No Need for Dark Matter in Galaxies?

The best evidence for mass discrepancies in galaxies comes from rotation curves of neutral hydrogen. However, in the case of our own Galaxy, the rotation curve cannot be traced beyond ∼ 20 kpc. It is the kinematics of stellar tracers of the distant halo – namely the bound dwarf galaxy satellites and distant globular clusters – that provide the mass estimates. The most recent analysis 1 found the total dynamical mass of the Galaxy is ∼ 2 × 10M . By contrast, the mass of the stellar disk is ∼ 6× 10M and of the stellar bulge and spheroid is ∼ 3 × 10M . There are only two alternatives. The first is that the Galaxy is largely composed of dark matter. This is matter whose existence is inferred solely from its gravitational effects. The second is that Newtonian gravity is incorrect and needs modification. There is no hard evidence that conventional theories of gravity are correct on scales greater than at most a parsec. But, to many astronomers, the second option is unattractive, as the hard-won understanding of large-scale structure formation would then be jeopardised . If dark matter exists, what is it? Microlensing surveys 3 and Hubble Space Telescope pencil-beam searches 4 strongly constrain the mass in dim stars and brown dwarfs. Suppose we rashly assume that all the lenses towards the Magellanic Clouds lie in the Galactic halo , then no more than ∼ 9 × 10M of the mass within 50 kpc lies in such objects. Yet, the dynamically inferred mass is roughly five times greater than this. So, there is firm evidence that faint or failed stars do not make up most of the Galactic halo. In contrast, non-baryonic particle dark matter is an essential component of theories of structure formation. First, the total dynamical mass density in the Universe