LUX: Results from another direct (non-)detection experiment for Dark Matter

On Wednesday, the Large Underground Xenon Detector (LUX), a direct detection experiment for Dark Matter, has announced its first results. Before the announcement there was the usual excitement, with Nature News titling “Final Word is near on dark-matter signal”. So, has Dark Matter finally been detected?

Some previous experiments had reported possible detections already. For example, the Cryogenic Dark Matter Search (CDMS) recently presented an impressive number of 3 possible dark matter events (compared to 0.7 they estimated to be background), while the Cryogenic Rare Event Search with Superconducting Thermometers (CRESST) has reported a larger-than-estimated-background number of possible dark matter events, too. In addition, DAMA/LIBRA has claimed a strong Dark Matter signal from annual modulation measurements for about a decade now, and finally the CoGeNT experiment has also claimed an excess of possible dark matter events with a possible annual modulation similar to that seen by DAMA. So, shouldn’t we rejoice and be convinced that first direct hints of Dark Matter have already been seen?

Well, unfortunately it isn’t that easy. As the plot below shows (adopted from the recent LUX publication), the properties of the possible Dark Matter particles claimed by the four experiments are inconsistent with each other. The plot shows the cross-section (i.e. the likelihood or probability) of interaction (compare to shooting at a coin at far distance, then the chance of hitting the coin can be expressed in terms of its physical area: the smaller the less likely an event) as a function of the mass of the weakly interacting dark matter particle, mWIMP, (“weakly interacting” means that the particle interacts with normal matter gravitationally and via the weak interaction). In the plot, the shaded areas correspond to the allowed regions for the different experiments, there is no point on which more than two of them overlap. In addition, everything above the red line has already been excluded by the XENON100 experiment.

WIMP Dark Matter cross section
Credit: adopted from: LUX collaboration, http://arxiv.org/abs/1310.8214 Inconsistencies of the dark matter particle properties claimed by different direct detection experiments. All excluded by the new results from LUX.

Nevertheless, if you look at the plot closely, you see that the CDMS area (and if you use a magnifying glass also the CoGeNT area) sticks out of the red line to the left. This has given many Dark Matter aficionados the hope that Dark Matter might be hiding in that region of the parameter space. A hypothesis was constructed, so-called ‘light dark matter’. The name is a bit confusing because it still refers to WIMPS (weakly interacting massive particles), but in a mass range below 10 GeV in contrast to the previously preferred range of about 100 GeV (note the use of units here: mass is measured by particle physicists in terms of energy per c2, where c is the speed of light. This comes from Einstein’s equation E=m c2. As a short cut physicists then simply refer to mass in terms of energy, as here in terms of GeV.)

The LUX experiment is 20 times more sensitive than the previous limits in this mass range. This allowed to predict the number of events expected if the light dark matter particle exists. The number of dark matter events LUX should have measured if the previously reported detections were due to dark matter is:

1550.

However, after background subtraction, it did measure (*drumroll*):

0.

That’s nothing. Not a single event. None. Well, ok, they set an upper limit of 2.4 events, but compared to a thousand that is essentially nothing and completely inconsistent with the expectation. Therefore, the light dark matter hypothesis has been ruled out by LUX.

In addition, this highlights that there must be a serious problem with the other direct detection experiments who have claimed dark matter detections. The new results clearly show that these were false detections (unless you claim, as some scientists suggest, that Dark Matter is Xenonphobic, i.e. does not interact with the Xenon-based experiments. But then you are still stuck with the inconsistency of the other experiments and have a contrived Dark Matter candidate).

In the large plot below an expanded view of the sensitivity of the various experiments is shown, also from the LUX paper.

Exclusion regions for WIMP Dark Matter from direct detection experiments.
Credit: LUX collaboration, http://arxiv.org/abs/1310.8214 Exclusion regions for WIMP Dark Matter from direct detection experiments.

What’s next? Well, obviously the hunt for dark matter goes on, even though observational data already falsify the cold dark matter paradigm on astronomical scales and there are possible alternatives which don’t need a new particle. Nevertheless, Xenon detectors one magnitude larger than the current ones are being planned. And once they don’t detect anything, people might simply try to build even larger ones, claiming that the Dark Matter cross section might be even lower. Unfortunately, this game can in principle continue to infinity (unless we run out of Xenon first), as the cross section might be infinitely small. However, there are natural limits. At some point, the detectors will reach into a background of neutrino interactions, which will hide any potential Dark Matter signal. At this point, the hypothesis of dark matter particles will become untestable by direct detection experiments.

Nevertheless, many colleagues are still betting on Dark Matter. But there is more talk about other types of Dark Matter particles, and there are many that can be imagined. These, in contrast to the WIMPS (remember: weakly interacting …) do not necessarily interact at all with baryons except gravitationally. Whether a non-interacting – and therefore by construction undetectable – particle is still a scientific hypothesis is another question we should start to discuss more seriously. Past centuries have shown which damage untestable hypothesis can do to human progress.

One motivation for preferring the WIMP hypothesis was that they would be a natural consequence of Supersymmetry (SUSY). But apparently the LHC does not see any evidence for SUSY: no deviations from the expectations of the standard model of particle physics for the decay of the B(s) meson, a very heavy Higgs boson only barely consistent with the minimally supersymmetric models and – maybe most important – no signs have been found for the expected supersymmetric particles in the mass range investigated to date. Taken together, this weakens both the SUSY and WIMP Dark Matter hypothesis, maybe opening up room (and minds) to consider completely different explanations to the Dark Matter phenomenon.

Perhaps, as often in the history of science, the answer to the Dark Matter conundrum will come to light through a different experiment than the one that was designed for solving the problem in the first place. For instance, a possible bet for an experiment that could change the game would be the ALPHA experiment at CERN, which if detecting anything like a “negative gravitational charge” would lead to an experimental probe of gravitational dipoles, which have been claimed by some to solve most problems of galaxy dynamics & cosmology, but are mostly ignored by the theoretical and cosmological community for the sole reason that the work is related to the MOND hypothesis.

 

(c) Marcel Pawlowski (Case Western, USA),  Pavel Kroupa (Bonn, Germany), Benoit Famaey (Strasbourg, France), Fabian Lüghausen (Bonn, Germany), 2013

See the overview of topics in The Dark Matter Crisis.

The Planck Results on the Cosmic Microwave Background

Guest contribution by Behnam Javanmardi

Prologue by Pavel Kroupa:

The much awaited Planck results on the CMB have been published recently. The results are consistent with those arrived at by using Wilkinson Microwave Anisotropy Probe (WMAP) measurements.

https://i0.wp.com/sci.esa.int/science-e-media/img/62/Compo_CMB_Planck_WMAP_v1_3k.jpg
Date: 20 Mar 2013
Satellite: Planck
Depicts: Cosmic Microwave Background
Copyright: ESA and the Planck Collaboration; NASA / WMAP Science Team: “This image shows temperature fluctuations in the Cosmic Microwave Background as seen by ESA’s Planck satellite (upper right half) and by its predecessor, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP; lower left half) A smaller portion of the sky is highlighted in the all-sky map and shown in detail below. With greater resolution and sensitivity over nine frequency channels, Planck has delivered the most precise image so far of the Cosmic Microwave Background, allowing cosmologists to scrutinise a huge variety of models for the origin and evolution of the cosmos. The Planck image is based on data collected over the first 15.5 months of the mission; the WMAP image is based on nine years of data.”

This agreement is excellent news, because it means that the two missions are consistent and thus the Planck data enhance our confidence in what we know about the CMB.

But, what do the results mean in terms of our physical understanding of the universe?

In this guest contribution by PhD student Behnam Javanmardi, who is studying cosmological models in Bonn since the Fall of 2012, some of the problems raised by the Planck CMB map are discussed:

Behnam Javanmardi, Bonn, 19.04.2013

Contribution by Behnam Javanmardi:

The European Space Agency (ESA) launched the Planck satellite on 14 May 2009 to the second Lagrange point of the Sun-Earth system (L2), at a distance of 1.5 million kilometers from the Earth, for observing the Cosmic Microwave Background (CMB), the afterglow of the Big Bang. On 21 March 2013, the Planck collaboration released the data with a series of papers on their scientific findings. Planck observed the CMB sky in different frequency bands, some of which are sensitive to the foregrounds (anything between us and that cosmic radiation, e.g. the disk of the Milky Way). This allows to remove the foregrounds and reach to an image of the Universe when it was very young.

Statistical analysis of this image (which shows small temperature fluctuations corresponding to small density contrasts at that time) gives us valuable information about our Universe. In the following, some major Planck’s results are reviewed with the main focus on the problems cosmologists now face, given these results. Technical details can be found in the Planck 2013 Results Papers.

The current Standard Cosmological Model (ΛCDM) has a set of parameters and the Planck collaboration reported the values for these parameters by fitting the model to the data. For example, the best fit ΛCDM parameters resulted in a 6% lower value for the density parameter of dark energy (Planck: ΩL=0.686±0.020 vs WMAP-9: ΩL=0.721±0.025) and an 18% higher value for the density parameter of dark matter (Planck: Ωm=0.314±0.020 vs WMAP-9: Ωm=0.279±0.025) than the results of the previous all sky CMB survey, i.e. WMAP. As can be seen from these numbers, the two parameters are consistent with each other within the measurment uncertainties. Thus, the Planck mission has nicely confirmed the WMAP fit to the standard model of cosmology.

https://i1.wp.com/sci.esa.int/science-e-media/img/67/Planck_anomalies_Bianchi_on_CMB_orig.jpg
Date: 21 Mar 2013
Satellite: Planck
Copyright: ESA and the Planck Collaboration: “Two Cosmic Microwave Background anomalous features hinted at by Planck’s predecessor, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP), are confirmed in the new high precision data from Planck. One is an asymmetry in the average temperatures on opposite hemispheres of the sky (indicated by the curved line), with slightly higher average temperatures in the southern ecliptic hemisphere and slightly lower average temperatures in the northern ecliptic hemisphere. This runs counter to the prediction made by the standard model that the Universe should be broadly similar in any direction we look. There is also a cold spot that extends over a patch of sky that is much larger than expected (circled). In this image the anomalous regions have been enhanced with red and blue shading to make them more clearly visible”.

The main interesting result from Planck was the confirmation of some features that have been revealed by WMAP data. Before Planck, there were some doubts about the cosmic origin of these features, but since the precision of Planck’s map is much higher than that of WMAP and the Planck collaboration was working nearly 3 years to carefully extract any foreground emission and those features are still present, we have to accept with a much higher confidence that these may be real features of the CMB sky.

These features or anomalies, which the standard model of cosmology did not expect, are significant deviations from large scale isotropy. But large scale isotropy is one of the two fundamental assumptions that form the Cosmological Principle and simply states that the Universe we observe must not be direction-dependent. Among these features found in the CMB one can mention a “Cold Spot” which is a low-temperature region much larger than expected. And, a “Hemispherical Asymmetry” has been detected: the northern ecliptic hemisphere has on average a significantly lower signal than the southern one. The latter leads to this question: why is the orientation of this asymmetry more or less aligned with the orbital angular momentum of the Earth? Is it a not-yet understood measurement bias or a data reduction bias or a coincidence? As the Earth orbits the Sun, its orbital angular momentum remains pointing into the same direction in the Milky Way. Perhaps a remnant Milky Way foreground contamination may play a role here.

The other assumption of the cosmological principle, i.e. that the initial temperature (and density) fluctuations had Gaussian distribution, has also been tested by the Planck collaboration and no significant deviation from it was reported, except for a few signatures which were interpreted to be associated with the above-mentioned anomalies.

Furthermore, the power-spectrum calculated using the Planck data (which is one of the main statistical tools for analyzing the CMB map) has a ≈2.7σ deviation from the “best fit ΛCDM model” at low-ℓ (ℓ ≤ 30) multipoles or large angular scales.

Regarding the test of inflation (a hypothesis which says that the early Universe was inflated by a factor of at least 10^(78) in less than 10^(-36) seconds), the models with only one scalar field are preferred by the Planck results and more complex inflationary scenarios do not survive. However, a recent paper by Ijjas et al (2013)  has gone through the problems of inflation considering the results from both the Planck satellite and the LHC,

The odd situation after Planck2013 is that inflation is only favored for a special class of models that is exponentially unlikely according to the inner logic of the inflationary paradigm itself

as they mention. The forthcoming results on polarization of the CMB from Planck will cast light on this issue.

As mentioned above, although the ΛCDM model is consistent with the overall picture as seen by Planck, it fails to account for these observed anomalies and the deviation of the power-spectrum at large scales. In addition, the three major elements of the ΛCDM model, i.e. dark matter, dark energy and inflation, still lack a firm theoretical understanding. Therefore, cosmologist should try to look for a model in which the recent observed features are no longer “anomalies” and are predicted by the model itself.

Epilogue by Pavel Kroupa:

The Planck data thus demonstrate that not all is well with our understanding of cosmology, that is, the CMB poses hitherto unanswered problems.  But even if the CMB had been in perfect agreement with the expectations from the current standard model of cosmology, what would this have implied for our physical understanding of cosmology?

First of all, an elementary if not trivial truth is that consistency of a model with a set of data does not prove the model. Thus, claiming that Planck establishes the existence of (cold or warm) dark matter and dark energy would be an unscientific statement. For example, the cosmological model by Angus & Diaferio (2011, see their fig.1)shows that the CMB can be reproduced with a non-CDM/WDM model, therewith proving the non-uniqueness of the models.

Furthermore, irrespective of any success or failure of the standard (or any other) cosmological model in reproducing some large-scale data, the highly significant problems encountered on the local cosmological scale of 100Mpc and below remain hard facts to be solved: See

Behnam Javanmardi’s final statement above,

“Therefore, cosmologist should try to look for a model in which the recent observed features are no longer “anomalies” and are predicted by the model itself.”,

emphasises that cosmology is one of the least understood of the physical sciences.

By Behnam Javanmardi and Pavel Kroupa  (22.04.2013): “The Planck results on the cosmic Microwave background” on SciLogs. See the overview of topics in The Dark Matter Crisis.

Scott Dodelson on dark matter and modified gravity (guest post)

Following the recent incident, we and the SciLogs team decided to invite a renown colleague to write a guest blog post. Thinking about possible guest bloggers who are experts in the field of cosmology and approach theories such as MOND with the necessary scientific skepticism, we arrived at Scott Dodelson as one candidate.

Scott is a very well-respected cosmologist. He is a scientist at Fermilab and  a professor in the Department of Astronomy and Astrophysics and the Kavli Institute for Cosmological Physics at the University of Chicago. His research focuses on the largest and smallest scales of the universe: the interplay of cosmology and particle physics. He investigates the nature of dark matter and dark energy, works on the cosmic microwave background and is also interested in modified gravity theories. In addition to his many papers, he has written the textbook “Modern Cosmology”.

We are very pleased that Scott Dodelson has accepted to write this guest post. Thank you, Scott!

 

Is modified gravity a viable alternative to dark matter? Or is dark matter so compelling that pursuits of modified gravity should be abandoned?

There are good reasons to believe in dark matter and to be optimistic about our chances of detecting it in the coming decade. Dark matter explains the flat rotation curves in galaxies; it accounts for the deflection of light far from the centers of galaxies and by galaxy clusters. Many aspects of galaxy clusters make sense only if dark matter is present. Perhaps most importantly, it is the key component in our modern story of how we got here: the standard cosmological model is called CDM or “Cold Dark Matter”. The small inhomogeneities captured in maps of the cosmic microwave background (CMB) grew to be the vast structure we see today via gravitational instability, but the story holds together only if dark matter is also present. The story works and it has been tested by observing the spectra of both the CMB and the distribution of matter on large scales. It is true that dark matter does not easily explain some phenomena on small scales, but there is a ready explanation for this: predictions on small scales are hard. Apart from the non-linearity of gravity, baryons play an important role on small scales, and incorporating these effects into numerical simulations is challenging. It is easiest to make predictions on large scales and those easy predictions have been confirmed with exquisite precision. Beyond all this lies the suite of experiments poised to detect dark matter. Thousands of scientists are now hunting for the particles that comprise dark matter by studying collisions at the LHC; by manning underground laboratories designed to detect it; and by launching satellites to observe the debris created when two dark matter particles in space collide and annihilate. We have reason to be optimistic.

Why then pursue modified gravity?

First, the people who study modified gravity (MG) tend to focus on small scale data rather than large scale data. They are serious, smart  scientists who make observations and fit MG models to the data. These fits tend to be pretty good,  often with very few free parameters and therefore the scientists gain confidence in their models. This focus on different data or different slices through the data presents a challenge to the dark matter model. Eventually, dark matter will have to explain these data sets as well. Slicing and combining things in different ways leads to different challenges than might otherwise arise. Even if you believe in dark matter, you want to confront the data in all forms. The simple (slightly condescending) way of saying this is to say that CDM must ultimately reduce to MONDian phenomenology on small scales.

More importantly, dark matter has not yet been detected. This is not the time to raise the barriers and decree that only those who accept dark matter are serious scientists. We are optimistic, but we have to accept the possibility that dark matter will not be detected in the next decade. Our initial feedback from the LHC shows no hint for the simplest model that contains dark matter, supersymmetry (although these early data are certainly not conclusive). There have been hints in direct and indirect detection experiments, but certainly nothing definitive. It is possible that we will need to think of something completely new. In so doing we are going to have to drop some assumptions, weight evidence differently than we do now. The MG community does this now by downweighting large scale data and focusing more on small scales. This may end up being the correct approach, or we may need to think of something even more radical. I do not know how to do this (How do we encourage a revolution?) but I am pretty sure suppressing alternatives is moving in the wrong direction.

The communities now are quite disparate and find it difficult to engage one another. Is the MG vs. dark matter dispute identical to the disagreements between people from different religions, say, virtually impossible to resolve because the two sides cannot communicate? Certainly not. We are scientists, and facts will change our minds. Some examples of things the vast majority of the MG community accepts or will accept:

  1. MG is not theoretically favored over dark matter because “dark matter is something new”. Both approaches are changing the fundamental lagrangian of nature by adding new terms and new degrees of freedom.
  2. The fact that Xenon100 or Fermi (or perhaps AMS in a few days) has not seen dark matter does not mean the theory is excluded. There is plenty of room in theories like supersymmetry and even more in other more generic models.
  3. If dark matter is detected unambiguously via direct and/or indirect detection, then MG would indeed fall outside the realm of reasonable scientific investigation.

On the other hand, our dispute does share similarities with those that divide adherents of religion. We are passionate, we come at things from different directions with different preconceptions, so it is sometimes difficult to speak the same language, to focus on a single question. At the end of the day, just like the devout in different religious traditions, we are all after the same goal, in our case, trying to understand nature. It is premature to state that our way is the only way.

 

Guest post by Scott Dodelson (07.03.2013): “Is modified gravity a viable alternative to dark matter? Or is dark matter so compelling that pursuits of modified gravity should be abandoned?”.

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.

Are there two types of dwarf galaxies in the universe?

Dwarf galaxies, that is galaxies less massive than a few billion solar masses, are expected to be formed through two processes. They might either be the luminous components of small dark matter halos, formed early in the universe when gas fell into the potential well of those halos. These dwarf galaxies are called primordial dwarf galaxies (PDGs) and are expected to be dominated by their dark matter content.

The other formation mechanism is a process observed even in the present-day universe. When two major disk galaxies collide, the gas and the stars in the disks are expelled by tidal forces induced by the encounter to large distances. An example for a very prominent structure that has been created through tidal interactions between disk galaxies is the ‘tail’ that extends to the upper right corner in the figure below. Within this tidal debris, new objects of dwarf galaxy mass form. This is why dwarf galaxies of this second type are called tidal dwarf galaxies, or TDGs.

Thus, TDGs form from the baryonic material in the galactic disks of the progenitor galaxies, but can they also contain dark matter? Even in a disk galaxy with a massive dark matter halo, the vast majority of the dark matter would be located outside the galaxy’s disks. Of the small amount of dark matter within the disk, only a tiny fraction would furthermore be moving in the same direction and would have the same velocity as the stars and the gas in the disks. The vast majority of the dark matter would therefore have different initial conditions regarding its location and motion than the gas and the stars. But during a galaxy collision, only material with similar initial conditions is thrown on similar trajectories by the tidal forces and has a chance of becoming bound to the gravitational field of a forming TDG. The vast majority of the dark matter, having different initial conditions, will therefore be thrown onto different trajectories. While the dark matter on such different trajectories may be able to cross the shallow gravitational field of a TDG, it would do so at a high relative velocity. Therefore, this dark matter cannot become bound to the TDG. As an analogy for an encounter between a TDG and a chunk of dark matter, consider two spaceships orbiting a planet. Even if they orbit the planet at the same altitude, they can only rendezvous if they follow each other on the same orbit. For all other possible choices of orbits (say one is flying to the south and the other is flying to the west), the spaceships would fly past each other quickly if they do not crash.

In summary, it is one of the major characteristics of TDGs that they cannot contain much dark matter, even if their progenitor galaxies did (e.g Bournaud 2010).

Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

If the standard model of cold dark matter is correct, there should be a co-existence of these two types of dwarf galaxies in the universe: dark-matter dominated PDGs and TDGs without significant dark matter content. This is the Dual Dwarf Galaxy Theorem (Kroupa 2012).As they would have very different compositions, the two types should fall into two easily distinguishable groups. The natural question to ask in order to test this prediction is:

Are there really two distinct populations of dwarf galaxies in the universe?

This is investigated in the article “Dwarf elliptical galaxies as ancient tidal dwarf galaxies” by Dabringhausen & Kroupa (2013). The principle of their study is simple: they just had to compare the observed properties of old dwarf galaxies with known tidal dwarf galaxies. For the comparison, they use two properties, which are easy to determine observationally. These properties are:

  • The stellar mass, i.e. only the mass in stars, without the mass in gas, dust or dark matter. It can be determined from the luminosity of the system (more stars = brighter object).
  • The projected half-light radius, which is a measure of how extended the system is.

There are extensive catalogs listing these two properties for so-called pressure-supported systems, i.e. systems of stars in which the stars move on chaotic orbits (in contrast to the ordered rotation of  disc galaxies). The following plot shows these data points.

Credit: Dabringhausen & Kroupa (2013)

These objects include globular clusters (GCs), ultra-compact dwarf galaxies (UCDs), massive elliptical galaxies (nEs), and dwarf elliptical galaxies (dEs). The first two types of objects (green points) appear to be free of dark matter, while the second two (red points) are generally assumed to sit in dark matter halos. The study of Dabringhausen & Kroupa is particularly interested in the dEs, as these are in the mass- and size-range of observed TDGs, but are generally assumed to be PDGs.

Adding Tidal Dwarf Galaxies

For a meaningful comparison, the properties of these dEs have to be compared with those of known TDGs. To be confident that an object is a TDG, it has to be associated with interacting galaxies (another possibility is to look at numerical simulations of galaxy collisions and extract the properties of TDGs formed in those models). However, this gives rise to a complication: TDGs associated with a pair of interacting galaxies are young, many of them are still forming some stars and such young TDGs can contain a lot of gas. The dEs, in contrast, are old systems without gas. So the observed properties of the young TDGs have to be aged before they can be compared to the dEs. As the TDGs age, they will loose their gas. The paper lists three possible processes:

  1. The gas is converted into stars.
  2. The gas is removed because the feedback of massive stars in the TDG heat it.
  3. The gas can be removed through ram-pressure stripping as the TDG moves through the intergalactic medium.

Because those gas-removal processes happen slowly, their major effect on the TDG properties is an increase of the system’s half-light radius: as (gas) mass is lost, the TDG will be less bound and the distribution of stars will expand. This allowed Dabringhausen & Kroupa (2013) to estimate where aged TDGs would show up in the figure:

Credit: Dabringhausen & Kroupa 2013

The TDGs (blue symbols) fit in quite nicely with the dEs. The lower points on the error bars represent the TDG properties as observed, i.e. still young. Their radii are a lower limit: the TDGs cannot shrink as they slowly loose their gas. The upper end of the error bars assumes that most of the TDG’s mass, 75% to be precise, has been lost. This coincides nicely with the upper end of the dE distribution, too. There is in principle no reason why a TDG couldn’t loose even more of its initial mass, but such TDGs are likely to be destroyed very easily (see further below).

So, the TDGs and the dEs populate the same region in the figure. What does this tell us?

Due to their different composition (PDGs being dark matter dominated, TDGs being dark matter free), one would expect to observe two distinguishable groups of dwarf galaxies. The opposite is found: dEs populate only one region in the plot, and the same region is covered by (aged) TDGs. Consequently, this suggests that the observed dEs are in fact old TDGs. But then there is no room for primordial, dark matter-dominated dwarf galaxies.

This finding is also consistent with the expected numbers of TDGs in the universe. Numerical simulations of close encounters between possible progenitor galaxies show that on average one or two long-lived, massive TDGs are created per such encounter (see Bournaud & Duc 2006). By considering the total number of encounters between possible progenitor galaxies until the present day, Okazaki & Taniguchi (2000) found that such a rate of TDG-production would already be enough to account for all dEs in the Universe.

The black lines in the second plot give another hint at a connection between dEs and TDGs. Because TDGs are formed by colliding galaxies, many of the TDGs will end up as satellite galaxies. When such satellites orbit around a much more massive host galaxy, they will be affected by tidal forces. If the satellite is too extended, its own gravity is not strong enough to keep it bound against the tidal forces of the host. The exact radius depends on the masses of the host and the satellite, as well as the satellite’s orbit. The black lines in the plot give an impression of the tidal radius of satellite galaxies, assuming they orbit at a typical satellite distance of 100 kpc around different host galaxies. For the lowermost line, the host is assumed to be heavy, while the uppermost line corresponds to a rather light host. Above a given line, a satellite of a galaxy with the corresponding mass is not stable anymore, but will be disrupted by tidal forces. So if a TDG loses so much mass that it expands above this line, it will be destroyed and vanish from the plot. Thus, if the dEs are indeed TDGs, the position and slope of the cutoff at large half-light radii is easily explained.

Conclusion

The results of Dabringhausen & Kroupa (2013), if confirmed by future studies, suggest that there is only one type of dwarf galaxies in the Universe. Virtually every galaxy that is classified as an old dwarf galaxy, i.e. a dE, would be an aged TDG which originated from the debris of interacting galaxies. We emphasize also that TDGs have been shown to lie on the baryonic Tully-Fisher Relation (Gentile et al. 2007), which they cannot if this relation is defined by dark matter. These results are very problematic for cold dark-matter based models, which predict that in addition to TDGs a plethora of primordial dwarf galaxies with a completely different composition exists as a second group of dwarf galaxies.  However, the result of Dabringhausen & Kroupa (2013) fits in nicely with the peculiarities of the Milky Way (e.g. Pawlowski et al. 2012) and Andromeda (Ibata et al. 2013) satellite galaxies: they co-orbit within thin planes, which is expected for a population of TDGs. But again this distribution is at odds with the predicted distributions of primordial galaxies.

When it comes to their properties and distribution, tidal dwarf galaxies seem to develop a lead over dark-matter dominated, primordial dwarf galaxies.

 

By Marcel S. Pawlowski and Pavel Kroupa  (07.03.2013): “Are there two types of dwarf galaxies in the universe?” on SciLogs. See the overview of topics in The Dark Matter Crisis.

This blog moves to SciLogs.com

Pavel and I have been too busy to blog for a while (my excuse being that I am in the final stages of my PhD studies). This is also why we did not announce this sooner: Our blog has moved from SciLogs.eu to SciLogs.com. The new site provides an improved blogging system and maybe more international visibility, as well as a pleasant neighborhood of science bloggers. The new URL for "The Dark Matter Crisis" is http://www.scilogs.com/the-dark-matter-crisis/. All future articles will be published there, but the old ones will remain accessible here on SciLogs.eu.

The first article on the ‘new’ blog deals with last week’s Nature article by Rodrigo Ibata and collaborators: "A Vast Thin Plane of Co-rotating Dwarf Galaxies Orbiting the Andromeda Galaxy". While the media currently focusses on the 15-year old co-author of the Nature study, the scientific implications of the study are no less spectacular. The co-rotating plane of satellite galaxies around Andromeda resembles the VPOS around the Milky Way and therefore similar formation scenarios are plausible, which we discuss in our article "Andromeda’s satellites behave as expected … if they are tidal dwarf galaxies".

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.