Missing Milky Way Dark Matter


Although dark matter is inherently difficult to observe, an understanding of its properties (even if not its nature) allows astronomers to predict where its effects should be felt. The current understanding is that dark matter helped form the first galaxies by providing gravitational scaffolding in the early universe. These galaxies were small and collapsed to form the larger galaxies we see today. As galaxies grew large enough to shred incoming satellites and their dark matter, much of the dark matter should have been deposited in a flat structure in spiral galaxies which would allow such galaxies to form dark components similar to the disk and halo. However, a new study aimed at detecting the Milky Way’s dark disk have come up empty.

The study concentrated on detecting the dark matter by studying the luminous matter embedded in it in much the same way dark matter was originally discovered. By studying the kinematics of the matter, it would allow astronomers to determine the overall mass present that would dictate the movement. That observed mass could then be compared to the amount of mass predicted of both baryonic matter as well as the dark matter component.

The team, led by C. Moni Bidin used ~300 red giant stars in the Milky Way’s thick disk to map the mass distribution of the region. To eliminate any contamination from the thin disc component, the team limited their selections to stars over 2 kiloparsecs from the galactic midplane and velocities characteristic of such stars to avoid contamination from halo stars. Once stars were selected, the team analyzed the overall velocity of the stars as a function of distance from the galactic center which would give an understanding of the mass interior to their orbits.

Using estimations on the mass from the visible stars and the interstellar medium, the team compared this visible mass to the solution for mass from the observations of the kinematics to search for a discrepancy indicative of dark matter. When the comparison was made, the team discovered that, “[t]he agreement between the visible mass and our dynamical solution is striking, and there is no need to invoke any dark component.”

While this finding doesn’t rule out the presence of dark matter, it does place constraints on it distribution and, if confirmed in other galaxies, may challenge the understanding of how dark matter serves to form galaxies. If dark matter is still present, this study has demonstrated that it is more diffuse than previously recognized or perhaps the disc component is flatter than previously expected and limited to the thin disc. Further observations and modeling will undoubtedly be necessary.

Yet while the research may show a lack of our understanding of dark matter, the team also notes that it is even more devastating for dark matter’s largest rival. While dark matter may yet hide within the error bars in this study, the findings directly contradict the predictions of Modified Newtonian Dynamics (MOND). This hypothesis predicts the apparent gain of mass due to a scaling effect on gravity itself and would have required that the supposed mass at the scales observed be 60% higher than indicated by this study.

23 Replies to “Missing Milky Way Dark Matter”

  1. There’s a typographical error on the very last phrase: “by 60% higher” should be “be 60% higher”.

  2. @Daniel: Thanks for that. I think I had “higher BY 60%” and “BE 60% higher” in my head at the same time and they got switched. Dyslexia does that.

  3. this study has demonstrated that it is more diffuse than previously recognized or

    Stars above! That is the 4th putatively related observation/prediction/article within the last week or so touching on sterile neutrinos:

    1) MiniBoone may have seen sterile neutrinos.

    2) Sterile neutrinos may predict matter/antimatter symmetry by CP violation (see link)

    3) Sterile neutrinos are suggested to be “warm” (neither cold nor hot) dark matter candidates and predict dwarf galaxy diffusive DM.

    4) … and now the Milky Way diffusive DM disk?

    I hear it also predicts how socks go missing in wash machines.

    (No, really, I’m cautiously optimistic, if one hypothesis is that predictive.)

  4. @Capper: Even if it doesn’t work for the disc, it still holds up remarkably well for the halo, galactic clusters, and many other cases. It’s more of a case of theories about its fine structure being flawed, but the overall picture still working.

  5. It’s more of a case of theories about its fine structure being flawed,

    And I believe we knew that, since (C)DM comes up seriously flawed precisely for galaxies in models, a physically realistic one is AFAIU not seen as of yet. They break down in the galaxy core, if nothing else. [Disclaimer: this is how I read one review on this, so it’s really armchair ‘understanding’ at best.]

    @ TLVL:

    Doesn’t anyone consider STVG, or is that lumped together with MOND?

    In this case I believe it is, it is effectively a realization of a hypothetical model-less MOND.

    Here is Starts With A Bang take:

    “In summary, MOND explains galactic rotation curves better than dark matter does, and pretty much nothing else. Dark matter explains the cosmic microwave background, large scale structure, galaxy clusters, gravitational lensing, the Bullet cluster and the cluster Abell 520, and also is consistent with a host of cosmological observations (e.g., nucleosynthesis, supernovae data, cluster estimates of the matter density) that MOND isn’t.”

    And as far as rejecting MOND goes, I understand from the same astrophysicist that it is really rejected by direct observation long since, but is kept around with artificial hope as far as galaxies goes:

    “This is the nail-in-the-coffin of modifying gravity. Why? Because you can’t have gravity where you don’t have mass. That doesn’t make sense, but that doesn’t stop people from making theories about it.

    These theories are called non-local theories of gravity, and you would need a different theory of gravity for every different configuration of colliding galaxy clusters.” [My bold.]

    Obviously you can’t have different theories in different DM situations, so I think MOND proponents have “settled” for MOND for DM in galaxies and “something else” elsewhere.

    Yeah, I don’t get that either.

    Nevertheless, even that “out” seems to close down as we write, not surprisingly considering the by now unphysical setting of the whole idea as described by SWAB.

  6. Also, whether or not MOND goes RIP from pursuing this particular area of research, I note that at this stage it is ironically back to being the ad hoc hypothesis for galaxy rotation curves it started out as. (Even if there now are more basic models like STVG that could make the same predictions.)

    That can’t be good.

  7. While you’re throwing out MOND, you may as well sh*t-can Dark Matter and Dark Energy, as they are both completely moronic concepts and totally erroneous postulations.

  8. There has been ample solid evidence from many independent calculations for some years now that disk galaxy rotation curves are fully consistent with Newtonian gravity and dynamics without any need for dark matter or dark energy. I will be glad to provide details to anyone interested.
    Please see these links:

    The following review may also be useful:
    although the reviewer became aware of much sensible work when the review was already almost at proof stage, and could not include it in the review or modify the review at places to take account of it.

    To be very frank & forthright about this issue, people have sort of jumped on the dark matter bandwagon w/o really examining if it was necessary, at least on single galaxy scale, and kept doing calculations and publishing and simulating and publishing for decades. Now the use of dark matter has accumulated a lot of “prestigious” “scientific” weight due to all this misconceived academic activity. I happened to go for a meeting discussing these & other related matters (IAU Symposium 254 on Galaxy Disk in Cosmological Context) and presented the paper given in the 2nd link above. What horrified me was that the people presenting solid observational results were sheepishly apologetic whenever their observations didn’t agree with results from large simulations!

    Are we going back to the dark middle ages, when people refused to look at anything against Aristotle’s theories? In solar physics, for example, sunspots were (co-)discovered by (if I remember right) Scheiner (a junior priest, I think), and when he told his superior about it, that worthy (the superior, I mean) immediately told Scheiner to go & examine his eyes or use better glasses or sth like that, since Aristotle had said / written that Sun is perfect & cannot have blemishes!

  9. Lars, The evidence for dark matter is huge, and same goes for dark energy. The excess mass is required to explain the velocity function with respect to galactic radius v(r) ~ constant, which is distinct from the Kepler law v(r) ~ 1/sqrt{r}. So there is plenty of evidence there must be some sort of physics at work here. Either there is some form of mass energy which accounts for this. Motion of a body inside a uniform distribution of matter has v(r) = const, which lead to the idea of DM halos. With respect to Dilip’s papers, the main question comes with whether there is sufficient luminous matter in the disk of a galaxy to get the result you claim. The problem is empirical, where that is not apparent. Your analysis appears to work fine, at least upon first look, but the matter density is probably only appropriate with the missing matter — aka dark matter.

    The MOND concept states that the force law of gravity F = GMm/r^2 must be modified for very small accelerations with F = ma. The idea is reasonable for phenomenology or curve fitting data. It is ridiculous to take it as some possible foundations of physics.


  10. Dark Matter: Pictures or it didn’t happen.

    The longer there is no observational evidence of its existence, the more we should look into what we got wrong about our basic understanding of gravity.

  11. There is plenty of evidence. Check out the Bullet Galaxy cluster, where DM is deduced by Einstein lensing.


  12. Several recent comments were held up in the Pending queue, for longer than normal; sorry about that.

    I was under the impression that Abell 520 couldn’t be explained until DM models were modified to make the particles weakly interacting.

    Anyway, I think I will sit on the fence until some conclusive evidence is found (or not found)

  14. I illustrate some aspects of weakly interacting matter particles (WIMPS) as DM in the blog entry on the Fermi Telescope data:


    The WIMP element is not terribly important with Abell 520, except for the fact that the DM in the galactic coalescence does not coalesce. The critical element there is that Einstein lensing, such as the Bullet or Abell 520 and other cases, measures the existence of a large amount of mass that is not visible in the EM spectrum. This is not luminous matter in the standard sense. The C in the CDM refers to cold, where an application of Boltzmann statistics for stable distributions of halos indicates the stuff is cold. If the particle motions were larger, or equivalently the matter was hotter, then halos would in a sense evaporate.


  15. Torbjorn Larsson OM: I guess I missed your comment MiniBoone may have seen sterile neutrinos. This is an interesting result, for it indicates something funny with CP violations, and some have suggested CPT violations — which would be even weirder. The FERMI results on possible DM decays also suggest a tauon decay channel, which seems to raise a question on whether there is some corresponding B-physics in QCD and CP violations. Kobayashi & Maskawa won the Nobel prize for CP violating physics in 2008, and this may have some implications here as well.

    Of course these MiniBoone and LSND data involves weak interaction physics, not gravity. Yet is may be that gravitation is an emergent field on a boundary which has a dual structure to conformal QFT. Of course we have yet to know for certain, but I think that from 10^{-17}cm to 10^{-33} cm there is a renormalization group flow of quantum fields, so gravity may play a type of role at the unification energy where the Higgs condensate produces particles. This renormalization flow may take a low energy field to the Planck scale at high energy with a continuous flow to quantum gravity, where nonlocality of field amplitudes likely holds. So there may be a loss of locality at the 10^{-17}cm scale which perturbs physics in a way which is unexpected.


  16. 1. Observation evidence for dark matter
    1) Galactic rotation curves
    2) Galaxy clusters and gravitational lensing
    3) WMAP
    4) Bullet cluster
    5) Structure formation

    2. In the Galaxy, No evidence for dark matter
    1) At the Earth : non-observation(Xenon100, CDMS-II…)

    2) At the solar system : non-observation

    3) At the center of galaxies : no evidence
    Greedy Supermassive Black Holes Dislike Dark Matter
    Astronomers Find Black Holes Do Not Absorb Dark Matter

    4) At the galactic plane : no evidence
    “No evidence for a dark matter disk within 4 kpc from the galactic plane”

    5) At the galactic halo : no evidence
    Globular Cluster problem
    “Evidence Against Dark Matter Halos Surrounding the Globular Clusters MGC1 and NGC 2419”

    Centripetal force effect in the galaxy from dark matter halo out of the galaxy!

    [Simulation vodeo]

    In the simulation,
    When negative mass’s shells exist at outline, collision time is shorter compared to existing only with positive mass’s shells. This implies that there exist additional effects of centripetal force by the dark matter halo at the galaxy’s outside in case of the galaxy having the axial symmetry(rotating galaxy).

  17. The dark matter issue does come from the anomalous motion of stars in the galaxy. It is not hard to show with the Poisson equation &_i&^i? = 4?G rho, &_i = 3 dimensional directional derivative, that for a region of constant density rho an integration over a volume is constant on the right hand side. One the left hand side one can use Stokes rule to get

    (&_iU)*A = 4?G rho V

    for A the area bounding the volume V, and U is the potential function of gravity. For A = 4?r^2 and the volume V = 4?r^3/3 you get the force F = &^iU ~ constant*r. So this is a spring force and gives dynamics similar to what we see with the motion of stars in a galaxy. This is different from the motion one expects of a central force of gravity with F = GMm/r^2. This is one reason that dark matter was initially inferred.

    The density of dark matter in the galaxy is about 10^{-24}kg/m^3. A spherical volume that encloses the solar system some 5e^{9}km out or 5e^{11}m is then a volume of about 5e^{34}m^3. Hence the mass contained in the solar system in the form of dark matter is only about 10^{10}kg. This is not a whole lot when compared to the mass of planets and certainly the sun. So this is why it does not make a significant contribution to the dynamics of the solar system.


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