Missing Milky Way Dark Matter

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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.

Jon Voisey

Jon is a science educator currently living in Missouri. He is a high school teacher and does outreach with the St. Louis Astronomical society as well as presenting talks on science and related topics at regional conventions. He graduated from the University of Kansas with his BS in Astronomy in 2008 and has maintained the Angry Astronomer blog since 2006. For more of his work, you can find his website here.

View Comments

  • @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.

  • 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.)

  • @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.

  • 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.

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