically disperse and become undetectable, whereas the dark galaxies remain with the correct high line width and detectable for the whole 5,000 million years of the simulation.


Though there are a couple of points to bear in mind. Firstly, the simulated debris at least moves in the right direction - it also has a higher line width than expected, just nowhere near high enough. Second the simulated dark galaxies don't agree perfectly with the real clouds - however, that's almost certainly because setting up a stable disc is hard. If we spent longer tweaking the initial conditions, we could get the simulated dark galaxies to be in much better agreement with the real clouds.
That, then, is that rarest of things : a sweeping and decisive victory for dark galaxies and a total defeat of the idea of tidal debris. But let's not get too cocky. Remember, our simulations don't include the intracluster gas, the gas in the streams is constrained to have the same temperature, none of the harassing galaxies have gas, and for our streams we didn't include the progenitor galaxies. These are all problems that do need to be addressed. However, the previous studies didn't include the intracluster gas, they kept its temperature constant, and their harassing galaxies were also gas free.

Since our results are often similar to the previous studies, that indicates that the lack of a parent galaxy probably isn't significant. For the first time we can quantify how likely the tidal debris theory is for clouds with high velocity widths : it isn't. That factor of three turns out to be a huge problem for the Duc and Bekki models - not the minor detail everyone seemed to think it was.


Mystery Solved ?

You might wonder why, if we'd got to the stage of dropping dark galaxies into the cluster, we didn't drop normal ones as well. That's surely the next logical step. The paper was 26 pages long at this point though - dangerously long, to the extent that people might not read it. And setting up a stable disc is not so easy, it isn't a matter of just tweaking the numbers of the discs we already used. So it's better to leave that to a second paper. It's tough to see it dramatically affecting the results though.

It's difficult to say how the other parameters would affect the results. The intracluster medium should, in my opinion, make the cloud's velocity widths if anything smaller. Any expansion velocity would be met with pressure acting to prevent it from expanding, slowing it down (though fluid effects are complicated and it's not always a good idea to guess what they'll do). Cooling of the gas has been shown to make a relatively small difference compared to harassment, so that probably won't change anything much either.

Smug as I am with this unexpectedly exciting result, the message I want to end on is rather different. Yes, we got a neat result. But we also know there are problems, and we should improve things accordingly. We're going to solve this one, dammit. But we're going to do it properly - not with hand-waving explanations that don't stand up to scrutiny. So far, the poo-poohed idea of dark galaxies is doing far better than the much more popular idea of "tidal debris". It's too important an issue to dismiss this with models that don't actually work and haven't been repeated. And we should always, always, always bear in mind that a model which works is not the same as a model which is correct.


Because humans are made of squishy neurons rather than silicon chips, belief occurs for both rational and irrational reasons. Scientists have irrational beliefs just as much as anyone else does. In this case, they've become so enamoured of the idea that the clouds could be tidal debris - which I think we've shown isn't even tenable anyway - that they've assumed that that's what they probably actually are. Yet the two situations are very different. Yes, I can run around naked hurling my own bodily fluids at people, but this doesn't mean I'm actually going to do it.

By the same token though, these results aren't enough to make me "believe" in dark galaxies. In fact if some masochist decided to whip me until I was forced to confess my preferred explanation (I dunno what kind of messed-up universe it would take for that to happen, but I'd like to avoid it), I'd probably still say "tidal debris". I couldn't give you a rational reason, but something about the idea of dark galaxies just doesn't smell right.

All we've really shown is that dark galaxies are more likely than was previously thought - nothing more, nothing less. It's a horrid cliché to end on, but often more research genuinely is needed. Given the dismissal of the alternative ideas thus far, if I can convince people that cheering, "tidal debriiiiiiis !" isn't a good enough explanation, I'll be happy.

Into Darkness (I)

My latest paper - sixth as first author - is now online. It's been in draft for about 18 months (as long as two pregnancies, so you decide which was the better choice...), taken over 200 simulations, and the resulting monster is 27 pages long. What great quest could possibly have warranted such fanaticism ? Find out in this and the next blog post. Or if you just don't care that much but are vaguely curious, read the super-short version here. If you don't want the background information, you can do directly to part 2 here.

To start, we have to go much further back than the last 18 months.


Everyone Should Have Their Own Life-Defining GIF

Back in the heady carefree days of 2006, I was a fresh-faced* and fancy free** Masters student about to embark on a PhD course. The direction this took was largely thanks to this, the single most important gif of my life so far :

*Spotty.
** Hapless.


I have, of course, explained such 3D data cubes many times before, so let's keep this simple. What you're looking at is a map of part of the Virgo galaxy cluster as seen through a radio telescope. Instead of looking at the stars, you're seeing the hydrogen gas. It's easier to explain in a 2D image. Let's compare what we'd see in "normal" visible light with these gas-sensitive radio wavelengths  :


The really bright big blob you see in the top gif spinning around is the gas in the spiral galaxy, NGC 4254. You can also see it has a long stream of gas, which apparently ends in a bit of rather denser gas that's the object known as VIRGOHI21. Pronounced, "Virgo H one twenty one". "HI" is really H Roman numeral 1, and is the symbol for neutral atomic hydrogen (as opposed to H2, which is molecular hydrogen, and HII, also pronounced "H two", which is ionised hydrogen). "21" is just the object's catalogue number in the survey.

If this is confusing, don't worry. Forget the molecular and ionised hydrogen, they're a bunch of smelly pants. We're only looking at HI . And the visible light from stars, but that's not so important either. The important point is that by measuring the gas, we can measure how fast everything's moving. If we're clever, we can use this to work out if things are rotating and the speed of rotation if they are.

The interesting thing about VIRGOHI21 is that it appears to be rotating. And if it's stable, just like ordinary galaxies it would also need a lot of dark matter to hold it together. So when I started my PhD, it was being widely touted as the first detection of a so-called "dark galaxy" - a dark matter cloud (or "halo" as they are usually known) containing a rotating disc of gas, but no stars.

Actually it wasn't the first time such a claim had been made at all, but I'll get back to that.

You might well be thinking, "Hang on a minute, you four-eyed beardy twerp. There's a great big complicated stream of gas coming out of NGC 4254, so how do you know this VIRGOHI21 thingy isn't just a kink in the stream ? Eh ? How do you know it's really a massive rotating disc ?". Well, we don't. To understand why we'd look at this object and say, "dark galaxy" and not "weird kink", we need a bit of backstory.


Galaxies Don't Need Stars, That's Just RACIST !

The theoretical basis for dark galaxies was that they could solve the "missing satellite" problem. Models predict about ten times more dwarf galaxies than we actually observe, so the idea was that maybe some galaxies just never have enough gas to form much (or any) stars. An elegant solution to a complicated problem that I'm grossly over-simplifying. It's one of the biggest problems in contemporary cosmology, so shouldn't be taken lightly. Any proposed solution has to be treated with strong skepticism*.

* And no, fellow Brits, I refuse point blank to spell it "scepticism". That would be pronounced like in septic, which would be unpleasant for all concerned.

The observational measurements just say that VIRGOHI21 could be a massive rotating disc. Or it could be a much less massive kink in the stream, which might have formed when another galaxy flew past and pulled the gas out of NGC 4254. The idea here is that it looks like it's rotating, but isn't really. But there was some pretty intriguing other evidence, at the time :
  • Unusually, NGC 4254 has one particularly prominent spiral arm. This could be the result of interacting with a massive object.
  • There's no obvious normal galaxy visible nearby that could be responsible for the prominent spiral arm or the gas stream.
  • The kink in the gas stream is very sharp compared to the rest of the stream. Why should only this one feature be so strange ? Few (if any) other streams show features like this.
If VIRGOHI21 really is rotating, it would be massive enough to explain the disturbance to NGC 4254. Galaxies this massive and dark would be controversial, but some models said they were possible. The very sharp kink in the velocity would be because VIRGOHI21 is a compact object, which is perfectly consistent with the observations. And based on what was known about galaxy interactions at the time, it was very difficult to see how an interacting galaxy could produce a stream with all these particular features.

My Masters project had been all about simulating the formation of a spiral galaxy without dark matter. Since this hadn't worked, my natural skepticism of dark matter had taken something of a blow. Not a mortal wound by any means, but enough to give pause for thought. In any case, VIRGOHI21 had turned up in a relatively low sensitivity survey of the cluster. Having access to much deeper AGES data (the Arecibo Galaxy Environment Survey, one of the most sensitive surveys of the cluster ever undertaken) as part of my PhD, the prospects for detecting other dark galaxy candidates looked good. Even if they turned out not to be real dark galaxies, it seemed pretty likely that we'd find lots of cool streams and other neat, weird objects.

It didn't really work out like that.


Where Are All These "Dark Galaxies" Then ?

The AGES data I was given charge of for my thesis consisted of two parts of the Virgo cluster. Well, it seemed like such an interesting place that surveying it twice was a sensible enough idea (observing the whole cluster would take much too long).

VC1 is the larger area in red while the smaller area is VC2.
VC2 contains pretty much bugger all. More accurately, it contains precisely bugger all that hadn't already been discovered. To be really accurate, the "bugger all"-ness is confined to new discoveries from the hydrogen data, but that's another story.

Fortunately, VC1 turned out to be a nice happy data cube full of lots and lots of interesting detections. But did it contain lots of hydrogen streams and dark galaxy candidates ? No, no it did not.

Whoa, steady on there, me ! It's true it didn't contain any streams. But the dark galaxy candidates statement is, like, totally way melodramatic. In fact we detected eight hydrogen clouds without any obvious galaxies present. Six of them seemed to be rotating very nearly as fast as VIRGOHI21. So why do I seem reluctant to say that we didn't find any dark galaxies ?

Regular readers will be screaming that this is the most boring visualisation I've ever done. Where are the glass cubes ? Where are the 3D movies ? Unfortunately, while HI is often stunningly beautiful, these particular detections are just too faint  and too small to show any details. 

There's No Way A Bunch Of Poxy Lines Are Going To Convince Me You've Solved The Missing Satellite Problem.

By this point the overall situation had changed. A rather convincing simulation had been published demonstrating that VIRGOHI21 could, in fact, just be the result of an ordinary encounter after all. You may well have been right to doubt my four-eyed beardy twerpiness proclaiming it to be a rotating disc, because this model said that it was indeed just a kink in the stream. It seemed that it's just an illusion of rotation and therefore totally uninteresting.

I'll save the details of that simulation for next time, though most people were rather swept away by the result. Cooler heads noted that the proposed model was far from perfect, but at the time, most people thought these flaws were minor details that needed ironing out - not gaping holes that meant the whole sorry thing should be burned and the ashes spat on and scattered to the four winds.


A perhaps even more serious problem was the extremely low numbers of these candidate dark galaxies. My supervisor's model had predicted that as many as 23% of our survey's detections should be dark. Even with these objects it was at most 1%, so that particular model was decisively ruled out. Which meant that dark galaxies didn't seem likely to explain the missing satellite problem, which was the reason they were proposed to exist in the first place.

There's a third reason. The Tully-Fisher relation is a very well-established correlation between how fast a galaxy is rotating and how much ordinary matter (gas and stars) it contains. In fact it's so well-established that if you wanted to define a galaxy, you could almost say, "anything that obeys the Tully Fisher relation". While even the faintest (but still optically visible) little smudges we'd found in the Virgo cluster with our survey seemed to obey this relation perfectly, these objects which were apparently totally dark do not.

The faster a galaxy appears to be rotating, the more massive it is. The "Tully-Fisher Relation" just refers to the precise numbers : knowing one, you can calculate the other. That's the red line in the picture. It works pretty well for regular galaxies, but doesn't work for the dark galaxy candidates.

And when I say faint smudges, I mean really faint stuff like this :


All of these pathetic little scraps of starlight sit on the TF relation just fine - even VC2_30 (bottom centre), which looks more like someone's sneezed on the lens than a proper galaxy. They have at least a hundred times less mass than giant objects, but they all obey the same relation. The fact that the truly dark objects don't follow this correlation, whereas the merely extremely dim ones do, marks them out as categorically different.

Taken together - the simulation showing VIRGOHI21 was tidal, the insufficient numbers of dark clouds, and the deviation from the Tully Fisher relation - the case against dark galaxies seemed pretty darn damning.



So Galaxies Have Stars After All Then. So Much For Galaxy Justice Warriors.

And yet... these clouds remained unexplained. Yes, there was one model showing how they might be formed - but only one. Public outreach articles will tell you that science values repetition. Well, sometimes this is if anything excessively true, but not always. In this case - for whatever reason - nobody saw fit to go back and re-examine the model and test it using other codes or explore its implications more fully. And while more clouds continued to be discovered from time to time, this one particular model was almost invariably cited to explain them.

Almost invariably. There were a few for which the basic idea of "tidal debris" (i.e. the gas was pulled out of a normal galaxy during an interaction) just didn't make any sense at all. I've previously described a couple of examples - the Smith Cloud and Keenan's Ring - in detail. At least one other example deserves a look in : HI1225+01, sometimes known as the Giovanelli & Haynes cloud.

Gas distribution shown by the white outline. I'd make a better image but I don't have the raw data to play with.
This enormous cloud is at least 650,000 light years long with a mass of around 4 billion Suns (the stupid poxy little clouds I found are at most 55,000 light years across and have masses no more than around 30 million Suns - more than ten times smaller and a hundred times less massive). True, HI1225+01 has a small galaxy at one end - but it's very small indeed. Far too small to be a credible source of all that gas. It only remains to choose which confused animal meme seems most appropriate.

This dog is exactly like the gas in HI1225+01. It has no idea where it is or what's going on. Also it's a bit damp and surrounded by adorable otters, which is probably where the similarities end.

Yes Alright, But The Universe Is A Jolly Big Place, And There Are Bound To Be A Few Weird Things We Can't Explain.

Indeed. But there seems to me to be enough weird objects that we should stop and take a good hard look at what's going on. To try and make sense of all those otters, I decided to catalogue them. That is, I did a literature search for all the really long hydrogen streams and dark galaxy candidates. Then, as best as I was able, I took all the available measurements and made a table of their properties.

The numbers aren't large - around 40 for both the streams and dark galaxy candidates. Even so, finding all of the damn things, combined with the need to re-measure just about everything, took several months. Still, the awkward fact remained that the numbers are thousands of times too low to explain the missing satellite problem.

But the world turned, and turned again. There were hints that there could be much larger populations of some objects in some regions - not as many as models predicted, but enough to make one wonder. A few new objects seemed to have signs of ordered motions - a really strong signature of rotation (the data for the clouds I found wasn't high enough resolution to show this even if present). Much larger cosmological simulations began reviving the idea that maybe not all dark matter halos contain enough gas to form stars. And then there were of course the discoveries of huge numbers of very large but very faint galaxies - not quite dark galaxies, but certainly very dim indeed.

So the mood of the community had apparently done a complete 360. Maybe several. It had gone from originally thinking that dark galaxies should be very common to dismissing them as unlikely when they weren't immediately found by the first large-area HI surveys. It had turned to outright hostility when they continued to not be found, to the point where alternative explanations for any weird objects became demanded rather than requested. Now it's swung back, at least in some quarters, into a tentative sort of toleration.


Make Up Your Bloody Minds !



If you look hard enough, you'll be able to find a qualified expert with a strong but ridiculous opinion about anything. There were and are certainly some people in this field fervently convinced that dark galaxies do and do not exist. And there's more than a small element of, "my dark galaxy candidate is much better than your dark galaxy candidate" at work. But the more I looked, the more and more it seemed to me that no-one had a really good idea of what was going on. This must, I imagine, be an area that's particularly difficult for journalists - lots of very strong opinions from confident-sounding experts, but no consensus.

Anyway all my cataloguing had left me convinced that my little AGES clouds were indeed particularly weird objects*. None of their nearest galaxies looked disturbed or showed any signs of extended Hstreams. Very few other clouds showed a similar deviation from the Tully-Fisher relation. If their velocity width wasn't due to stable rotation, they ought to have disintegrated so quickly we'd be unlikely to detect them at all. They could be held together by their gas alone... but that would mean they'd have to be incredibly small and dense and form stars like nobody's business. At a more reasonable size, they probably couldn't survive for very long in the chaotic Virgo cluster without dark matter.

* Obviously, I'm not guilty of the my-candidate-is-better-than-your-candidate problem at all, because I'm a special snowflake.

The thing is, none of the original problems with dark galaxies were by themselves fatal. True, not enough had been detected. But we don't really have a great understanding of how gas gets into halos, so maybe most of the dark matter clouds don't even contain detectable amounts of gas. It's a bit like saying, "you told me there were three Easter eggs in this room, but I only found one, so whatever the heck it is I've found, this big chocolatey egg-shaped thing can't possibly be an Easter egg".

THIS IS NOT A MAGRITTE JOKE.
Likewise we don't really understand the Tully-Fisher relation all that well either. Actually the fact that faint galaxies lie on the TFR is surprising ! And these dark little clouds don't, so that becomes very interesting indeed. At least one of the optically faint but not totally dark galaxies also deviates in the same way. So defining galaxies as "anything that obeys the TFR" may be a teensy-weensy bit over-zealous. And here's the really interesting thing : lots of new, extremely faint galaxies have recently been discovered in clusters, and at least of them (probably more !) seems to be deviating from the TFR just like the clouds do.

Which only leaves that pesky simulation showing that objects like VIRGOHI21 could just be tidal debris. Now here I have to tread a very careful line indeed, because the authors of that study are certainly vastly more experienced than me when it comes to galaxy simulations. And yet the more I examined their findings, the more doubts I had about their conclusions. Their own figures don't show anything resembling VIRGOHI21. Worse, they were so preoccupied with whether or not they could explain VIROGHI21 they never stopped to think if their mechanism would. That is, a possible explanation does not mean it's a very likely one, let alone correct.


Summary

For a topic this controversial, even this long-winded post is a simplification. But it boils down to a few simple points :
  • If galaxy formation models are correct, then there must be a lot of dark matter halos which don't contain any stars and gas. Some might contain just enough gas to be detectable but not enough to form stars. 
  • But on the other hand, the models aren't that sophisticated. Although all models predict lots of dark matter halos, the numbers predicted to have just enough gas to be detectable but no stars vary wildly depending on the exact values used.
  • Gas-rich galaxies with no stars appear to be extremely rare... but, tentatively, a few might exist. That means the models of galaxy formation might be basically correct after all. So even discovering just a few dark galaxies could solve the missing satellite problem.
  • But we don't have a great model for the origin of galaxies and alternatives say that what appear to be dark galaxies could just be the relics of old interactions. So using a very few detections to infer huge numbers of truly dark, undetectable halos is rather dodgy, at best.
  • Galaxies with very few stars and no gas do exist in large numbers with certainty. Those stars must have formed from gas at some point, so truly dark galaxies should exist.
  • But we have absolutely no idea when they converted their gas into stars, or what caused some to remain dark and others to light up. 
  • A very few objects are known which are rotating more quickly than expected based on their observed mass - meaning they have extremely high amounts of dark matter. That would explain why they're not on the Tully-Fisher relation.
  • Yes, but we don't know if those AGES clouds are rotating because the observations don't have enough resolution. Tidal debris is at least as good an explanation. And even really dim galaxies follow the Tully-Fisher relation (despite predictions that they shouldn't), which suggests that totally dark galaxies should as well.
Let me put it another way :


We don't have a good idea of what's going on here. Anyone who says otherwise is guilty of wishful thinking. The only way to proceed is incrementally, point by point, slowly and very carefully tweaking the nipples of knowledge. Or something. But clearly, understanding these objects relies as much on theory as it does observation. So next time we'll head deeper into darkness and look at one of those key points : the popular idea that the objects could be some form of tidal debris.