Dynamics of Local Group galaxies: Evidence for a past Milky Way–Andromeda Flyby?

Indranil_BanikThe following is a guest post by Indranil Banik. Indranil is a PHD student at the University of Saint Andrews, part of the Scottish Universities’ Physics Alliance. He was born in Kolkata, India and moved to the UK with his parents a few years later. Indranil works on conducting tests to try and distinguish between standard and modified gravity, especially by considering the Local Group. Before starting his PhD in autumn 2014, he obtained an undergraduate and a Masters degree from the University of Cambridge with top grades. There, he worked on understanding the dynamics of ice shelves, and on a Masters project on the thick disk of the Milky Way, as well as on a few other problems.

Figure_1I recently won the Duncombe Prize from the American Astronomical Society’s Division on Dynamical Astronomy for a detailed investigation into the Local Group timing argument. This was to present a recently accepted scientific publication of mine (link at bottom of article) at their annual conference in Nashville, Tennessee.

The timing argument takes advantage of the fact that the Universe has a finite age of just under 14 billion years. Thus, everything we see must have started at a single point at that time, which we call the Big Bang. Due to the finite speed of light, by looking very far away, we are able to look back in time. In this way, we observe that, shortly after the Big Bang, the Universe was uniform to about one part in 100,000. Thus, we know that the expansion of the Universe was very nearly homogeneous at early times. This means that any two objects were moving away from each other with a speed almost proportional to the distance between them. This is called the Hubble law.Figure_2new

The Hubble law also works today, but only on large scales. On small scales, the expansion of the Universe is no longer homogeneous because gravity has had a long time to change the velocities of objects. As a result, our galaxy (the Milky Way, MW for short) and its nearest major galaxy, Andromeda (or M31) are currently approaching each other. This implies that there must have been a certain amount of gravitational pull between the MW and M31.

Although this has been quantified carefully for nearly 60 years, my contribution involves analysing the effects of the MW and M31 on the rest of the Local Group (LG), the region of the Universe where gravity from these objects dominates (out to about 10 million light years from Earth). Recently, a large number of LG dwarf galaxies have been discovered or had their velocity measured for the first time (McConnachie, 2012). We took advantage of this using a careful analysis.

We treated the MW and M31 as two separate masses and found a trajectory for them consistent with their presently observed separation. We treated the other LG dwarf galaxies as massless, which should be valid as they are much fainter than the MW or M31. For each LG dwarf, we obtained a test particle trajectory whose final position (i.e. at the present time) matches the observed position of the dwarf. The velocity of this test particle is the model prediction for the velocity of that galaxy.

Figure_3The basic feature of the model is that the expansion of the Universe has been slowed down locally by gravity from the MW and M31. At long range (beyond 3 Mpc or about 10 million light years), this effect is very small and so objects at those distances should essentially just be following the Hubble law. But closer to home, the results of this model are clear: the MW and M31 are holding back the expansion of the Universe, and objects within about 1.5 Mpc should be approaching us rather than moving away (see figure above). By comparing the detailed predictions of our model with observations, we were able to show that, for all plausible MW and M31 masses, a significant discrepancy remains. This is because a number of LG galaxies are flying away from us much faster than expected in the model.

An important aspect of these models is that the MW and M31 have never approached each other closely. Although one can in principle get them to have a past close flyby in Newtonian gravity if they are assigned very high masses, there are several problems with this. Such high masses are unreasonable given other evidence. More importantly, if there had been such a flyby, the dark matter halos of the MW and M31 would have overlapped, leading to a substantial amount of friction (of a type called dynamical friction, which is reliant only on gravity). This would have caused the galaxies to merge, contradicting the fact that they are now 2.5 million light years apart.

I was aware of an alternative model for galaxies called Modified Newtonian Dynamics (MOND – Milgrom, 1983). This is designed to address the fact that galaxies rotate much faster than one would expect if applying Newtonian dynamics to their distributions of visible mass. The conventional explanation is that galaxies are held together by the extra gravitational force provided by a vast amount of invisible dark matter. Many galaxies need much more dark matter than the amount of actually observed matter. But, so far, this dark matter has not been detected directly. What MOND does is to increase the gravitational effect of the visible matter so that it is enough to explain the observed fast rates of rotation. In this model, there is no longer any need for dark matter, at least in halos around individual galaxies. You can find out more about MOND here on McGaugh’s MOND pages and here on Scholarpedia.

Figure_4In MOND, the MW and M31 must have undergone a past close flyby (Zhao et al, 2013). In this model, the absence of dark matter halos around galaxies means that there need not have been any dynamical friction during the flyby (remember that the disks of the MW and M31 are much smaller than their hypothetical dark matter halos, which are only needed if we apply Newton’s law of gravity).

The high relative speed of the MW and M31 at this time (about 9 billion years ago) would probably go a long way towards explaining these puzzling observations. This is because of a mechanism called gravitational slingshots, similar to how NASA was able to get the Voyager probes to gain a substantial amount of energy each time they visited one of the giant planets in our Solar System. The idea in this case would be for the MW/M31 to play the role of the planet and of a passing LG dwarf galaxy to play the role of the spacecraft.


This mechanism is illustrated in the figure above. In the left panel, there is a small galaxy moving at 1 km/s while a much heavier galaxy moving at 5 km/s catches up with it. The massive galaxy sees the dwarf approaching at 4 km/s (right panel). The trajectory of the dwarf is then deviated strongly, so it ends up receding at 4 km/s back in the direction it approached from. Combined with the velocity of the massive galaxy (which is almost unchanged), we see that the velocity of the dwarf has been increased to 5 + 4 = 9 km/s.

We do in fact observe many LG dwarf galaxies moving away from us much faster than in the best-fitting dark matter-based model (see figure below, observed radial velocities are on the y-axis while model-predicted ones are on the x-axis). Moreover, based on the distances and velocities of these objects, we can estimate roughly when they would have been flung out by the MW/M31. This suggests a time approximately 9 billion years ago, which is also when one expects the MW and M31 to have been moving very fast relative to each other in MOND as they were close together.


These high-velocity LG dwarfs would have been flung out most efficiently in a direction parallel to the velocity of whichever heavy galaxy they interacted with. Naturally, the MW and M31 have not always been moving in the same direction. But it is very likely that they were always moving within much the same plane. Thus, one test of this scenario (suggested by Marcel Pawlowski) is that these high-velocity dwarfs should preferentially lie within the same plane.

There is some evidence that this is indeed the case. Moreover, the particular plane preferred by these objects is almost the same as what would be required to explain the distribution of satellite galaxies around the MW and M31. This is described in more detail towards the end of this lecture I gave recently about my work.

Even without this evidence, there is a strong case for MOND. One of the astronomers heavily involved in making this case is Professor Stacy McGaugh. I was very pleased to meet him at this conference. We discussed a little about his current work, which focuses on using rotation curves of galaxies to estimate forces within them. For a modified gravity theory which does away with the need for dark matter, it is important that these forces can be produced by the visible matter alone. Stacy was doing a more careful investigation into estimating the masses of galaxies from their observed luminosities and colours (which give an idea of the mix of different types of star in each galaxy, each of which has its own ratio between mass and luminosity, old stars being red and young ones blue). The success enjoyed by MOND in explaining dozens of rotation curves is one of the major reasons the theory enjoys as much support as it does.

This brought us on to discussing how we came to favour the theory over the conventional cosmological model (ΛCDM) involving Newtonian gravity and its consequent dark matter. Stacy explained how it was particularly his work on low surface brightness galaxies which convinced him. This is because such galaxies were not known about when the equations governing MOND were written down (in the early 1980s). Despite this, they seemed able to predict future observations very well. This was somewhat surprising given that the theory predicted very large deviations from Newtonian gravity. In the ΛCDM context, the presence of large amounts of invisible mass makes it difficult to know what to expect. As a result, it is difficult for the theory to explain observations indicating a very tight coupling between forces in galaxies and the distribution of their visible mass – even when most of the mass is supposedly invisible (a feature called Renzo’s Rule). A broader overview of what the observations seem to be telling us is available here (Famaey & McGaugh 2012) and here (Kroupa 2015).

I then explained my own thinking on the issue. I was aware of some of the observations which persuaded Stacy to favour MOND and I was aware of the theory, but I did not favour it over ΛCDM. Personally, what got me interested in seriously considering alternatives to ΛCDM was its missing satellites problem. The theory predicts a large number of satellite galaxies around the MW, much larger than the observed number. Although it is unclear if MOND would help with this problem, that does seem likely because structure formation should proceed more efficiently under the modified gravity law. This should lead to more concentration of matter into objects like the MW with less being left over for its satellites.

Although this suggested MOND might be better than ΛCDM, my initial reaction was to consider warm dark matter models. Essentially, if the dark matter particles were much less massive than previously thought (but the total mass in the particles was the same), then they would behave slightly differently. These differences would lead to less efficient structure formation at low masses, reducing the frequency of low-mass halos and thus making for less satellite galaxies. I hoped this would explain a related problem, the cusp-core challenge which pertains to the inner structure of satellite galaxies.

What finally convinced me against such minor alterations to ΛCDM and in favour of MOND was the spatial arrangement and internal properties of the MW and M31 satellite galaxies. Much has been written in previous posts to this blog about this issue (for example, here), with this 2005 paper by Kroupa, Theis & Boily pointing out the discrepancy between observations and models for the first time.

Figure_7I have summarised the results in a flowchart (left). Essentially, the hypothetical dark matter halos around the MW and M31 need to be distributed in a roughly spherical way. This is unlike the disks of normal (baryonic) matter in these galaxies. The reason is that baryons can radiate and cool, allowing them to settle into disks. As a result, in an interaction between two galaxies, the baryons with their ordered circular motions in a disk can get drawn out into a long dense tidal tail that then collapses into small tidal dwarf galaxies. But these would be free of dark matter, and they would also be mostly located close to a plane: the common orbital plane of the interacting galaxies. You can see more about this scenario here.

The argument goes that it is difficult to form such planes of satellites in any other way (for example, see Pawlowski et al, 2014). Just such satellite planes are in fact observed around both the MW and M31. Supposedly free of dark matter, they should have quite weak self-gravity and thus low internal velocity dispersions/rotate very slowly. Yet, their observed velocity dispersions are quite high, signalling the need for some extra force to stop them flying apart.

Because the spatial arrangement of these satellites suggests a violent origin, it is unlikely that they have much dark matter. Thus, I became convinced of the need to modify our understanding of gravity. It turns out that exactly the same modification that can help explain galaxy rotation curves without dark matter could also help address this problem (McGaugh & Milgrom, 2013). Although the dark matter plus Newtonian gravity worldview might just about be able to explain galaxy rotation curves (although detailed tests are showing this not to have succeeded: Wu & Kroupa 2015), I do not think it can explain the satellite plane problem. This eventually convinced me to investigate this issue further. I explain some of the more compelling reasons for favouring MOND over ΛCDM in this lecture I gave recently.

The paper for which I won the Duncombe Prize is available here: http://arxiv.org/abs/1506.07569

The peer-reviewed version has appeared in the Monthly Notices of the Royal Astronomical Society, volume 459, issue 2, pages 2237 to 2261.

Dark Matter in the innermost regions of the Milky Way?

Spiral galaxies rotate too fast. If they would only consist of the visible (baryonic) mass we observe in them and Newton’s Law of gravity is correct, then they would not be stable and should quickly fly apart. That they don’t has been one of the first indications that the galaxies (and the Universe as a whole) either contains large amounts of additional but invisible “dark matter”, or that the laws of gravity don’t hold on the scales of galaxies. One possibility for the latter, Modified Newtonian Dynamics (MOND), proposes that gravity needs to be stronger in the low acceleration regime present in galaxies (for more details see the extensive review by Famaey & McGaugh 2012 and Milgrom’s Scholarpedia article). That the rotation curve (i.e. the function of circular velocity of the galactic disc with radius) of our Milky Way galaxy follows the same trend as the rotation curves of other spiral galaxies has been known for a long time, too. So it appears to be a bit surprising that the Nature Physics study “Evidence for dark matter in the inner Milky Way” by Fabio Iocco, Miguel Pato and Gianfranco Bertone makes such a splash in the international press. That the MW should contain dark matter is not news, but nevertheless the paper got a huge amount of press coverage.

Rotation curves of two spiral galaxies (images in the background). The black line illustrates the Newtonian expectation for the rotation curve based on the observed baryons (stars and gas), the blue line is the MOND fit.
Rotation curves of two spiral galaxies (images in the background). The black line illustrates the Newtonian expectation for the rotation curve based on the observed baryons (stars and gas), they are clearly not high enough to explain the data points (small circles). The blue line is the MOND fit for which the mass-to-light ratio is the only free parameter. Credit: Stacy S. McGaugh, private communication

One thing emphasized a lot by the press articles (and press releases) is that the authors claim to have found proof for the presence of dark matter in the ‘core‘, ‘innermost region‘, or even ‘heart of our Galaxy1, not just in the intermediate and outer regions. This might be worrisome for modified gravity theories like MOND, which predict that regions very close to the center of the Milky Way should be in the classical Newtonian regime, i.e. the rotation curve should be consistent with that predicted by applying Newton’s law to the observed mass distribution. The underlying reason is that due to the higher density of baryonic matter in the center of the Milky Way the gravitational acceleration of the baryons there already exceeds the low-acceleration limit. But only once the acceleration drops below a certain threshold the non-Newtonian gravity effect kicks in. Interpreted naively (i.e. assuming Newtonian dynamics), this would mimic dark matter appearing only beyond a certain radial distance from the Galactic Center.

Without even going into the details of checking their assumed Milky Way models, the way the observational data is combined and whether there are systematic effects, a simple look at figure 2 in Iocco et al. already reveals that their strong claim unfortunately is not as well substantiated as I would wish.

Credit: Fig. 2 of Ioco et al. (2015).

The plot’s upper panel is what is of interest here. It shows the angular circular velocity in the Milky Way disk versus the Galactocentric radius. The red points with error bars are observed data for different tracers. The grey band is the range of velocities allowed for the range of baryonic mass distributions in the Milky Way considered by Iocco et al. (that are all consistent with observations). If there would be only baryonic matter and Newtonian Dynamics, the rotation curve of the Milky Way should lie somewhere in this area.

First of all, the figure shows that they did not consider any data in the region within 2.5 kpc. That makes sense because that region will be dominated by the bar and bulge of the Milky Way. Stars in the bulge don’t follow circular orbits, so one can’t measure circular velocities there.

So, what is the core, heart or ‘innermost region’ of the Milky Way? Lets try to come up with something motivated by the structure of our Galaxy. The Galactic disk is often modeled by an exponential profile, with a scale length of about 2.2 kpc. What if we say the core of the MW is everything within one scale length? Immediately there’s a problem with the claim by Iocco: They are not even testing data on this scale.

Lets ignore the phrase ‘core’ or ‘heart’ of the Milky Way and focus on the more general formulation they also use in their paper’s title: “Evidence for dark matter in the inner Milky Way”. Looking at their Figure again, we can see that the data start to leave the grey band at a distance of about 6 kpc from the MW center. Thus, within 6 kpc (almost three scale radii of the Milky Way disk!) the purely baryonic models encompass the data. Consequently, here is no need to postulate that dark matter contributes significantly to the dynamics. The figure clearly shows that there is no need, and therefore no evidence for dark matter within 6 kpc of the Galactic Center, which is as generous a definition of ‘inner Milky Way’ as it gets in my opinion. The authors themselves even write that ‘The discrepancy between observations and the expected contribution from baryons is evident above Galactocentric radii of 6-7 kpc’. In this regard it doesn’t matter whether the majority of the possible baryonic models predict a lower rotation curve: as long as the data agree with at least one baryonic model that is consistent with the observed distribution of mass in the Milky Way, there can not be evidence for dark matter.

I really don’t understand why they then claim to have found proof of dark matter in the innermost regions of the Milky Way. My suspicion is that the authors and their press releases seem to have a (literally) quite broad interpretation of the term ‘innermost region’. Judging from the context, they seem to subsume everything within the solar circle of ~ 8 kpc (the distance of the Sun from the Galactic Center) as ‘innermost’. I don’t think it is an appropriate definition, after all it makes the vast majority of the baryonic mass of the Milky Way part of the innermost region. Half the light of an exponential disk is already contained within less than 1.7 scale length (1.7 x 2.2 kpc = 3.7 kpc for the Milky Way), and all of the bulge/bar is in there, too. But if we nevertheless roll with it for the moment we can see that yes, between 7 and 8 kpc there seems to be need for dark matter … or for a MOND-like effect.

Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line). Note the excellent prediction of the observed rotation curve given the observed distribution of baryons only which is achieved in MOND; the Galaxy appears entirely Newtonian within the innermost 2 kpc.
Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line). Note the excellent prediction of the observed rotation curve given the observed distribution of baryons only which is achieved in MOND; the Galaxy appears entirely Newtonian within the innermost 2 kpc. Credit: McGaugh (2008)

So, lets have a look at one MOND rotation curve constructed for the Milky Way (from McGaugh 2008) to see where we expect to find a difference in Newtonian and MONDian circular velocities. The expected Newtonian rotation curve is shown as a black line in the plot, equivalent to the purely baryonic rotation curves making up the grey band in the figure of Iocco et al.. The rotation curve predicted by MOND is shown as a magenta line and the observed circular velocities are the small squares.

The plot immediately reveals that a discrepancy between the Newtonian and the MONDian rotation curves is expected already at small radii, well within 6 kpc. The findings of Iocco et al. that there appears to be some mass missing within the solar circle therefore do not disagree with the MONDian expectation, in contrast to what one of the authors is quoted saying in a Spektrum article. Furthermore, the plot demonstrates that the need for dark matter (or MOND) in the region inside the solar circle was already well known before this new study.

So, in summary, the study doesn’t show all that much new or surprising, the claimed ‘evidence’ for dark matter in the innermost Milky Way is not present in their data (unless you define ‘innermost’ very generously) and some apparent dark matter contribution within the solar circle is not even unexpected based on MOND predictions.


1: The press releases of the TU Munich and Stockholm University even call it a ‘direct observational proof of the presence of dark matter in the innermost part our Galaxy’ (which is clearly wrong, there is obviously nothing direct about it and the innermost part would imply the very center of the Milky Way).


See the overview of topics in The Dark Matter Crisis.

Pavel Kroupa on "The vast polar structures around the Milky Way and Andromeda "

In case you, like me, have missed Pavel Kroups’s recent talk at the Joint Astronomical Colloquium in Heidelberg, you now have the opportunity to watch a movie of the event and download the slides. The movie is quite long (more than an hour), but it is worth watching it to the end. While the talk is titled “The vast polar structures around the Milky Way and Andromeda”, Pavel talks about much more, starting with tidal dwarf galaxies and ending with a discussion of indications for an alternative model of gravity.

This presentation is very similar and in most parts identical to Pavel’s presentations held at Monterey at the conference “Probes of Dark Matter on Galaxy Scales” and in Durham at the “Ripples in the Cosmos” conference. The latter talk resulted in quite a discussion on Peter Coles’ (aka Telescoper) blog “In the Dark”, following his criticism of Pavel’s talk as being “poorly argued and full of grossly exaggerated claims”. The video of a very similar presentation now offers everybody the opportunity to develop their own opinion on the issue. Given the numerous questions Pavel got during his talk and afterwards, people must have thought that it was worth the effort to argue with him, in contrast to Peter’s opinion.


See the overview of topics in The Dark Matter Crisis.

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:


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


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.

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.


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.