Dark Matter on Small Scales; Telescopes on Large Scales

This article reviews recent progress in observational determination of the properties of dark matter on small astrophysical scales, and progress towards the European Extremely Large Telescope. Current results suggest some surprises: the central DM density profile is typically cored, not cusped, with scale sizes never less than a few hundred pc; the central densities are typically 10 − 20GeV/cc; no galaxy is found with a dark mass halo less massive than ∼ 5.10 7 M ⊙. We are discovering many more dSphs, which we are analysing to test the generality of these results. The European Extremely Large Telescope Design Study is going forward well, supported by an outstanding scientific case, and founded on detailed industrial studies of the technological requirements. 1 Dark Matter on small scales Are dSph haloes cusped or cored? Debate continues to rage about whether the cusped haloes always created in CDM simulations are in conflict with observations of rotation curves of Low Suface Brightness galaxies. The gas-free nature of dSphs makes them kinematically clean systems in which to test theoretical predictions. Stellar velocities may also be used to place constraints on the steepness of any possible central cusp, whether due to a black hole, the intrinsic physical properties of the CDM 7 , or possibly even CDM as modified by a central black hole 6. In the last few years several groups have obtained large kinematic data sets, determining the line-of-sight velocity dispersion across teh face of many of the local dSph galaxies. These data, together with surface brightness profiles defining the scale length on which the light is distributed, allow standard stellar velocity pressure vs gravitational pressure gradient analses through application of the Collisionless Boltzmann equation, or more commonly the moment equations known as Jeans' equations. As a general result, in all cases with sufficient data we rule out (King model) mass-follows-light models. King models are not an adequate description of these galaxies, all of which have high mass-to-light ratios (in solar visual band units), and extended dark matter halos. Even in the inner regions mass does not follow light, while including outer data commonly we find a most likely global mass to light ratio which is very high, being for Draco ∼ 440, 200 times greater than that for stars with a normal mass function (Fig. 1). The Draco halo models favoured by the data contain significant amounts of mass at large radii, leading …