No dark matter in the solar neighbourhood

A recent study of the motion of about 400 stars in the solar neighbourhood suggest that our region of the Milky Way is devoid of dark matter.  As discussed in the 6 April post, while we cannot yet detect dark matter, we believe it must exist because of its gravitational effect.  In the Milky Way (and most other spiral galaxies) the inner part of the Galaxy rotates like a solid body, which means that the velocity increases with distance from the galaxy centre, but in the outer parts of the galaxy the rotation curve is flat, so the velocity remains constant with increasing distance from the galaxy centre. This means that stars in the outer parts of spiral galaxies must be feeling the pull of more gravity that stars in the inner part of the galaxy. And yet from observations we know there are less stars and less gas in the outer parts of spiral galaxies – hence flat the rotation curve of the Milky Way and other spiral galaxies suggests the existence of a dark matter halo surrounding and permeating them.

Rotation curves of spiral galaxies. (Credit: © 2012 Pearson Education, Inc., publishing as Addison Wesley.)

Using results of a kinematic study of 412 red giant stars in the thick disk of the Milky Way within 4.5 kpc from the galactic plane, Christian Moni Bidin of the Universidad de Concepción in Chile and collaborators have been able to calculate the total mass of material in a region of the solar neighbourhood four times larger in volume than previously considered. By studying the motion of the stars, especially those moving away from the galactic plane, the team can then calculate the mass within that volume since the motion of the stars are effected by the mutual gravitational interaction between all material present – stars, gas, dust and dark matter. To their surprise, the mass of the observable matter in the volume exactly matched the dynamical mass required, indicating that there is no dark matter in this volume of space. They go on to conclude that Earth-based experiments that are trying to detect dark matter particles will almost certainly fail.

The search for dark matter in the solar neighbourhood of the Milky Way. (Credit: ESO/L. Calçada)

However, other astronomers are skeptical.  As Moni Bidin et al. point out, the analysis assumes that the Milky Way’s dark matter halo is spherical.  The results could be explained by a very elongated dark matter halo surrounding the Milky Way with little dark matter in the vicinity of the Sun. Furthermore, Moni Bidin et al. assume a constant average orbital speed of stars in the Milky Way (i.e. the average orbital speed does not depend on distance from the galactic center), which influences stellar motions in the vicinity of the Sun. As Avi Loeb of Harvard University points out this assumption of contact average stellar orbital speed actually requires dark matter, unless gravity is modified from the Newtonian inverse-square law.  The theory of modified Newtonian dynamics (MOND) can be used to explain the rotation curves of spiral galaxies by having an extra gravitational “kick” at the edge of galaxies.  However, MOND doesn’t work so well for elliptical galaxies, galaxy groups and clusters, and larger-scale structure. Perhaps less-massive warm dark matter particles (rather than more massive cold dark matter particles) could be help salvage the problem by having the dark matter distributed on  wider scales in the inner parts of the Galaxy than on the outer parts.   However, in terms of cosmology, the less massive warm dark particles form galaxies too quickly to explain observations of the early Universe.

For mode detail, see

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2 Responses to No dark matter in the solar neighbourhood

  1. Chris Flynn says:

    This is a really difficult measurement — the authors are attempting something not tried before, and it’s not clear if the method being applied really works. The motions of the stars which are being used to detect the presence of dark matter are sensitive to a great deal more besides — and one has to get everything just right.

    One way to test the method would be to apply it to the stars seen in simulations of disk galaxies — exactly how much dark matter is included and we can test to see if the method recovers the correct answer. The authors of the article are planning to do that in the future, but I’ve tried this myself, and the initial results were worrisome — the method seems to underestimate the amount of dark matter present, or even miss it completely. Why this should be the case is unclear.

    Here are some more links to comments on this result:

    It’s worth pointing out that there has been one other attempt to measure the amount of dark matter due to the Milky Way halo near the Sun — by Garbari, Read and Lake, 2011.

    These authors apply related (but much simpler) methods to measure the amount of ordinary matter (in gas and stars) and the amount of dark halo matter (of unknown composition) near the Sun, by analysis of how nearby stars are moving. Garbari and co-authors do find dark matter near the Sun, and pretty much in the expected amount… throwing up a challenge as to why the newer study seems to miss it entirely.

    Chris Flynn
    Swinburne Astronomy Online

  2. Pingback: On the Dearth of Dark Matter in the Solar Neighbourhood « In the Dark

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