Astronomy

How to explain tidal tails and dark matter sub-halos in simple terms?

How to explain tidal tails and dark matter sub-halos in simple terms?


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There is the exciting news by ESA entitled Is the nearest star cluster to the Sun being destroyed?

Data from ESA's Gaia star mapping satellite have revealed tantalising evidence that the nearest star cluster to the Sun is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy.

I heard about dark-matter sub-halos and tidal tails before, but I am by no means an expert and Wikipedia is not too helpful for my question: I would like to use ESA's animation on Hyades tidal tails to explain the terms "dark matter halo" and "tidal tails" in 2 minutes. Consider that my audience probably heard the term "dark matter" before, but they have to be reminded about it. The term "galaxy" may be used, maybe even "star cluster", but not much more. May you help me, please?

References

  • Tereza Jerabkova et al.: The 800 pc long tidal tails of the Hyades star cluster

How to explain tidal tails and dark matter sub-halos in simple terms? - Astronomy

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51. The Crisis in Cosmology is now catastrophic

(by Pavel Kroupa, 10th Nov. 2020, 09:00)

We have not blogged for some time and an update on some of the developments concerning The Dark Matter Crisis has been posted here. Below are recent scientific developments which strongly suggest that the standard model of cosmology (the SMoC) which relies on the existence of cold or warm dark matter (C/WDM) particles is not a correct description of the observed Universe. Note that the SMoC which is based on the hypothesis that cold dark matter particles exist comprises the currently widely accepted LCDM cosmological model, while the SMoC which assumes warm dark matter particles exist constitutes the currently less popular LWDM cosmological model. The difference of both models in terms of structure formation and the type of galaxies formed is minimal, which is why both are referred to as the SMoC.

Why has the Cosmology Crisis become catastrophic?
  1. First of all, C/WDM particles have still not been found after more than 40 years of searching! The account of the situation published on October 11th, 2020, on the Triton Station by Stacy McGaugh is worth reading. Stacy writes “… the field had already gone through many generations of predictions, with the theorists moving the goal posts every time a prediction was excluded. I have colleagues involved in WIMP searches that have left that field in disgust at having the goal posts moved on them: what good are the experimental searches if, every time they reach the promised land, they’re simply told the promised land is over the next horizon?“. In view of the available evidence challenging the existence of C/WDM particles, it is stunning to read “ The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is by now well established ” in Sec. 26.1.1 of the 2018 version of the Review of Particle Physics. Some five years ago I had dared to suggest to the editors and section authors to change this very statement to “ The existence of Dark (i.e., non-luminous and non-absorbing) Matter (DM) is currently a leading hypothesis ” or similar, but the short reply was quite unpleasant. It is unfortunate that only the cosmological argument leads one to the C/WDM particle hypothesis, there being no independent (non-cosmological and non-astronomical) evidence. Such evidence could have come from indications in the Standard Model of Particle Physics, for example, but this is not the case. Put in other words, if we had not known about cosmology or galaxy rotation curves, we would not be contemplating C/WDM particles. Thus, by the astronomical evidence having gone away (follow the Dark Matter Crisis), the physicists are left with nothing apart from belief. I would argue that the words “belief” and “opinion” should be banned from the language of natural sciences. Note that the situation is different for the fast collisionless matter (FCM, or “hot dark matter”) which appears in MOND-cosmological models (Angus 2009). Independetly of the astronomical evidence, the experimental fact that neutrinos have mass and oscillate suggests the existence of an additional sterile neutrino. Candidates for FCM particles thus arise independently of astronomy or cosmology. FCM particles do not affect galaxies as they are too low mass, so even at their maximum allowed phase space density as set by the Tremaine-Gunn limit, they cannot be dynamically relevant to the masses of galaxies. Returning to the SMoC: the lack of experimental verification of C/WDM particles comes in hand with additional failures of the SMoC:
  2. Testing for the presence of the speculative C/WDM particles through the very well understood physical mechanism of Chandrasekhar dynamical friction leads to the conclusion that the dynamical friction through the putative dark matter halos around galaxies which are, in the SMoC, made up of C/WDM particles, is not evident in the data (Angus, Diaferio & Kroupa 2011 Kroupa 2015 Oehm & Kroupa 2017). That is, a galaxy which falls towards another galaxy should be slowed down by its dark matter halo, and this slow-down is not seen. The galaxies pass each other with high velocities, like two stars passing each other on hyperbolic orbits, rather than sinking towards each other to merge. This evidence for the non-existence of C/WDM halos around galaxies is in-line with the above mentioned lack of experimental detections (point 1 above). Customarily, an image of two strongly interacting galaxies is automatically interpreted as being a galaxy merger. But this is an over-interpretation of such images, since the implied mergers are not happening in the frequency expected in the standard dark-matter-based theory. Renaud et al. (2016) calculate ant document the theoretical description of an observed strongly interacting galaxy pair in the C/WDM framework and in MOND. Indeed, that the population of galaxies does not evolve significantly since a redshift of one has been found by Hoffmann et al. (2020) and has already been described by Kroupa (2015). This lack of evolution and the hugely vast preponderance of disk galaxies, of which a large fraction is without bulges, means that galaxies merge rarely as mergers nearly always transform the involved disk galaxies into earlier types of galaxies (disks with massive bulges, or even S0 or elliptical galaxies). is now much discussed. The Hubble Tension comes about as follows: the Hubble constant we should be observing today can be calculated assuming the standard dark-matter based SMoC is correct and that the Cosmic Microwave Background (CMB) is the photosphere of the Hot Big Bang (but see also point 6 below). The actually measured present-day value, as obtained from many independent techniques including supernovae 1a standard candles, gravitational lensing time delays, and mega-masers, comes out to be significantly larger though. The evidence is compiled in Haslbauer et al. (2020). The observer today sees a more rapidly expanding Universe than is possible according to the SMoC. More on the Hubble tension below (point 7).
  3. The planes of satellites (or disk of satellites) problem has worsened: Our own Milky Way has been found to have a more-pronounced disk of satellite galaxies around it than thought before (Pawlowski & Kroupa 2020 Santos-Santos, Dominguez-Teneiro & Pawlowski 2020). Andromeda has one (Ibata et al. 2013, Sohn et al. 2020) and the nearby Centaurus A galaxy too (Mueller et al. 2018). The majority of other galaxies also show evidence for such planes or disks of satellites (Ibata et al. 2015). That the three nearby major galaxies simultaneously show such disks of satellite galaxies, and that disks of satellite systems are indicated by the majority of more distant galaxies, where the SMoC expects such satellite planes only in very rare cases (Pawlowski et al. 2015 Pawlowski 2018), eliminates with de facto complete confidence (i.e. much more than 5sigma) the SMoC, given that the satellites are in the great majority of cases ancient and void of gas such that they must have orbited their hosts many times. The Milky Way satellites also seem to be on almost circular orbits, strongly suggestive of a dissipative origin (Cautun & Frenk 2017) similar to the process that forms solar systems.
  4. Astronomical data have uncovered, with extremely high confidence (more than 5sigma), that the strong equivalence principle is violated on the scale of galaxies (Chae et al. 2020 ), exactly in-line with a central expectation by MOND (Milgrom 1986), and in contradiction to the SMoC. While apparently not receiving much attention (e.g. via news coverage), this work by Chae et al. (2020) is a game-changer, a break-through of the greatest importance for theoretical physics. Independent evidence for the violation of the strong equivalence principle is also evident in asymmetrical tidal tails around globular clusters (Thomas et al. 2018). Gravity therefore behaves non-linearly on galaxy scales, preventing a simple addition of the fields contributed by different masses. This is a consequence of the corrected, generalised Poisson equation (Bekenstein & Milgrom 1984) which these authors point out is also found in classical theories of quark confinement.
  5. Possibly a “nuclear bomb” nuked standard cosmology: Although the SMoC is only valid if the Universe is transparent, observations show there to be dust between galaxies. This intergalactic dust is ancient, and it radiates as it is heated by photons from the surrounding galaxies. Vaclav Vavrycuk (2018) has added all photons from this dust in an expanding Universe (i.e., in the past the intergalactic dust density was higher in a warmer Universe) and found the photon emission received by us to be very (nearly exactly) comparable to the measured CMB with the correct temperature of about 2.77K. For an explanation of his research paper see this YouTube video by MSc student Rachel Parziale at Bonn University. Note that the measured weak but large-scale magnetic fields around galaxy clusters and voids produce a correlated polarisation signal. The total number of infrared photons received at Earth is an integral over the time evolving density distribution along the line of sight such that the observed mass distribution within a small redshift around us should not correlate with the overall fluctuation of photon intensity seen in projection on the sky. The calculations by Vavrycuk thus suggest that CMB=cosmological dust emission , rather than being the photosphere of the Hot Big Bang. CMB research comprises an incredibly precise science, but the role of intergalactic dust needs to be considered very carefully and by avoiding pre-conceptions. Note that even if only a few per cent of the CMB were to be due to ancient intergalactic dust, then this would already bring down the SMoC.
  6. The Universe around us contains far too few galaxies out to a distance of about 0.3 Gpc . This Keenan-Barger-Cowie (KBC) void falsifies the SMoC at more than 6sigma confidence. The KBC void kills the SMoC because the SMoC relies on the Universe starting off isotropically and homogeneously with the observed CMB fluctuations at the redshift z=1100 boundary condition about 14Gyr ago and cannot evolve density differences to the observed KBC under-density at z=0 which is the present time. Combined with the Hubble tension, the SMoC is falsified with more than 7sigma confidence. Newtonian gravitation plus the hypothetical C/WDM particles are together nowhere near strong enough to generate the observed density contrasts and the observed velocity differences between neighbouring Gpc-scale volumes . The next blog by Moritz Haslbauer will explain this situation. Note that here we still treat the CMB as the photosphere of a Hot Big Bang, but this may need to be reconsidered (see point 6 above).
  7. The SMoC relies on the Universe having no curvature, but Di Valentino, Melchiorri & Silk (2020) find the enhanced lensing amplitude in CMB power spectra to imply a closed and thus curved Universe. However, this could be related to structure formation being more efficient than is possible in the SMoC (see point 7 above).
  8. Cosmic isotropy is challenged at the 5sigma confidence level by X-ray selected galaxy clusters (Migkas et al. 2020), with the implication that the Universe appears to expand faster in a certain direction. A discussion of this evidence is provided by Scientific American. Cosmic isotropy is also challenged by the significant evidence for a dipole in the number counts of quasars beyond redshift one (Secrest et al. 2020). Independently of this, Javanmardi et al. (2011) also found evidence for a directionally dependent expansion rate.
  9. Last for now but not least, the observation of massively interacting galaxy clusters such as the El Gordo cluster at high redshift (z=0.87) independently falsifies the SMoC with more than 6sigma confidence. In the SMoC, galaxy clusters cannot grow to such masses by this redshift – there is not enough time, or alternatively, Newtonian gravitation is too weak even with the help of the hypothetical C/WDM particles. This is shown by Asencio, Banik & Kroupa (2020). Elena Asencio is researching for her MSc thesis in the SPODYR group in Bonn.

Combining the above KBC void/Hubble Tension/El Gordo falsifications with the previously published tests (Kroupa et al. 2010, Kroupa 2015 see the figure below taken from Kroupa 2012) means that it has become, by now, wrong to still consider the standard dark-matter based cosmological model, the SMoC, as being relevant for describing the Universe. The falsification of the SMoC has reached well above the 7 sigma confidence — Remember: the Higgs Boson was accepted as having been discovered once the experimental confidence rose to 5sigma. It is important to emphasise that independent tests on very different scales lead to the same result, the SMoC being ruled out by many tests with more than 5sigma confidence.

The loss of confidence until 2012 in the Standard Model of Cosmology (SMoC) with each documented failure (numbered here from 1 to 22 and explained in Kroupa 2012) which has never, to date, been resolved. Thus, if each such failure (meaning the SMoC prediction is falsified by observational data) is assumed very conservatively to lead to a loss in confidence of only 30% that the SMoC is valid, then, by today (including the catastrophic >6sigma falsifications described in this blog) the statement that the SMoC describes the real Universe can be defended with a confidence=epsilon , with epsilon being arbitrarily close to zero (taken from figure 14 in Kroupa 2012).

The above list, but more importantly, the very high significance of the results, seem to indicate that a paradigm change may be under way in the sense that our current understanding of the Universe may be entirely rewritten at a very fundamental level. This is already indicated by gravitation being Milgromian. The paradigm shift would be epochal (see also this previous blog on the historical context) if the suggestion by Vavrycuk concerning the physical nature of the CMB were correct (point 6 above) because in this case our very concept of a Hot Big Bang and the origin of matter would be up in the air. There is independent evidence that a once-in-a-century paradigm shift may be under way: the Universe is much more structured than allowed by the SMoC . Thus, the Local Group of Galaxies (on a scale of 3Mpc across, Pawlowski, Kroupa & Jerjen 2013 ) shows a frightening symmetry in its matter arrangement (I call this frightening because there is currently no known theory to explain this distribution of matter). The arrangement of galaxies (Peebles & Nusser 2010) in the nearby cosmological volume (20Mpc across) does not correspond to the SMoC model and these very galaxies show a history of star-formation which appears to be far too tuned and non-varying (Kroupa et al. 2020). This begs the question how they manage to do so? The entire local Universe appears to be engaged in a significant bulk flow generated by major voids and over-densities (Haslbauer et al. 2020 Hoffmann et al. 2020).

Galaxies provide formal and precise observational data that allow us to correct the work of Newton and Einstein on gravitation, who did not have these data at their disposal. Rather, they formulated the currently assumed theories of gravitation subject to Solar System constraints only, which are now many decades if not centuries old. In his book “A Philosophical Approach to MOND“, David Merritt (2020)addresses the formal philosophical measures concerning how the Newtonian/Einsteinian formulation of gravitation needs to be assessed in terms of its success in describing the observed Universe in comparison with the correction to the law of gravitation through incorporation of galaxy data as formulated by MilgrOmiaN Dynamics (MOND) . (Next sentence added Jan 3rd, 2021:) In Merritt (2017) we read his conclusion “The use of conventionalist stratagems in response to unexpected observations implies that the field of cosmology is in a state of ‘degenerating problemshift’ in the language of Imre Lakatos.” This would tend to close a circle: if Newtonian/Einsteinian gravitation needs to be revised, then we cannot use Einsteinian gravitation to formulate the evolution of the Universe, which opens the whole issue of how it started, what are the boundary conditions and how does it evolve? The Catastrophic Crisis in Cosmology (i.e. the fact that the observational data do not fit to the SMoC) is thus merely exactly the statement that we may well be in the process of a very major paradigm shift.

The big challenge for the future will be to find out how the Universe truly does work. The next blog by Moritz Haslbauer will indicate how a step towards this goal might have been achieved by Haslbauer, Banik & Kroupa (2020).

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


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.

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.


Title: TIDAL STREAM MORPHOLOGY AS AN INDICATOR OF DARK MATTER HALO GEOMETRY: THE CASE OF PALOMAR 5

This paper presents an example where the morphology of a single stellar stream can be used to rule out a specific galactic potential form without the need for velocity information. We investigate the globular cluster Palomar 5 (Pal 5), which is tidally disrupting into a cold, thin stream mapped over 22 deg on the sky with a typical width of 0.7 deg. We generate models of this stream by fixing Pal 5's present-day position, distance, and radial velocity via observations, while allowing its proper motion to vary. In a spherical dark matter halo we easily find models that fit the observed morphology. However, no plausible Pal 5 model could be found in the triaxial potential of Law and Majewski, which has been proposed to explain the properties of the Sagittarius stream. In this case, the long, thin, and curved morphology of the Pal 5 stream alone can be used to rule out such a potential configuration. Pal 5-like streams in this potential are either too straight, missing the curvature of the observations, or show an unusual morphology which we dub stream-fanning: a signature sensitive to the triaxiality of a potential. We conclude that the mere existence of other thin tidal streamsmore » must provide broad constraints on the orientation and shape of the dark matter halo they inhabit. « less


Massive Dark Matter Monster May Be Destroying Stars According to New Scientific Study

Stars in the nearby Hyades cluster are disappearing, with many believing that a massive amount of dark matter with the mass of 10 million Suns is the culprit.

Something massive in space is making clusters of stars vanish. The mysterious invisible cosmic mystery has scientists scrambling for answers. The Hyades cluster, which is closest to our sun and 153 lightyears from Earth, is losing stars at an incredible rate, and scientists believe that it is a "dark matter substructure, a relic that contains the mass of 10 million Suns and is made of a mysterious non-luminous substance." One part of the Hyades cluster has been left bare, devoid of stars.

Tereza Jerabkova is leading the team of scientists at the European Space Agency (ESA) who came across the "Galactic Lump" by using data collected by ESA's Gaia satellite. "This is the amazing thing about the data from the Gaia satellite-we have the chance, for the first time in history, to search for stellar structures that are hiding in the huge amount of field stars in the galaxy," Jerabkova said in an interview with Vice. A lot of the stars in the Hyades cluster are visible to the naked eye from Earth, which means that we can even see their absence without a high powered multimillion dollar telescope.

Over the past 700 million years, the Hyades cluster has undergone several changes and many of its brightest stars can still be seen on Earth "in the V-shape at the head of the constellation Taurus." The massive amount of changes are due to "interior cluster dynamics as well as gravitational forces from the larger Milky Way galaxy." The outside forces have developed what scientists have called "tidal tails," that sweep out into the galaxy, showing older and new stars in the process.

Now, these "tidal tails" are being ripped apart by something massive. According to the new study from Tereza Jerabkova and her team at the ESA, "a close encounter with a massive Galactic lump can explain the observed asymmetry in the tidal tails of the Hyades." Jerabkova adds, "We see that stars that belong to the nearest star cluster are moving in a way they should not be moving if we apply our known and widely used models." She goes on to note that their models could either be way off, "or the motions are changed due to a dark matter lump, and this would also be an important discovery."

Tereza Jerabkova states that the mystery "may be a dark matter substructure, also known as a sub-halo." The massive dark matter objects "emerge in the early years of galactic formation," and have been studied by scientists for years now. A black hole would simply gobble up the stars, but that isn't what's happening here. Jerabkova says. "the orbits of the stars in the Galaxy are being affected/changed by the encounter," which sees some of the "tidal tails" being torn apart. As for this massive galactic beast taking away our sun, scientists say that it's not something that we should be worrying about. The interview with Tereza Jerabkova was originally conducted by Vice.


Of course there are data which constitute anomalies for both theories. Two examples (both taken from Merritt, 2020): (i) the measured abundances of lithium-7 and deuterium imply, via the equations of big bang nucleosynthesis, very different numbers for the mean density of nuclei (‘baryons’) in the universe. This anomaly is independent of assumptions about dark matter and exists with equal force in both theories. (ii) The observed dynamics of galaxy clusters is difficult to explain under either theory. Under the standard model, dark matter is invoked to explain the cluster data, but only with limited success. Merritt (2020) also presents a list of anomalies that exist under the standard model but not under Milgrom’s theory (the ‘core-cusp’ problem, the ‘too big to fail’ problem, the ‘problem of the satellite planes’ etc.).

Skordis & Złosnik were not the first to demonstrate empirical equivalence of this sort. At least two earlier versions of Milgrom’s theory (Angus 2009 Berezhiani & Khoury, 2015) successfully accounted for all large-scale cosmological data, but they did so by postulating forms of dark matter. Skordis & Złosnik were the first to achieve this without invoking any form of dark matter.

Feyerabend does not state explicitly, in either of the cited articles, what he means by “second-order effects of motion”. I believe that “second-order” here means order V 2 /c 2 where V is the speed of the observer relative to the Ether and c is the speed of light. The famous Michelson and Morley experiments were of second-order in this sense. Cei (2020, Chapter 7) remarks that “by 1895 the genuinely troubling results [from the standpoint of Lorentz’s theory] were only the ones of second order.”.

Feyerabend (1987, p. 293). The misspelling of Worrall’s name is Feyerabend’s.

Nevertheless there do exist relativistic versions of Milgrom’s theory that are, apparently, as successful as the standard model at explaining data from the cosmic microwave background, the matter power spectrum on cosmological scales etc. See Angus (2009) and Skordis and Złosnik (2020) for two examples.

Even some normal matter is expected to be ‘dark’ for instance, the black hole and neutron star remnants that are believed to be produced during the late evolution of massive stars. Astrophysicists (both standard-model and Milgromian) typically try to account for the presence of these objects when computing the gravitational force from the normal matter.

Indeed there is growing momentum, on the part of standard-model cosmologists, to define ‘galaxy,’ quite generally, as ‘a stellar system containing dark matter’ see Willman and Strader (2012).

Standard-model cosmologists sometimes invoke, in this context, the so-called ‘WIMP miracle’: the fact that the self-annihilation cross-section needed to obtain the correct cosmological abundance of dark matter via thermal production in the early universe is similar to what is expected for a new particle (a ‘WIMP’) that interacts via the electroweak force. However there is an emerging consensus that this paradigm for dark matter has already been experimentally ruled out (e.g. Siegel, 2019). Karl van Bibber, in the Summary talk of the July 2016 Identification of Dark Matter (IDM2016) meeting in Sheffield, England, encouraged the experimenters in his audience not to be discouraged: “No hand-wringing over fraying of the ‘WIMP miracle’! … Often a deceptively too simple argument is just what’s required to get the ball rolling.”.

Milgromian researchers prefer the name ‘dwarf over-prediction problem.’.

This remarkable fact—established already in the 1970s (Kunkel & Demers, 1976 Lynden-Bell, 1976)—was all but ignored by standard-model cosmologists until quite recently, in spite of (or perhaps because of) the fact that it is so clearly at odds with the predicted distribution of dark sub-halos.

The fact that the satellites lie spatially in a thin planar structure already implies a great deal of velocity correlation, unless one postulates that we are observing the structure at a special time.

“Massive” means here that the mass of the body is much greater than the mass of a single dark-matter particle, i.e. M.

Dwarf galaxies are traditionally named after the constellation in which they sit. This naming scheme has become cumbersome as the number of identified dwarves has increased, e. g. Bootes I, Bootes II, Bootes III etc.

For comprehensive reviews of the successes and failures of the two cosmological models, see Famaey and McGaugh (2012), Sanders (2019), Merritt (2020) and Skordis and Zlosnik (2020).

E. g. Popper (1959, p. 108): “We choose the theory … which not only has hitherto stood up to the severest tests, but the one which is also testable in the most rigorous way.”.

See, for instance, Laudan (1989, p. 316, note 33) or Worrall (1978, pp. 308–309 and 1991, pp. 343–344). Both authors note that Feyerabend’s rule directs us to choose the theory that has (in Laudan’s words) more “heuristic potential” in a Lakatosian sense that is, greater potential for generating confirmed novel predictions.


32. Does filamentary accretion of dark matter sub-halos naturally produce a VPOS-like structure?

In the previous post we discussed the VPOS, the vast polar structure of satellite objects around the Milky Way. One of the suggested origins within the cosmological cold dark matter paradigm is that the satellites have been preferentially accreted along large, cosmic filaments. These are long, thread-like structures which arise naturally during the formation of structure in the cosmos. The movie below shows how they come about:

One work suggesting that filamentary accretion can solve the VPOS-problem is Lovell et al. (2011). Its abstract claims that:

“All [six] haloes [of the Aquarius simulations] possess a population of subhaloes that rotates in the same direction as the main halo and three of them possess, in addition, a population that rotates in the opposite direction. These configurations arise from the filamentary accretion of subhaloes. Quasi-planar distributions of coherently rotating satellites, such as those inferred in the Milky Way and other galaxies, arise naturally in simulations of a ΛCDM universe.

Note the part we marked in bold face. This statement of theirs suggests that a structure like the VPOS is a natural outcome of cosmological simulations, which arises due to the filaments around a dark matter halo. That filaments can lead to anisotropies in the direction from which sub-halos are accreted onto larger halos is obvious, and it was a good idea that this might be a way to form anisotropic distributions of subhalos. However, there are several reasons to doubt this scenario.

First of all, the filaments are way too thick. For example, in Vera-Cirro et al (2011) it is shown that the filaments in the Aquarius simulations are very wide, of the order of 0.5 – 1 Mpc (see figure below). The VPOS has a thickness of only about 50 kpc. There is no way it can have been formed out of a much bigger filament. The filaments are in fact larger than the halo of the main galaxy (virial radius 200-250 kpc). Vera-Cirro et al (2011) write:

“[…] when the surrounding filament is sufficiently wide, i.e. of comparable or larger cross-section than the virial radius of the halo, the infalling particles will appear to be more isotropically distributed on the sky […]. We have argued […] that [this] case is characteristic of the late stages of mass assembly in 10^12 Msun objects.”

This statement is contrary to the Lovell et al. (2011) one. In the simulations Vera-Cirro et al. (2011) discuss, the filaments are much wider than the central halo for most of the time of the simulation (from about 5 Gyr on). Thus, for about the past 9 Gyr, the accretion must have been more isotropically. Interestingly, the Vera-Cirro et al. (2011) work is based on the Aquarius simulations, the same set of cosmological simulations as the Lovell et al. (2011) paper. And it has been accepted for publication before the Lovell et al. (2011) paper.

Caption: Part of figure 4 of Vera-Ciro et al. (2011). It illustrates the size of a cosmic filament around a Milky Way like halo. The virial radius of the central halo is shown by the white ellipse. The thickness of the VPOS is less than one 10th of the white line at the bottom giving the scale of about 700 kpc.

In addition to this, the orientation of the preferred direction of orbits of subhalos is at odds with the expectations. Lovell et al. (2011) show that there is a slight over-abundance of subhalos orbiting in the same direction as the main halo (and in some cases also in the opposite direction). However, the galaxies forming in the main halos preferentially spin in the same direction as the main halo, so the sub-halo over abundance lies in the same plane as the galactic disc. In the case of the VPOS around the MW, the orientation is perpendicular.

Finally, Lovell et al. (2011) did not test the rather strongly worded statement of their abstract quantitatively. From their figures, it is already obvious that the before mentioned over-abundance of co-orbiting subhalos is small, only a factor of about 2 compared to the isotropic case in the bin closest to the main halo spin. The majority of subhalo orbital directions is distributed more evenly around the main halo.

Caption: The directions of angular momentum vectors, called orbital poles, of sub-halos coming from a cosmological cold-dark matter simulation (upper) and satellite galaxies of the Milky Way (lower). The question we addressed in our recent paper was: how likely is it that a distribution like the observed (lower) one can arise when drawing from the modeled (upper) one.

To allow a fair comparison, we have developed a method to test this claim. It is described in our recent paper “Can filamentary accretion explain the orbital poles of the Milky Way satellites?” (by Marcel S. Pawlowski, Pavel Kroupa, Garry Angus, Klaas S. de Boer, Benoit Famaey and Gerhard Hensler). In it, we determine how likely it is to find sets of angular momenta in model data (e.g. upper plot in the figure above) which are as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles (lower plot in the figure above). We have applied the method to both cosmological simulation data as well as models of galaxy collisions resulting in polar distributions of tidal debris.

The results are clear. They unambiguously disfavor the cold dark matter models.

Caption: A part of Fig. 3 of our paper, illustrating the results of one of our criteria. The plot shows how likely it is that orbital poles derived from models can be at least as concentrated as the observed value of 35.4 degree. The integral below the curves within the shaded region give the probability that randomly drawn orbital poles from the model are as concentrated as is observed. The two cosmological simulations (Aquarius D2 and Via Lactea 1, upper panels) show curves which are very similar to that for an isotropic distribution of satellite galaxies (thin line), it is unlikely that they fall into the shaded area. The lower panel shows the results for tidal debris of a galaxy collision, which is much more concentrated towards the left. In this latter case, it is most likely to draw orbital poles as concentrated as observed.

Using data from high-resolution cosmological simulations of halos that should host Milky-Way-like galaxies, we were able to show that the sub-halo orbits do not naturally produce the observed properties. In contrast, models in which the satellite galaxies are formed as tidal dwarfs from the debris of a galaxy-collision can easily reproduce the observed distribution of orbital poles. The claim that cosmological ΛCDM simulations naturally produce satellite distributions as inferred in the Milky Way has therefore been falsified. At the same time, this shows that the tidal scenario passes the test.

For more details, please read our paper (accepted by MNRAS). It is available as a preprint.

By Pavel Kroupa and Marcel Pawlowski (15.05.2012): “Does filamentary accretion of dark matter sub-halos naturally produce a VPOS-like structure?” on SciLogs. See the overview of topics in The Dark Matter Crisis.


Is the nearest star cluster to the Sun being destroyed?

Data from ESA’s Gaia star mapping satellite have revealed tantalising evidence that the nearest star cluster to the Sun is being disrupted by the gravitational influence of a massive but unseen structure in our galaxy.

If true, this might provide evidence for a suspected population of ‘dark matter sub-halos’. These invisible clouds of particles are thought to be relics from the formation of the Milky Way, and are now spread across the galaxy, making up an invisible substructure that exerts a noticeable gravitational influence on anything that drifts too close.

ESA Research Fellow Tereza Jerabkova and colleagues from ESA and the European Southern Observatory made the discovery while studying the way a nearby star cluster is merging into the general background of stars in our galaxy. This discovery was based on Gaia’s Early third Data Release (EDR3) and data from the second release.

The team chose the Hyades as their target because it is the nearest star cluster to the Sun. It is located just over 153 light years away, and is easily visible to skywatchers in both northern and southern hemispheres as a conspicuous ‘V’ shape of bright stars that marks the head of the bull in the constellation of Taurus. Beyond the easily visible bright stars, telescopes reveal a hundred or so fainter ones contained in a spherical region of space, roughly 60 light years across.

A star cluster will naturally lose stars because as those stars move within the cluster they tug at each other gravitationally. This constant tugging slightly changes the stars’ velocities, moving some to the edges of the cluster. From there, the stars can be swept out by the gravitational pull of the galaxy, forming two long tails.

One tail trails the star cluster, the other pulls out ahead of it. They are known as tidal tails, and have been widely studied in colliding galaxies but no one had ever seen them from a nearby open star cluster, until very recently.

The key to detecting tidal tails is spotting which stars in the sky are moving in a similar way to the star cluster. Gaia makes this easy because it is precisely measuring the distance and movement of more than a billion stars in our galaxy. “These are the two most important quantities that we need to search for tidal tails from star clusters in the Milky Way,” says Tereza.

Previous attempts by other teams had met with only limited success because the researchers had only looked for stars that closely matched the movement of the star cluster. This excluded members that left earlier in its 600–700 million year history and so are now travelling on different orbits.

To understand the range of orbits to look for, Tereza constructed a computer model that would simulate the various perturbations that escaping stars in the cluster might feel during their hundreds of millions of years in space. It was after running this code, and then comparing the simulations to the real data that the true extend of the Hyades tidal tails were revealed. Tereza and colleagues found thousands of former members in the Gaia data. These stars now stretch for thousands of light years across the galaxy in two enormous tidal tails.

But the real surprise was that the trailing tidal tail seemed to be missing stars. This indicates that something much more brutal is taking place than the star cluster gently ‘dissolving’.

650 million years ago until now
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Running the simulations again, Tereza showed that the data could be reproduced if that tail had collided with a cloud of matter containing about 10 million solar masses. “There must have been a close interaction with this really massive clump, and the Hyades just got smashed,” she says.

But what could that clump be? There are no observations of a gas cloud or star cluster that massive nearby. If no visible structure is detected even in future targeted searches, Tereza suggests that object could be a dark matter sub-halo. These are naturally occurring clumps of dark matter that are thought to help shape the galaxy during its formation. This new work shows how Gaia is helping astronomers map out this invisible dark matter framework of the galaxy.

“With Gaia, the way we see the Milky Way has completely changed. And with these discoveries, we will be able to map the Milky Way’s sub-structures much better than ever before,” says Tereza. And having proved the technique with the Hyades, Tereza and colleagues are now extending the work by looking for tidal tails from other, more distant star clusters.


Research Progress on Dark Matter Model Based on Weakly Interacting Massive Particles ☆,

The cosmological model of cold dark matter (CDM) with the dark energy and a scale-invariant adiabatic primordial power spectrum has been considered as the standard cosmological model, i.e. the ΛCDM model. Weakly interacting massive particles (WIMPs) become a prominent candidate for the CDM. Many models extended from the standard model can provide the WIMPs naturally. The standard calculations of relic abundance of dark matter show that the WIMPs are well in agreement with the astronomical observation of ΩDM h 2 ≈0.11. The WIMPs have a relatively large mass, and a relatively slow velocity, so they are easy to aggregate into clusters, and the results of numerical simulations based on the WIMPs agree well with the observational results of cosmic large-scale structures. In the aspect of experiments, the present accelerator or non-accelerator direct/indirect detections are mostly designed for the WIMPs. Thus, a wide attention has been paid to the CDM model based on the WIMPs. However, the ΛCDM model has a serious problem for explaining the small-scale structures under one Mpc. Different dark matter models have been proposed to alleviate the small-scale problem. However, so far there is no strong evidence enough to exclude the CDM model. We plan to introduce the research progress of the dark matter model based on the WIMPs, such as the WIMPs miracle, numerical simulation, small-scale problem, and the direct/indirect detection, to analyze the criterion for discriminating the “cold”, “hot”, and “warm” dark matter, and present the future prospects for the study in this field.