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.
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.
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
- Serious Blow to Dark Matter Theories? ESO news
- Dark Matter Is Missing in Sun’s Neighborhood?, National Geographic News, Ker Than
- If Not Dark Matter, Then What? Space.com, by Natalie Wolchover
- Kinematical and chemical vertical structure of the Galactic thick disk II. A lack of dark matter in the solar neighborhood, Moni-Bidin et al. (2012), ApJ, in press