The distribution of dark matter in galaxies: Constant-density dark halos envelop the stellar disks

In this paper we review the main and the most recent evidence for the presence of a core radius in the distribution of the dark matter around spiral galaxies. Their rotation curves, coadded according to the galaxy luminosity, conform to an Universal profile which can be represented as the sum of an exponential thin disk term plus a spherical halo term with a flat density core. From dwarfs to giants, these halos feature a constant density region of size ro and core density po related by po = 4.5 x 10(-2) (r(o)/kpc)(-2/3)M(circle dot)pc(-3). At the highest masses po decreases exponentially, with ro revealing a lack of objects with disk masses > 10(11)M(circle dot) and central densities > 1.5 x 10-2(ro/kpc)(-3)M(circle dot)pc(-3), which implies a maximum mass of,: 2 x 10(12)M(circle dot) for halos hosting spirals. The fine structure of dark matter halos is obtained from the kinematics of a number of suitable low-luminosity disk galaxies. The inferred halo circular velocity increases linearly with radius out to the edge of the stellar disk, implying a constant dark halo density over the entire disk region. The structural properties of halos around normal spirals are similar to those around dwarf and low surface brightness galaxies; nevertheless they provide far more substantial evidence of the discrepancy between the mass distributions predicted in the Cold Dark Matter scenario and those actually detected around galaxies.