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.

The Weizmann Experience: discussions on the future of cosmology

Together with Francoise Combes, who was recently appointed as a professor in the most prestigeous institution in France, Le College de France, and Benoit Famaey, who is an expert on Milgromian dynamics and its deeper foundations (e.g. Famaey & McGaugh 2012), we were invited by Mordehai (Moti) Milgrom to spend a whole week at the Department of Particle Physics and Astrophysics in the Weizmann Institute in Rehovot, Israel. A link to the video (dubbed in English) of the inaugural lecture given by Francoise Combes for her new chair and the introduction by Serge Haroche (Nobel Prize 2012 in physics) is available here (alternatives to the dark matter approach are explicitly mentioned by both).

I met Benoit at Frankfurt airport in the very early morning (he was heading in some random direction) since we had booked the same Lufthansa flight to Tel Aviv. We arrived on Sunday, March 6th, and met Moti at his office in the late afternoon.

In the entrance hall of the department. From left to right: Einstein's field equation without Lambda, Francoise Combes, Mordehai Milgrom, Pavel Kroupa and Benoit Famaey.
In the entrance hall of the Department. From left to right: Einstein’s field equation without Lambda, Francoise Combes, Mordehai Milgrom, Pavel Kroupa and Benoit Famaey.

Coming to know the place and first discussions

I am very impressed by the size and beautiful campus of the whole Weizmann Institut, and how pleasant the entire ambiente is.

Chairs and a pond infront of th Department.
Chairs and a pond infront of the Department.

The people are very friendly and helpful. And interested. I was staying at the spacious and luxurious San Martin Faculty Clubhouse. At night the various buildings and park areas in the Weizmann Institute are illuminated beautifully, with warm lights setting accents and emphasizing a welcoming atmosphere.

The highly-ranked  Weizmann Institute consists of many departments of various natural sciences and seems to be perfectly created for academic pursuit, including leisure areas. Its success in the pursuit of basic research in the natural and exact sciences and in acquiring funding is evident through the architecture, spaciousness, and general design.

There was no planned agenda for us, apart that Benoit was to give a talk on Wednesday, 9th of March, at 11:15, and for Francoise Combes to give a departmental colloquium on Thursday, 10th of March at 11:15. In between these talks we could do either nothing and hang about enjoying the sunshine and exquisite weather and pool, or engage in intense discussions. Perhaps due to the ambiente and of course our comparable research interests, we largely chose the latter.

On Monday, 7th of March, we had a very relaxed day, meeting with Moti at the Department in the late morning and spending our time debating. Typical discussion points (largely between Francoise, Benoit and myself) throughout the visit were the local major underdensity and its possible implications on the value of the cosmological Lambda, the underlying theory of MOND and whether it is due to a “dark” fluid which behaves like dark matter on large scales (e.g. Luc Blanchet’s dipoles and Justin Khoury’s condensate)

Given that Lambda was missing in the equation displayed in the entrance hall of the Department (see first photo above), we began to discuss it. And this is where the “local” underdensity now plays a possibly important role, see this figure from Kroupa (2015),

The underdensity is significant, according to the shown data, and may challenge any cosmological model. From Kroupa (2015).


and in contrast the very recent work by Whitbourn & Shanks where the authors explicitly state agreement with the previous survey by Kennen et al. (2014). The independent finding by Karachentsev (2012) on the local 50 Mpc scale appears to naturally continue the trend evident from the Kennan et al. data (see the figure on the left), IF one assumes the same baryonic to dark-matter ratio as at larger distances. The actually measured stellar density remains similar to the Keenan et al. value at small distance. So the baryonic density (assuming the gas to star ratio and the contribution by dwarf galaxies to remain unchanged out to distances of 800 Mpc [redshift of 0.2]) then within 300 Mpc there is at least a decrease in the baryonic density by factor of two. Conversely, taking Karachentsev’s measurement, we would see a disappearance of dark matter nearby to us since the stellar density remains similar to the Kennen measurement within 150 Mpc while the dark matter density decreases further. So the measurements appear to imply the following picture: within 400 Mpc the luminous (and thus baryonic) matter density decreases significantly by a factor of two. At the same time, the ratio of dark matter to baryonic matter decreases even more. Both findings violate the cosmological principle.

The work by David Wiltshire (his lecture notes) and Thomas Buchert already indicates that inhomogeneities could possibly make the Universe appear to an observer situated within such an underdensity as if it’s expansion is accelerating, although in truth it is not. That is, the inhomogeneities appear to be of the correct magnitude to eliminate the need for Lambda, Lambda (dark energy) merely being an apparent effect mis-interpreted by the supernova type 1a data. The reason lies in that a distant object’s observed redshift depends in reality on the exact paths the photons travel in a universe which consists of time-changing voids and over-densities, and this is a different redshift computed assuming a homogeneous and isotropic expanding Universe.

But we need more detailed calculations taking into account the constraints from the observed under-density shown in the figure to be assured that Lamba=0. It is certainly true that Lambda=0 may be more in line with theoretical ideas than the very small value deduced to explain an apparently accelerating Universe, because it is actually predicted, from quantum field theoretical calculations of the vacuum (for details see e.g. Padilla 2015), to have a value some 60 to 120 orders of magnitude larger. It should be emphasized, though, that “MOND likes Lambda“, in the words of Moti. The reason is that the Lambda derived from astronomical observations (e.g. from supernovae of type 1a observations) and Milgrom’s constant a_0 appear to be naturally related, and MOND may be derivable from vacuum processes (Milgrom 1999).

Within about 300 Mpc, where we can say that we have the best measurements, the Universe is nicely consistent with MOND. The mass-to-light ratios of galaxy groups are less than 10 (Milgrom 1998 and Milgrom 2002), i.e. there is only baryonic matter. The observationally inferred increased density of baryonic matter at distances larger than 300 Mpc would then perhaps be due to cosmological models being inappropriate, i.e. that the currently used red-shift–distance relation may be wrong.

We also debated galaxy evolution, the fraction of elliptical galaxies and the redshift dependence of this fraction. Notably, fig.7 in Conselice (2012)  shows that the observed fraction of massive galaxies does not evolve although the LCDM model predicts a strong evolution due to merging. This is consistent with the independent finding by Sachdeva & Saha (2016) that mergers are not a driving mechanism for galaxy evolution, and this is in turn consistent with the independent findings reached by Lena et al. (2014)  on the same issue.

We further talked about how LCDM is faring on large, intermediate and small  scales, how stellar populations change with physical conditions, the variation of the IMF, as well as political topics. The discussions were far from reaching consensus, we had different views and data sets we could quote on various problems, and time flew by such that we barely noticed.

However, Moti managed to drag us away from his Department, and showed us around the Weizmann institute. An particular station was the famous landmark tower which once housed the Koffler Accelerator and which now houses, in its “bubble”,

The tower which housed the Koffler Accelerator and which now houses a conference room (in its “bubble”) and the Martin S. Kraar Observatory.

a conference room and also the Martin S. Kraar observatory which is also used in international top-level research projects. The director of the observatory, Ilan Manulis, kindly explained to us in much detail its functionality and design for full remote-observations without human interference.

Viewing the lands from the top of the Koffler Accelerator Building. From left to right: Benoit Famaey, Francoise Combes and Mordehai Milgrom.
Part of the Weizmann Institute as viewed from the top of the Koffler Accelerator.
Part of the Weizmann Institute as viewed from the top of the Koffler Accelerator Building.
The "bubble" housing the conference room in the tower of the Koffler Accelerator.
The “bubble” housing the conference room in the tower of the Koffler Accelerator.
The Group at the Koffler Accelerator. From right to left: Benoit Famaey, Francoise Combes, Mordehai Milgrom and Pavel Kroupa.
The Group at the Koffler Accelerator. From right to left: Benoit Famaey, Francoise Combes, Mordehai Milgrom and Pavel Kroupa.

On this Monday Moti took us to lunch at the Lebanese restaurant Petra located in Nes-Ziona, a town 5 minutes drive from the Weizmann Institute. The Lebanese cuisine was fabulous, and I ate far too much.


A diversion to history

And, on Tuesday, 8th of March, Moti and his wife Ivon took us on a drive-around nearby Israel. This trip, involved about 4 hours of driving by Moti, and while driving we discussed, amongst other topics, the new study by Papastergis et al. (2016) in which they use 97 gas-dominated galaxies from the ALFALFA 21cm survey to construct their estimate of the baryonic Tully-Fisher relation showing excellent agreement with the expectations from Milgromian dynamics.

The drive was incredible, as we saw places with many thousands of years of history dating back to the Caananite peoples. It is this land which took the central role in the evolution of the Mediteranean-Sea-engulfing Roman Empire to a Christian empire. It contains the scars of the episodes of the invasion by a newer religion of christian lands, christian reconquest, and reconquest by the newer religion, till the foundation of Israel, issues which remain current to this day.

We visited Caesarea:

Caesarea, once a thriving port for many centuries, from where Paulus was imprissioned and sent to Rome for his hearing at the emperor's court, was wiped out in the 13th century.
Caesarea, once a thriving port for many centuries, from where Paulus was imprissioned and sent to Rome for his hearing at the emperor’s court, was wiped out in the 13th century.

The thriving thousand-yearold medieval city of Caesarea, named by King Herod after Octavian (i.e. Augustus Caesar) and which was once the main port in his kingdom, was finally obliterated from existence after a siege by a Mamluk army in the thirteenth century.

The ruins of Caesarea. King Herodot had his palace here.
The ruins of Caesarea. King Herod is supposed to have had his palace here.
The author amongst the ruins of Caesarea. "What was the fate of Caesarea's inhabitants when it fell to the Mamluks?"
The author amongst the ruins of Caesarea. “What was the fate of Caesarea’s inhabitants when it fell to the Mamluks?”
The Group in front of the Roman ampitheater in windy Caesarea, nearly but not quite ready.
The Group in front of the Roman ampitheater in windy Caesarea, nearly but not quite ready.
The Group in Caesarea, ready. From right to left: Mordehai Milgrom, Francoise Combes, Benoit Famaey, Pavel Kroupa.
The Group in Caesarea, ready. From right to left: Mordehai Milgrom, Francoise Combes, Benoit Famaey, Pavel Kroupa.



Acre, once a blossoming port and a gate-way to the holy lands for christian pilgrims.
Acre, once a blossoming port and a gate-way to the holy lands for christian pilgrims.

Acre: the chief port in Palestine during the crusader epoch still boasting major remains of the huge crusader’s fortress:






Acre: the remains of the Crusader port.
Acre: the remains of the Crusader port.
Acre was under the administration of the Knight's Hospitaller who helped arriving pilgrims and food was served in this Crusaders Refectory.
Acre was under the administration of the Knight’s Hospitaller who helped arriving pilgrims and food was served in this Crusaders Refectory.

After a wonderful dinner at the seashore between Tel Aviv and old Jaffa at the restaurant Manta Ray, where some action happened just before we arrived judging from the large number of police and other forces around, we visited very beautiful Old Jaffa:

Old Jaffa, which dates back to a history of 4000 years and where alrady the Egyptian empire stationed a garrison.
Old Jaffa, which dates back to a history of 4000 years and where alrady the Egyptian empire stationed a garrison.
Old Jaffa.
Old Jaffa.

The restoration of the archeological sites of Caesarea, Acre and of Old Jaffa brings to mind how incredibly rich and beautiful the thousand year old places are along the Mediterranean coast throughout the middle East and northern Africa, if upheld with the corresponding desire to show this history.


Back to science

On Wednesday, 9th of March, we spend the whole day in discussions with staff of the Institute. It began with Benoit Famaey’s presentation on the latest numerical results of modelling the Sagittarius satellite galaxy and its stream in Milgromian dynamics by Strasbourg-PhD student Guillaume Thomas. Natural solutions appear to emerge and this will, once published, clearly add spice to the discussions, given that the only solutions available in LCDM by Law & Majewski (2010) are unnatural in that the dark matter halo of the Milky Way needs to be oblate at right angle to the Milky Way, a solution which poses severe dynamical instabilities for the Milky Way disk. Notably, this polar oblate dark matter halo of the Milky Way alignes with the vast-polar structure (the VPOS) of all satellite galaxies, young halo globular clusters and stellar and gas streams.

In these discussions with the staff members during the aftenoon, we dealt with supernova rates and explosions and types in different galaxies, the relevance to the variation of the IMF in various environments (e.g. metal-poor dwarf galaxies vs metal-rich massive galaxies and the dependency of the IMF on density and metallicity), and cosmological problems such as the local massive under-density mentioned above.

An important point I tried to emphasize repeatedly is that if Milgromian dynamics is the correct description of galactic dynamics, then we must keep an open mind concerning the possibility that all of cosmological theory may have to be rewritten and the large-redshift data may need to be reinterpreted in terms of different redshift–distance and redshift–age relations.

In the evening of Wednesday I tried out the swimming pool on campus, and their sauna as well. I had access to this swimming pool by staying in The San Martin Faculty Clubhouse and the Hermann Mayer Campus Guesthouse – Maison de France. I must admit, that the day was near to being perfect with the sunshine and a closing dinner with Francoise and Benoit again in our meanwhile standard kosher restaurant (Cafe Mada) nearby the San Martin guest house.

On Thursday, 10th of March, Francoise Combes gave her interdepartmental presentation on “The Molecular Universe” which was well visited, and afterwards we went together with some staff of the Weizmann Institute for lunch at Cafe Mada, where a lively and very entertaining discussion ensued on religeos questions. In the late afternoon we joined the Whisky lounge, in which anyone traveling back to Rehovot from abroad can bring a duty-free bottle of Whisky to and donate it to this lounge.

The Local Group of galaxies is highly symmetrical, with all non-satellite dwarf galaxies lying in two planes symmetrically and equidistantly situated around the axis joining the Milky Way and Andromeda. From Pawlowski et al. (2013).
The Local Group of galaxies is highly symmetrical, with all non-satellite dwarf galaxies lying in two planes symmetrically and equidistantly situated around the axis joining the Milky Way and Andromeda. From Pawlowski et al. (2013).

Young researchers meet every Thursday (remember, this is in Israel the end of the week) to sip Whisky and thereby to elaborate on various problems, such as in our case on the local underdensity, or how the two critical constraints we have from the highly organized structure of the Local Group of galaxies and the CMB together constrain the cosmological model.

An interesting statement made was that while one needs about ten LCDM Universes to get one Bullet cluster (Kraljic & Sarkar 2015), an infinite number of LCDM Universes will not give a single Local Group with its symmetries.

At least these are some of the questions we discussed while there on this Thursday. We were also impressed by all the connections of this Department with Princeton, Caltech and Harvard.

Friday and Saturday

Shops begin to close down and it becomes a challenge to find food and Francoise left for France. In the morning I went for a swim and sauna, and for luch Benoit and myself had to go out of the Weizmann Institute (exit Main Gate and turn left) to find a sandwich place.

The Basha Bar in Tel Aviv.


After some work and then in the evening and at about 18:00 we decided to take a taxi to Tel Aviv. We arrived at the Basha Bar by about 18:30 and stayed for three hours (see photo).


The Basha Bar, enjoying a three-hour shisha smoke and many Tuborg beers.
The Basha Bar, enjoying a three-hour shisha smoke and many Tuborg beers.

On Saturday, the kosher breakfast in the guest house was as excellent as ever, but it was interesting for me to note that neither the toaster nor the coffee machine were to be used, while the water boiler was on so we could still have hot Turkish coffee (which we also drink in Bohemia, by the way, so not much new for me here). Nearly everything is closed. Benoit and myself met for lunch and walked outside the Main Gate turning right, over the bridge to reach the Science Park finding bistro Cezar for lunch.

In the evening Moti picked us up for a dinner at his home with Ivon, where we had a long discussion also on the dynamic situation in Germany, Europe and the future.

At the home of Moti in Rehovot.
At the home of Moti in Rehovot. From right to left: Moti, Benoit and the author.


Final comments

Benoit and myself stayed on until Monday, joining the astrophysics journal club which serves lunch at the Department on Sunday. I spent most of the afternoon discussing with Boaz Katz how star clusters may be relevant for type 1a supernovae. In the evening of Monday Benoit and I went again to Cafe Mada for a final dinner and drinks. On Monday, 14.03., we flew out around 16:00, taking a taxi to the Tel Aviv airport at 13:00 from the Department. We shared the same flight back. Again the 4+ hour long Lufthansa stretch without personal-screen-based entertainment system! But, this gave Benoit and myself a chance to further discuss at length the above mentioned Khoury condensate and the Blanchet dipoles as models for galaxy-scale MOND and cosmology-scale dark-matter-like behaviour. But I note that these are not dark matter models. During pauses my thinking was that as the coastal line of Tel Aviv receded in the setting Sun we left a small fraction of the Levant and northernmost Africa, all once pat of the Roman Empire, at a level of civilisation mirrored by the clear, brllliantly lit vast and dynamic power- and resource-hungry central-European night with full autobahns, radiant towns and illuminated football fields in nearly every village. In Frankfurt our ways parted after a last small dinner in the train station, Benoit taking a bus to Strasbourg at about 21:30, and me starting my odessey to Bonn at the same time using the available train connections (German trains all too often run late, these days).

The visit was most memorable for all of us, and Benoit and myself agree that we would like to return. We did not reach any conclusions but we came to know many new people and perhaps helped to underscore the very seriousness of alternative concepts to dark matter and the many failures of the LCDM model.

In closing it is probably fair to say that Milgrom contributed the greatest advance on gravitational physics since Newton and Einstein.


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

First Workshop on Progress in Modelling Galaxy Formation and Evolution in Milgromian dynamics — first results achieved with the Phantom of Ramses (PoR) code

[Note: This web-page is being updated continuously:
current status: 26.09.15]

Observatoire astronomique de Strasbourg, Universite de Strasbourg, CNRS UMR 7550, Sept. 21st - 25th 2015

Below are provided
ORGANISERS: Benoit Famaey (Strasbourg) and Pavel Kroupa (Bonn)

1.BACKGROUND / MOTIVATION: Galaxy-scale data seem to be in accordance with the hypothesis that the extrapolation of Newtonian gravitation by orders of magnitude below the Solar system space-time curvature breaks down completely, and that collisionless astronomical systems behave according to space-time scale-invariant dynamics, as postulated by Mordehai Milgrom (2015). The classical theories of dynamics and gravitation underlying this symmetry, often referred to as MOND  theories, show a richer dynamical behaviour with new phenomena which appear non-intuitive to a Newtonian mind. Very successful analytical results have been obtained in this dynamics framework, such as accounting for the hitherto not understood properties of polar-ring galaxies (Lueghausen et al. 2013), accounting for the Bullet cluster (Angus, Fmaey & Zhao 2006Angus & McGaugh 2008) and the properties of disk galaxies (MOND reviews by Scarpa 2006; Famaey & McGaugh 2012;Trippe 2014) and elliptical galaxies (Sanders 2000; Milgrom & Sanders 2003; Scarpa 2006).

But little understanding of the dynamical behaviour of live Milgromian systems has been gathered. Live calculations, i.e. simulations of galaxies, are required in order to test, to possibly refine or to falsify this approach. The implications for fundamental physics are major in any case!

A series of Milgromian-dynamics workshops is planned to begin remedying this situation.

With this first “Phantom of Ramses” (PoR) meeting, the aim is to bring together the pioneers who have been daring footsteps into applying Milgromian dynamics to simulate live galaxies. First simulations of galaxies within MOND have been achieved with the first Milgromian Nbody code without gas (Brada & Milgrom 1999). Tiret & Combes (2007) re-visited this problem with their own code. The PhD thesis of Tiret is available here (in French). For spheroidal geometries MOND simulations have become possible with the NMODY code by Nipoti, Londrillo & Ciotti (2007), see e.g. the application of this code to the phase-transition of spheroidal systems on radial orbits (Wu & Kroupa 2013). A MOND code has also been developed for studies of cosmological structure formation by Ilinares, Knebe & Zhao (2008). While being highly successful in their ability to represent observed galaxies, all of these attempts have died-off due to a lack of long-term sustainability.

Now much more involved and more numerous studies has become possible with the first publicly available Milgromian dynamics computer code including star formation, i.e. baryonic physics (Lueghausen, Famaey & Kroupa 2015) with which even full-scale simulations of cosmological structure formation have become achievable, PoR being an official patch to Teyssier’s RAMSES code. A similar computer code (RAyMOND) has been developed independently by a Chilean research group (Candlish, Smith & Fellhauer 2015).

Because non-linear Milgromian dynamics is largely non-intuitive for researchers trained to think within the framework of linear Newtonian gravitation, this group of pioneers needs to find the chance to discuss, in as great depth as is required, the issues arising with initialising, setting-up and evolving Milgromian galaxies in virial equilibrium, including gas dynamics and star formation. The first scientific results which have already been achieved with the PoR code will be discussed at this occasion, but research related to Milgromian dynamics (e.g. by adoption of zeroth-order approximations by adding dark matter particles to Newtonan systems) will also be discussed.

The meeting will take place at the Observatoire astronomique de Strasbourg. We are planning a whole week for this event, whereby there will be one to two (at most three)  presentations per day interrupted with long discussion breaks to dwell upon problems that have been encountered and that may need solutions. Also, the breaks are intended to allow new persons to learn using PoR. The meeting will take place in the *MEETING ROOM* (with a capacity of about 20) at the Observatoire, and the presentations can be of any duration, but must have a break after the first 45 minutes if longer. After the last presentation each day discussions may continue at will, and Strasbourg offers many excellent culinary opportunities for the evening entertainments.

Please register by sending an e-mail to Benoit Famaey <benoit.famaey_at_astro.unistra.fr> and to Pavel Kroupa <pavel_at_astro.uni-bonn.de>.

Note that this meeting does not have invited talks. The attendance is limited to 20.
3.PARTICIPANTS (preliminary):

Garry Angus (Brussel, Belgium)
Indranil Banik (St. Andrews, UK)
Christian Boily (Strasbourg, France)
Joerg Dabringhausen (remotely from Concepcion, Chile)
Benoit Famaey (Strasbourg, France) [SOC]
Martin Feix (Paris, France)
Hector Flores (Paris, France)
Alistair Hodson (St. Andrews, UK)
Rodrigo Ibata (Strasbourg, France)
Tereza Jerabkova (Praha, Czech Rep.)
Pavel Kroupa (Bonn, Germany) [SOC]
Fabian Lüghausen (Bonn, em.; tbc)
Marcel Pawlowski (Cleveland, USA)
Florent Renaud (Surrey, UK)
Jean-Babtiste Salomon (Strasbourg, France)
Ingo Thies (Bonn, Germany)
Guillaume Thomas (Strasbourg, France)
Yanbin Yang (Pairs, France)
HongSheng Zhao (St. Andrews, UK)

Conference Photo (24.09.2015):

Left to right:  Yanbin Yang, Indranil Banik, Ingo Thies, Guillaume Thomas, Garry Angus, Jean-Babtiste Salomon, Tereza Jerabkova, HongSheng Zhao, Rodrigo Ibata, Marcel Pawlowski, Hector Flores, Alistair Hodson, Florent Renaud, Benoit Famaey, Fabian Lueghausen, Pavel Kroupa

Hotel Esplanade
ETC Hotel
Hotel Roses
Au Cerf d’Or
des Princes
The programme, abstracts and list of participants are available here as a pdf file:

PROGRAM (with downloadable presentations):  

First Workshop on Progress in Modelling Galaxy Formation and Evolution in Milgromian dynamics —
first results achieved with the Phantom of Ramses (PoR) code.
At the Observatoire astronomique de Strasbourg, 21.09.-25.09.2015.

PoR-code talks are scheduled for the afternoons allowing for discussion and learning time.  A few scientific talks relevant to the mass-deficit problem are scheduled for the mornings.

******* Sunday, 20th September

evening, approximately 18:00-
Meet for drink and food at Au Brasseur
******* Monday, 21st September 10:00 MORNING COFFEE 10:30 Welcome/Introduction/First presentation and discussion: Setting the scene: 1. Kroupa_PoR.pdf: Why is the dark-matter approach ill-fated? (Pavel Kroupa) 2. Famaey.pdf: The basics of Milgromian dynamics/MOND (Benoit Famaey) LUNCH (12:15-14:45) 15:00-16:15 1. Lueghausen_PoR.pdf: The PoR code (Fabian Lueghausen) 2. Thies_PoR.pdf: Setting up a stable disc galaxy in PoR (Ingo Thies) 16:30 AFTERNOON TEA 17:00-18:00  Open Discussion ******* Tuesday, 22nd September 10:00 MORNING COFFEE 10:45-11:15 (30 minutes) Angus_PoR.pdf: The DiskMass Survey’s implications for MOND, CDM and itself  (Garry Angus) LUNCH (12:15-14:45)   14:45-15:15 (30 minutes) Banik.pdf: The External Field Effect In QUMOND: Application To Tidal Streams (Indranil Banik) 16:10 AFTERNOON TEA 16:30 Thomas_PoR.pdf: Simulating Tidal Streams with PoR (Guillaume Thomas) PoR Movie (dSph Sgr, slide 19 in presentation): YouTubelink 17:00-18:00  Open Discussion - decision to set up PhantomWIKI ******* Wednesday, 23rd September 10:00 MORNING COFFEE 10:45-11:15 Yang_PoR.pdf: (30 minutes) Reproducing properties of MW dSphs as descendants of DM-free TDGs (Yanbin Yang) MEETING PHOTO  (12:15) LUNCH (12:20-14:45) 14:15-14:45 Angus2_PoR.pdf: The sub-subhalo connection to M31’s plane of satellites (Garry Angus) 14:45-15:15 Pawlowski_PoR.pdf: (30 minutes) Small-scale problems of cosmology and how modified dynamics might address them (Marcel Pawlowski) 16:00 AFTERNOON TEA 16:30 Renaud_PoR.pdf: Gravitation-triggered star formation in interacting galaxies (Florent Renaud) 17:30-18:00  Open Discussion 18:30--  Workshop dinner at Au Brasseur
******* Thursday, 24th September 10:30 MORNING COFFEE 10:45-11:15 Hodson_PoR.pdf: (30 minutes)  EMOND (Extended MOND) and effective galaxy cluster masses (Alistair Hodson) 11:30-12:00 Preliminary results on QMOND forces between point masses (HongSheng Zhao) LUNCH (12:15-14:45) 14:45-15:15  Salomon_PoR.pdf: The tangential motion of the Andromeda System (Jean-Babtiste Salomon) 15:15-15:45 Dabringhausen_PoR.pdf: Early-type galaxies in Milgromian dynamics (Joerg Dabringhausen, remotely from Concepcion, Chile) 16:15 AFTERNOON TEA 16:45-17:15 Banik2_PoR.pdf: Evidence for Dynamical Heating in The Local Group (Indranil Banik) 17:15-18:00  Open Discussion ******* Friday, 25th September 10:00 MORNING COFFEE 10:30-12:00 Kroupa_IMF_Strasbrourg.pdf Main Seminar of the Observatory: Is the stellar IMF a probability distribution function, or is star formation highly regulated? (Pavel Kroupa) LUNCH (12:15-14:45) 15:00 Final discussion and FAREWELL

This wiki is dedicated to supporting the research making use of the “Phantom of RAMSES” (PoR) patch.

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.