61. The crisis in the dark matter problem becomes a historically unparalleled failure in the scientific method

This year, Pavel Kroupa was asked to hold a Golden Webinar in Astrophysics on the dark matter problem. This contribution provides the link to the recording of this presentation which has now become available on YouTube. In this presentation, Pavel Kroupa argues that the dark matter problem has developed to become the greatest crisis in the history of science, ever. This contribution also provides links to recordings available on YouTube of previous related talks by the same speaker from 2010 (the Dark Matter Debate with Simon White in Bonn) and 2013 (in Heidelberg). This might allow some insight into how the debate and the research field have developed over the past dozen or more years.

1) Golden Webinar: “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”

On April 9th, 2021, Prof. Pavel Kroupa presented a talk in the Golden Webinars in Astrophysics series on “From Belief to Realism and Beauty: Given the Non-Existence of Dark Matter, how do I navigate amongst the Stars and between Galaxies?”. The talk is now available on Youtube:

The slides to the talk without the fictitious story can be downloaded here:

If you are interested in other talks presented during The Golden Webinars in Astrophysics series, you can find the record of those already presented on their Youtube Channel, and here is a list of upcoming talks. The Golden Webinars are provided as a free public service and have no registration fees.

2) The vast polar structures around the Milky Way and Andromeda

In November 2013, Prof. Pavel Kroupa presented “The vast polar structures around the Milky Way and Andromeda” in the Heidelberg Joint Astronomical Colloquium. In the talk he discussed the failures of the Standard model of cosmology and the implications for fundamental physics.

A blog entry from 2012 on the vast polar structure (VPOS) of satellite objects around the Milky Way can be found here.

3) Bethe-Kolloquium “Dark Matter: A Debate”

In November 2010, Prof. Simon White (Max Planck Institute of Astrophysics, Garching) and Prof. Pavel Kroupa (University of Bonn) debated on the concept and existence of dark matter during the Bethe Colloquium in Bonn. Their presentations and the subsequent debate are available here:

a) Presentations by Prof. White and Prof. Kroupa

Summary of both presentations:

b) The Debate

The German-language television channel 3sat produced a TV report on the Bethe Colloquium, which can be also found on Youtube (available only in German):

Part I

Part II


In The Dark Matter Crisis by Moritz Haslbauer, Marcel Pawlowski and Pavel Kroupa. A listing of contents of all contributions is available here.

39. Question E: The Dark Matter Crisis continues: on the difficulties of communicating controversial science

(Continuation of the series A-E)

There has been an unsuccessful attempt to close down The Dark Matter Crisis. Here is the story (and an email by Jim Peebles): UPDATE: The guest post is now online.

As regular readers of our blog know, and first-time readers may be able to guess from this blog name, Pavel and I mostly write about the problems and shortcomings of the dark matter hypothesis. One aspect of our research is to test dark matter models on cosmologically small scales such as the Local Group of galaxies. Over the past years, our research and those of others has revealed that numerous model expectations of the dark matter hypothesis are not met by observations. This led us to the conclusion that we should consider a paradigm shift in how we understand the dark matter phenomenon. Maybe, we thought, a modification of the laws of gravity, one possible approach being Mordehai Milgrom’s MOdified Newtonian Gravity (MOND), could solve these issues.

Doing research that identifies shortcomings in a widely-held assumption and that is skeptical of a mainstream hypothesis is certainly a very interesting and rewarding endeavor for a scientist. It is closely connected to the fundamental scientific method of falsification and holds potential for groundbreaking discoveries. However, working on a controversial scientific topic also has its downsides. For one, papers criticizing basic assumptions are less attractive to be cited in mainstream publications. And before publication, controversial science already faces a more challenging peer-review process. For example Ashutosh Jogalekar explains in his blog The Curious Wavefunction:

“[…] reviewers under the convenient cloak of anonymity can use the system to settle scores, old boys’ clubs can conspire to prevent research from seeing the light of day, and established orthodox reviewers and editors can potentially squelch speculative, groundbreaking work.”

In addition to these ‘formal’ scientific interactions via academic publishers, there is also communication amongst scientists. For instance, early PhD students, who are still in the process of learning about the business of doing science, may be looking for advice from mentors and other more experienced scientists. Unfortunately, when the talk comes to controversial areas of science, students are often discouraged from getting involved in non-mainstream research (note, however, Avi Loeb‘s opposite advice). This begins with the commonly expressed belief that such research might “hurt your career”, but sometimes even more direct warnings are made. For example, a few years ago a professor told me that he would never hire someone who has published even a paper on MOND. A fellow PhD student got a similar piece of “advice” while visiting a different university, where one scientist advised him that he should only publish results which are negative for MOND, but nothing in support of it.

For people who are just starting in science, especially, such comments may be alarming. Graduate students do not yet know much about the job market. They therefore tend to believe what the ‘old boys’ tell them. To researchers who have a bit more experience, such warnings are often incomprehensible since they know by then (if they didn’t already initially) that it is entirely unscientific to withhold research results that do not fit a pre-determined picture.

The difficulties of working in a controversial field of research do not stop here. Communicating such science to a wider audience can also result in problems. While the public is generally very interested in the challenges faced by prevailing theories, there are difficulties to overcome. One of them is the question of how to differentiate completely unscientific things (the paranormal, creationism, …), from actual, albeit controversial, science.

A promising approach to overcome this difficulty is to discuss controversial science publicly. This way, the public can follow and be part of the debate, learn that arguments are backed by references to peer-reviewed research and see that hypotheses need to be tested through comparison with observational data—essentially the public gets to view the scientific process as it is applied in any branch of research. By demonstrating that scientists stick to facts, respond to opposing arguments and do not resort to emotionally driven rhetoric, we can adequately demonstrate the strengths of science.

The strength of the scientific method over dogmatic beliefs should always prevail in order to be able to contemplate the possibility of paradigm shifts. This is indeed a complex idea to explain, and presenting research results as absolute truth is something scientists should be prepared not to do. Unfortunately, this is not always the case. Sometimes, some people profess the ideas they subscribe to as the scientific or absolute truth. Such claims of absolute truth completely contort the nature of science. It is certainly going too far when science bloggers, in an attempt to protect their preferred mainstream theory, demand that a scientists’ blog be closed because their views differ. Scientists who publish their research in scientific journals, who go through the peer-review process and who in the end publish slightly unorthodox but nonetheless valuable ideas, should not be censored from the science blogosphere.

Unfortunately, this is what happened to our blog, The Dark Matter Crisis.

A popular science blogger demanded that SciLogs.com discontinue our blog and has, for a short time, succeeded. We would like to use this occurrence as an example of the reactions and difficulties faced when doing online communication of controversial science topics. The incident demonstrates why debate in science must be based on objective facts and not be driven by personal opinions. It illustrates the dangers of mixing scientific convictions with personal goals and emotions.

Why we started the Dark Matter Crisis blog

In late 2009, Pavel and I wrote an invited article for the German popular science magazine Spektrum der Wissenschaft about dwarf galaxies as tests of cosmology. During the process, Spektrum asked us to also start an accompanying science blog on SciLogs.eu, to provide a place for discussions that might arise due to the controversial nature of our work. We were very hesitant initially, but after talking to students and colleagues we agreed to start a blog. What convinced us to blog was the possibility to get in touch with readers, which would allow immediate feedback and discussions, and the ability to continuously provide current information about our active field of research. When the Spektrum article was published in July 2010, the blog The Dark Matter Crisis went online, too. We blogged on it for about two years, and then agreed to move The Dark Matter Crisis to the new SciLogs.com network. The first article on the SciLogs.com blog was published on January 3, 2013.

The discontinuation of The Dark Matter Crisis

On January 28, we received an email from the SciLogs.com community manager. The email informed us that our blog had been discontinued and that we would no longer be able to update it, although the blog’s archive would remain on the site. The short explanation provided was that the “thesis pushed by The Dark Matter Crisis is now overwhelmingly considered incorrect by the scientific community and as such cannot be considered sound enough to be promulgated by SciLogs.com”.

As we blog mostly about our own and related research, such a justification not only attacks our blogging but also hits at the very heart of our scientific work. Consequently, the first reaction to this email was shock, quickly followed by many questions. Which “theses pushed” by our blog “is now overwhelmingly considered incorrect”? That the currently prevailing hypothesis of cold dark matter has serious problems? This certainly is not considered overwhelmingly incorrect, as there are many scientists working on addressing these problems, both within the framework of standard cosmology (e.g. Mutch et al. 2013, Fouquet et al. 2012), as well as by modifying it (e.g. Lovell et al. 2012, Macció et al. 2012) or even by taking a completely different approach (e.g. Famaey & McGaugh 2012). Also, we were invited to start the blog because of the controversial nature of this topic.

Furthermore, at the time of discontinuation, the SciLogs.com version of The Dark Matter Crisis had only one blog post thus far. The sole post presents the recent discovery of a co-rotating plane of satellite galaxies around Andromeda reported in Ibata et al. (2013, Nature). It discusses possible implications which are right now actively debated among scientists. In fact, that blog post was, as far as I can tell, the only one on the web to provide a detailed explanation as to why the Nature paper might be a threat to Einstein’s theory of gravitation, which was explicitly alluded to by numerous publications, but explained by none (most articles in classical media focussed on the 15-year-old co-author of the study). Surely, it is not the aim of SciLogs.com, as a service to provide information to the public, to censor a blog that was communicating science to the public. Therefore, we concluded that this blog post could not have been the reason for the discontinuation.

But even expanding the scope to the old SciLogs.eu blog, we cannot see where we push a thesis which is not scientifically sound. Our blog posts are full of references to peer-reviewed publications. While we often discuss non-mainstream interpretations, we always remain within the realm of science and discuss an active field of research. For example, we frequently mention alternatives to dark matter which try to explain the missing mass phenomenon by non-Newtonian gravity laws. As an active scientist in this field, one can certainly not say that this is not scientifically sound and “overwhelmingly considered incorrect”. Just looking at the number of citations to the first paper about MOND by Milgrom, shows a citation count that has been constantly rising over the last few years and is currently at 1066.

So, what might have triggered the decision to discontinue our blog?

What Who has triggered our blog’s discontinuation?

Digging around on Twitter revealed several interesting discussions which were obviously related to the discontinuation of The Dark Matter Crisis. It turns out that a former-scientist-turned-blogger, who had spent a few years doing research in cosmology (publishing 5 first-author papers with now 88 citations), demanded the discontinuation.

The blogger (@StartsWithABang) contacted @scilogscom on January 24 by replying to a 15-day old tweet that announced our blog’s move to the new domain. He tweeted “Bummed that @scilogscom is in the business of promoting contrarian scientist viewpoints.”, and asks the SciLogs.com community manager (@notscientific) “[Why] are you allowing @scilogscom to promote contrarian voices that undermine public understanding of [science]?”, adding “You have taken on “Dark Matter Crisis” blog, whose mission is to undermine all of physical cosmology & promote MOND.”

The two agreed to discuss the issue via email, with the blogger adding that he was “*personally* worried that you are promoting clicks & false controversy over quality science content”, and states that he is “very, VERY disappointed about this move that @scilogscom has made”.

By now the SciLogs.com community manager has explained to us what happened after these tweets. He and the publishing director responsible for SciLogs.com unfortunately assumed that the blogger’s criticism was justified. They decided to close our blog without conferring with others or asking us for a statement. After we complained about the discontinuation, they performed an internal investigation, which involved reaching out to astrophysicists and other people, and have realized that discontinuing our blog was a big mistake. We attribute SciLogs.com’s poor judgement to two factors: neither the community manager nor the publishing director has an (astro)physical background, it was the first time that SciLogs.com had experienced an attack against one of its blogs.

So, the result was that four days after the tweets about The Dark Matter Crisis were posted, our blog was discontinued. Interestingly, only a few hours later the blogger who complained about our blog tweeted: “Shout out to the @SciLogscom  team, esp. @notscientific  and @laurawheelers, for stepping up & vetting their #science blogs for quality!”. (@laurawheelers was not involved in the decision to discontinue our blog. She only referred @StartsWithABang to SciLogs.com’s community manager.) @StartsWithABang added “They are storing the archives, but the blog is inactive and will not be continued”. While until then this situation was only an example of one blogger attacking our blog and our research with contorted accusations, the reactions of a few other Twitter users  were disheartening.  Some of them, science communicators and even an active astronomer, welcomed the blog’s discontinuation. One would have hoped that they would see the value of our science blog, regardless of their own opinions on the controversial topic we blog about.

Some slightly earlier attacks

The incident seems to be related to a recently published paper by us: Kroupa, Pawlowski & Milgrom (2012). When the paper appeared on the preprint server arXiv on January 18, this lead to a short discussion on Twitter, during which the same blogger who would later led to the short-timed discontinuation of our blog, made some pretty harsh accusations against “the MOND zealots”, whom he seems to call a mix of skeptics and liars and deniers who trot out misinformation and undermine confidence in science. In reaction to our paper, he published a blog post in which he claims to rule out MOND with one graph. Unfortunately, his blog post does not address any of the issues discussed in our recent paper, nor does it address those discussed in many other papers over the recent years.

In reaction to the accusations and contorted depiction of our research, I submitted a comment to the blog post. It asks for a clarification of the accusations and tries to start an objective discussion. There was no reason to censor it. Nevertheless, the comment was not published the first time, so I submitted it again the following day. Again, it was not published. I then decided to ignore the issue and the blogger in the future, as a factual debate seemed to be undesired and emotion-laden quarreling on the web is a waste of time. However, as our blog was actively attacked only a few days later by that very same blogger, the comment is being published here for transparency:

“When I understand your Twitter tweets from yesterday correctly, you think that “Kroupa and some of the other MOND zealots” are, at least to a certain extend, liars and deniers who “trot out misinformation & undermine confidence in science”. Is this what you were saying or did I misunderstand something? My honest opinion is that this would be unnecessarily aggressive, insulting, unprofessional and unscientific as it does not help to establish a well-founded discussion of the scientific issues.

The fact that you do not address the numerous problems of LCDM, many of which are mentioned in the recent paper, does not help shaping a discussion. In your blog post, you base your argumentation on only one problem of MOND: the the strong oscillations in the matter power spectrum. However, according to e.g. Famaey & McGaugh (2012), this problem is not as clear-cut as you claim. They write: “the non-linearity of MOND can lead to mode mixing that washes out the initially strong signal by z = 0”, and even suggests a more robust test.

More fundamentally, basic logic tells us that falsifying one hypothesis does not provide information about the validity of an opposing one. Just to give an example: Disproving that the world is a disk does not prove that the guy who is claiming that the earth is donut-shaped is right. As it turns out, the earth is neither a disk nor a donut, but essentially a sphere. Nevertheless, you jump from this graph to a conclusion about “MOND, MOG, TeVeS, or any other dark-matter-free alternative”. In addition, if you would consider the numerous failures of the LCDM model in a similar way like those of MOND, according to your argumentation we would have to give up on both, modified gravity theories and dark matter.

As a last note, I’d like to point out that in our recent paper we do not present MOND as the final answer. The fact that there is not a single “MOND”, but many different attempts to construct a full theory of modified gravity (see Sect. 6) already demonstrates that more work needs to be done. But in order to search for a solution of the many problems LCDM has on scales of many Mpc and below (where MOND is very successful), scientists should be encouraged to investigate this possibility. That is what a paradigm shift is, in my opinion: acknowledging that there are problems and being open-minded for new or alternative explanations, without hiding the problems that these alternatives may themselves face. As we acknowledge in the paper, mass discrepancies in galaxy clusters and building a consistent cosmology are real challenges for MOND, but there exist more or less convincing answers to these problems in the various effective covariant theories that have been proposed to date (see e.g. the list of theories in Famaey & McGaugh 2012 and their Section 9.2). Even if most of these tentative new explanations will turn out to be unsuccessful, I am sure there still is much to learn about the Universe. We have made this clear in the final sentences of our paper, too: “Understanding the deeper physical meaning of MOND remains a challenging aim. It involves the realistic likelihood that a major new insight into gravitation will emerge, which would have significant implications for our understanding of space, time and matter.”

So, I don’t think there is any lying, denying or misinformation involved on part of us as active scientists. It is just that the Universe is a hard nut to crack. Having the strength to admit that none of the current models are the final answer should in fact increase our confidence in science.”

It is ironic that in a comment on this very blog post, the blogger suggests to a critical reader that if he does not like his way of blogging, the reader could get his own blog. Only a few days later the blogger seems to have worked towards the discontinuation of our blog …

The aftermath and an upcoming guest post

After being informed about the discontinuation and after having discovered the background story on Twitter, we got in touch with the staff responsible for SciLogs.com. As mentioned before, they quickly realized that the discontinuation of The Dark Matter Crisis was a mistake. After discussing the issue with Richard Zinken, the publishing director of Spektrum der Wissenschaft (who is also responsible for the SciLogs.com blog network), he and the community manager apologized for the incident. We have accepted the apology and understand that mistakes can happen. During the last weeks, we worked together with the SciLogs.com team, thinking about what would be the best way to re-open the blog and how to handle the recent events in a constructive way. Together with Richard and the community manager we developed this blog post on the difficulties faced when communicating controversial research.

Together, we also decided to invite a guest blogger to The Dark Matter Crisis, preferably a cosmologist who is skeptical about our views. We hope that this helps to shape the debate and keep it at a scientific level, in contrast to the seemingly emotionally driven attacks which misshape the public’s view of how science handles controversial research. We have asked a few colleagues for such posts, and are content that one experienced scientist has agreed to act as our guest blogger. We know that he is well-respected in the field. His guest post will go online tomorrow.

UPDATE (March 09 2013): In a recent blog post, supposedly trying to shut off people working on dark matter alternatives forever, the blogger attacking us wrote: “Courtesy of Scott Dodelson, I present to you the one graph that incontrovertibly settles the matter.” We now rather offer you a guest blog post on that matter by … Scott Dodelson.

In the meantime, Jim Peebles, Albert Einstein Professor Emeritus of Science at Princeton University, gave us his explicit permission to publish the full, unedited email in which he explains that he would not like to be our guest blogger. We would like to thank him for this and, given our recent experience, fully understand that he prefers to not start blogging:

“Hello Pavel

Sorry for the delay. I have been thinking about your email, and have decided that I will not contribute a commentary on your situation.

I agree with many of your points. The behavior of [SciLogs.com] is silly; this is not the way of science. As you indicate, the community is remarkably optimistic about galaxy formation within the standard LCDM cosmology. I consider this an example of the human herd instinct. With you I distrust talk of precision cosmology; we are still seeking an accurate cosmology. But I think we differ on the weight of evidence for LCDM. I am deeply impressed by the variety of independent lines of evidence that point to LCDM, and conclude that the case for LCDM as a useful approximation to reality on the scale of the Hubble length is about a good as one gets in physical science. No one can prove that there is not another cosmology without dark matter that fits the data as well as LCDM, and no one can prove that there is not another theory that works as well as quantum mechanics. I expect we both put the odds on the latter as too low to matter. I feel close to the same about the former.

You are entirely entitled to take the approach I see in your blog, but I do not want to state my opinion on your blog. I don’t want to take up [blogging] anywhere!

Regards, Jim”

In addition, you can have a look at a recent article in New Scientist: “Dark matter rival boosted by dwarf galaxies”. The article mentions James Binney, from the University of Oxford, who says that he “believes that some sort of MOND-like behaviour may manifest itself on small scales”, while Avi Loeb, of Harvard University, being skeptical about MOND, nevertheless states that: “The theory deserves a lot of respect.”

We believe that all astronomers, whether skeptical or not of our controversial research, are able to agree with Loeb’s statement, and it is in this spirit that we would like to continue our endeavours in online science communication.

By Marcel S. Pawlowski and Pavel Kroupa  (08.03.2013): “The Dark Matter Crisis continues: on the difficulties of communicating controversial science” on SciLogs. See the overview of topics in The Dark Matter Crisis.

36. Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies

Today’s issue of Nature contains a very exciting study by Rodrigo Ibata et al. which might be a game-changer in the research areas of galaxy formation and near-field cosmology. It is titled “A vast, thin plane of corotating dwarf galaxies orbiting the Andromeda galaxy” and already now should be seen as a candidate for the most-exciting paper of 2013.

UPDATE Jan. 4th: The article is now also available on the arXiv.

Pavel Kroupa and I have been waiting for this paper to appear for quite some time. Several months ago we’ve heard the first rumors that Ibata from the University of Strasbourg has detected, with great significance, a plane of satellite galaxies around our neighboring spiral galaxy Andromeda (M31). My curiosity even made me look into available data, which supported what we had heard. Chatting with Rodrigo during a recent N-body meeting in Bonn (after his paper was accepted) finally confirmed these rumors. Seldom have I been looking forward to a paper this curiously, while at the same time being aware of its essential content already.

The Facts

So, what is it all about? Ibata and his collaborators have performed the Pan-Andromeda Archeological Survey (PandAS, lead by Alan McConnachie), an extensive observational campaign of the region around the Andromeda galaxy. This survey has unveiled many of Andromeda’s satellite galaxies and allowed the team to measure the distances to these satellite galaxies in a homogeneous manner (Conn et al. 2012). They then looked at the spacial distribution of the satellite galaxies around their host, motivated by the distribution of satellite galaxies of our own Galaxy. Around the Milky Way, the satellites are distributed and orbit in a thin plane, which we recently termed a vast polar structure (VPOS, Pawlowski et al. 2012a). In fact, the satellite objects are correlated to a degree which is at odds with cosmologically motivated expectations.

Now Ibata et al. find that out of the 27 satellite galaxies in their sample, 15 lie in a common plane. They report that this plane has a thickness of only 13 kpc (40,000 light years), while it has a diameter of at least 400 kpc (1.3 million light years), possibly reaching further out beyond the PAndAS survey region. They can rule out that a chance-alignment is responsible for this configuration with very high confidence, the likelihood that such a well-pronounced structure appears at random is only 0.13 per cent.

An illustration of the Andromeda satellite galaxies which belong to the co-orbiting satellite plane. The top-right vie shows the satellites plane edge-on, as seen from the Milky Way, while the bottom left shows the plane rotated by 90 degrees (the orientations of these two views are indicated in the lower right). The top-left is a optical picture of the Andromeda galaxy. Image Credit: Ibata et al.

But it is not only the existence of this plane which is stunning. The plane is aligned perfectly with the Milky Way, in a way such that we see it edge-on. This fortunate orientation allowed Ibata et al. to also look at a kinematical coherence. We can measure the radial velocities of the satellite galaxies, which lie within the plane due to the planes orientation. This reveals that 13 out of the 15 satellite galaxies in the plane show a common sense of rotation. This, again, is similar to the VPOS around the Milky Way, in which at least 8 satellites orbit in the same sense, while at least one is counter-orbiting in the same plane (Pawlowski 2012). The authors state that including this kinematic information into their analysis increases the significance of the satellite plane to 99.998 per cent. This is just amazing.

Here you can find a very nice video animation illustrating the structure’s orientation with respect to the Milky Way.

Unfortunately the letter itself is behind Nature’s pay-wall, so you can only access it if you have a Nature subscription. I’ll update this blog post if a freely accessible arXiv version becomes available. For the meantime, please be referred to the accompanying press releases. UPDATE Jan. 4th: The arXiv version of the article can be found here.

The Interpretation

In my opinion, the importance of this discovery can not be over-stated, which is in line with Nature publishing a comment on the discovery in the same issue (“Astronomy: Andromeda’s extended disk of dwarfs” by R. Brent Tully) and even making the letter its cover story. The about-the-cover text already hints at the study’s importance:

“Recent studies of the dwarf galaxies of the Milky Way have lead some astronomers to suspect that their orbits are not randomly distributed. This suspicion, which challenges current theories of galaxy formation, is now bolstered by the discovery of a plane of dwarf galaxies corotating as a coherent pancake-like structure around the Andromeda galaxy”

I suppose that due to the restrictive space constraints set by Nature (4 pages, 30 references), the letter is short and does not discuss the study’s implications in extensive detail. In their letter, Ibata et al. mention two broad ideas which might lead to an explanation for the structure’s existence.

  • Either all the satellites in the plane were accreted together, which is unlikely because the very small thickness of the satellite plane restricts the size of an accreted group to less than 14 kpc. Such groups are not observed in the universe.
  • Or the satellites within the plane were formed in place around Andromeda, for example as tidal dwarf galaxies.

Overall, the authors prefer not to make any strong conclusions, instead stating that “the formation of this structure around M31 poses a puzzle”, which is also the prevailing tone of the press release. This is why I would like to share some of my thoughts on the discovery and also highlight some very relevant publications that obviously did not make it into the letter.

Filamentary Accretion?

The letter by Ibata et al, but also the comment by Tully, discusses that the accretion of dwarf galaxies along cosmic filaments might be responsible for the structure. However, there are several reasons why this idea does not work. First of all, the filaments found in cosmological simulations are too thick. They would need to be as thin as the observed structures (< 14 kpc) to have a chance to explain the planes, but their size typically is on the order of 500-1,000 kpc. This is supported by studies like Vera-Ciro et al (2011), who, analyzing the behavior of dark matter particles in cosmological simulations, conclude that

“[…] at later times the cross-section of the filaments becomes larger than the typical size of Milky Way like haloes and, as a result, accretion turns more isotropic […]”.

Consequently, the satellite structure can not be both: of filamentary origin and young, which contradicts the argument in Tully’s comment.

In Pawlowski et al. (2012b) we have also shown that even in case of the VPOS of the Milky Way satellites, a filamentary accretion origin can be ruled out because the coherence of the orbital poles of the sub-halos in cosmological high-resolution simulations is not strong enough to explain the alignment of the MW satellite orbits. The filament might initially lead to a preferred direction of infall, but does not produce a thin, co-rotation plane of sub-halos but a prolate distribution. And now the Andromeda satellite disc is even thinner and more coherent than the VPOS. For more details, please have a look at my blog post on filamentary accretion.

Tidal Dwarf Galaxies

In contrast to the often mentioned accretion along cosmic filaments, the tidal dwarf galaxy scenario is a much more natural explanation for co-orbiting discs of satellite galaxies. In this scenario, two galaxies interact, such that the tidal forces rip out matter from the galactic discs, which form spectacular tidal tails. Within this tidal debris new galaxies (tidal dwarf galaxies or TDGs) form, a process which is observed to happen in the universe and also reproduced by simulations. As the TDGs form from a common tidal tail, they share a common orbital direction and are generally found in a thin plane. Just as it is observed around the Milky Way and now Andromeda.

In fact, this TDG scenario can also explain the existence of counter-orbiting satellites, of which there seem to be two in the Andromeda disc and at least one around the Milky Way (Pawlowski et al. 2011). There is even a study proposing that Andromeda experienced such an galaxy encounter (Hammer et al. 2010), during which TDGs have been formed. These might even be responsible for the VPOS of the Milky Way (Fouquet et al. 2012), in which case the Milky Way should lie within the satellite plane around Andromeda … which is indeed the case. Unfortunately, all these very relevant papers did not make it into the short Nature letter.

All this is also why I have to disagree with a sentence in R. Brent Tully’s discussion of the letter (which of course got picked up by the media …). He states that

“No theorist of galaxy formation would have dared to predict such a situation”.

This is not quite true. I would also argue that the authors of Fouquet et al. (2012) have been expecting such a situation in their tidal dwarf galaxy scenario and that most researchers working on tidal dwarf galaxies would probably predict such an orientation for TDGs. Even I wrote about this in my 2012 paper on the Milky Way VPOS:

“The M31 satellites are preferentially distributed in a structure extending approximately from north to south in Galactic coordinates, just as the MW VPOS extends in the north–south direction. A common direction of the satellite distributions of both galaxies is expected in a tidal scenario that formed both satellite populations together, as TDGs form in a plane defined by the orbit of the interaction.”

There is one major argument against the tidal dwarf galaxy scenario: tidal dwarfs do not contain a significant amount of dark matter, while some of the observed satellite galaxies seem to be completely dark matter dominated. This argument is based on two major assumptions which, however, might both be questioned:

  1. The dwarf galaxies are dynamically relaxed, gravitationally bound systems. If they are not and do not contain dark matter, high mass to light ratios might be derived from their velocity dispersion by mistake (e.g. Kroupa 1997, Klessen & Kroupa 1997).
  2. The underlying gravity law is Newtonian. If the gravity law is modified, e.g. In the low acceleration regime, most satellite galaxies would not need dark matter (e.g. Famaey & McGaugh 2012).

Conclusion

Because of the new study we now know that both satellite galaxy systems for which we have full three-dimensional positions available show strong planar alignments. This coherence is also supported by the available kinematic data: the objects in the VPOS around the Milky Way and in the disc of satellites around Andromeda mostly co-orbit in the same direction.

Such a phase-space coherence is expected if the satellite galaxies were born as tidal dwarf galaxies, but completely at odds with all current cosmological simulations in which the satellites are assumed to be represented by dark matter dominated sub-halos. Therefore, the discovery by Ibata and collaborators, in my opinion, supports the tidal dwarf galaxy scenario and will contribute to a paradigm shift in the field of galaxy formation. We might have to re-consider what we know about near-field cosmology and will have to develop a new understanding of the origins of dwarf satellite galaxies. In the end, this publication might even have an impact on our understanding of the laws of gravity.

The cosmological implications of VPOS-like structures are discussed at length in our paper Kroupa et al. (2010) “Local-Group tests of dark-matter Concordance Cosmology: Towards a new paradigm for structure formation” and in the review by Kroupa (2012) “The dark matter crisis: falsification of the current standard model of cosmology”.

By Marcel Pawlowski and Pavel Kroupa  (03.01.2013): “Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies” on SciLogs. See the overview of topics in  The Dark Matter Crisis.