The Intriguing Distribution of Dark Matter in Galaxies
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We review the most recent evidence for the amazing properties of the density 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 ρ 0 related by ρ 0 = 4.5 × 10-2(r 0/kpc)-2/3M⊙pc-3. At the highest masses ρ 0 decreases exponentially with r 0, revealing a lack of objects with disk masses > 1011 M⊙ and central densities > 1.5 × 10-2 (r 0/kpc)-3 M⊙pc-3 implying a maximum mass of ≈ 2 × 1012 M⊙ for a dark halo hosting a stellar disk. The fine structure of dark matter halos is obtained from the kinematics of a number of suitable low-luminosity disk galaxies. The 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 properties of halos around normal spirals provide substantial evidence of a discrepancy between the mass distributions predicted in the Cold Dark Matter scenario and those actually detected around galaxies.
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