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

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

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

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

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

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

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

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

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

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

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

a) Presentations by Prof. White and Prof. Kroupa

Summary of both presentations:

b) The Debate

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

Part I

Part II


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

58. The tidal stability of Fornax cluster dwarf galaxies in Newtonian and Milgromian dynamics

(Guest post by Indranil Banik and Elena Asencio, August 2nd, 2021)

A directly-related presentation by Elena Asencio is available here:

The tidal stability of Fornax cluster dwarf galaxies in Newtonian and Milgromian gravity

The slides of the presentation can be downloaded here:

A large number of dwarf galaxies in the Fornax cluster (Figure 1) appear to be disturbed, most likely due to tides from the cluster gravity. In the standard cosmological model (ΛCDM) , the observable structure of the dwarfs is barely susceptible to gravitational effects of the cluster environment, as the dwarfs are surrounded by a dark matter halo. Because of this, it is very hard to explain the observations of the perturbed Fornax dwarfs in this theory. However, these observations can be easily explained in MOND, where dwarfs are much more susceptible to tides due to their lack of protective dark matter halos and the fact that they become quasi-Newtonian as they approach the cluster center due to the external field effect.

Figure 1: Fornax galaxy cluster. The yellow crosses mark all the objects identified in the Fornax deep survey (FDS) for this region of the sky, the black circles are masks for the spikes and reflection haloes, and the red crosses mark the objects that pass the selection criteria to be included in the FDS catalog. Image taken from Venhola et al. 2018.

The impact of tides on what the dwarfs look like is illustrated in Figure 2, which shows the fraction of disturbed galaxies as a function of tidal susceptibility η in ΛCDM and MOND, with η = 1 being the theoretical limit above which the dwarf would be unstable to cluster tides. Moreover, there is a lack of diffuse galaxies (large size and low mass) towards the cluster center. This is illustrated in Figure 3, which shows how at low projected separation from the cluster center, dwarfs of any given mass cannot be too large, but larger sizes are allowed further away. Figure 3 thus shows a clear tidal edge that cannot be explained by selection effects, since the survey detection limit would be a horizontal line at 1 on this plot such that dwarfs above it cannot be detected. Diffuse dwarf galaxies are clearly detectable, but are missing close to the cluster center. Another crucial detail in Figure 3 is that dwarfs close to the tidal edge are much more likely to appear disturbed, which is better quantified in Figure 2 in the rising fraction of disturbed galaxies with tidal stability η. The tidal edge is also evident in Figure 2 in that the dwarfs only go up to some maximum value of η, which should be close to the theoretical stability limit of 1. This is roughly correct in MOND, but not in ΛCDM.

Figure 2: Fraction of disturbed galaxies for each tidal susceptibility bin in MOND (red) and ΛCDM (blue). Larger error bars in a bin indicate that it has fewer dwarfs. The bin width of the tidal susceptibility η is 0.5 in MOND and 0.1 in ΛCDM (each data point is plotted at the center of the bin). Notice the rising trend and the maximum η that arises in each theory.

Figure 3: Projected distances of Fornax dwarfs to the cluster center against the ratio Re/rmax, where Re is the dwarf radius containing half of its total stellar mass, and rmax is the maximum Re at fixed stellar mass above which the dwarf would not be detectable given the survey sensitivity. The dwarfs are classified as “disturbed” (red) “undisturbed” (blue). The black dashed line shows a clear tidal edge – at any given mass, large (diffuse) dwarfs are present only far from the cluster center. This is not a selection effect, as the survey limit is a horizontal line at 1 (though e.g. some nights could be particularly clear and allow us to discover a dwarf slightly above this).

We therefore conclude that MOND and its corresponding cosmological model νHDM (see blog post “Solving both crises in cosmology: the KBC-void and the Hubble-Tension” by Moritz Haslbauer) is capable of explaining not only the appearance of dwarf galaxies in the Fornax cluster, but also other ΛCDM problems related to clusters such as the early formation of El Gordo, a massive pair of interacting galaxy clusters. νHDM also better addresses larger scale problems such as the Hubble tension and the large local supervoid (KBC void) that probably causes it by means of enhanced structure formation in the non-local universe. These larger scale successes build on the long-standing success of MOND with galaxy rotation curves (“Hypothesis testing with gas rich galaxies”). MOND also offers a natural explanation for the Local Group satellite planes as tidal dwarf galaxies (“Modified gravity in plane sight”), and has achieved many other successes too numerous to list here (see other posts). Given all these results, the MOND framework appears better suited than the current cosmological model (ΛCDM) to solve the new astrophysical challenges that keep arising with the increase and improvement of the available astronomical data, which far surpass what was known in 1983 when MOND was first proposed.


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

44. 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.

Milky Way rotation curve in MOND

Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line).

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.

wc_fit

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.

Milky Way rotation curve in MOND

Rotation curve of the Milky Way: Observed velocities (squares), baryons + Newtonian Dynamics (black line) and MOND rotation curve (magenta line).

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.

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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.

43. 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.

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

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

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

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

The Facts

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

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

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

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

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

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

The Interpretation

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

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

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

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

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

Filamentary Accretion?

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

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

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

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

Tidal Dwarf Galaxies

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

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

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

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

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

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

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

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

Conclusion

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

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

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

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