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 r_0 and core density rho_0 related by rho_0= 4.5 10^{-2} (r_0/kpc)^{-2/3}M_sol pc^{-3}. At the highest masses rho_0 decreases exponentially, with r_0 revealing a lack of objects with disk masses > 10^{11}M_sol and central densities > 1.5 10^{-2}(r_0/kpc)^{-3} M_sol pc^{-3}, which implies a maximum mass of ~2 10^{12} M_sol 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.