We use high‐quality optical rotation curves of nine low‐luminosity disc galaxies to obtain the velocity profiles of the surrounding dark matter haloes. We find that they increase linearly with radius at least out to the edge of the stellar disc, implying that, over the entire stellar region, the density of the dark halo is about constant. The properties of the mass structure of these haloes are similar to those found for a number of dwarf and low surface brightness galaxies, but provide a more substantial evidence of the discrepancy between the halo mass distribution predicted in the cold dark matter scenario and those actually detected around galaxies. We find that the density law proposed by Burkert reproduces the halo rotation curves, with halo central densities (ρ0∼1–4×10−24 g cm−3) and core radii (r0∼5–15 kpc) scaling as ρ0∝r0−2/3.

We show that the small scatter around the fundamental plane (FP) of massive elliptical galaxies can be used to derive important properties about their dark and luminous matter. The central velocity dispersion σ0 (appearing in the fundamental plane) is linked to photometric, dynamical and geometrical properties of (luminous and dark) matter. We find that, inside the effective radius Re, the matter traced by the light must largely dominate over the dark matter (DM) in order to keep the ellipticals close enough to the FP. This recalls analogous findings for spiral galaxies. In particular we also find that cuspy DM distributions, as predicted by numerical simulations in ΛCDM cosmology, are unable to explain the very existence of the FP; in fact, according to this theory, the structural properties of dark and luminous matter are so interwoven that a curved surface is predicted in the log‐space (σ0, Re, L), rather than a plane. To agree with the FP implies that CDM haloes have values in the range of 5–9 for the concentration parameter c (i.e. values significantly lower than the current predictions). Assuming a more heuristic approach and allowing for cored DM haloes, we find that the small intrinsic scatter of the FP yields: (i) an average value of about 0.3 for the dark‐to‐light‐traced mass ratio, (ii) a mass‐to‐light of the matter traced by the light increasing with spheroidal luminosity: Msph/Lr∝L0.2r (Gunn‐r band), with a value of 5.3 at L*r≡ 2.7 × 1010 Lr⊙.

Abstract. In this paper we present a test case for the existence of a core in the density distribution of dark halos around galaxies. DDO 47 has a rotation curve that increases linearly from the first data point, at 300 pc, up to the last one, at 5 kpc. This profile implies the presence of a (dark) halo with an (approximately) constant density over the region mapped by data. This evidences the inability of standard Λ Cold Dark Matter scenario to account for the dark matter distribution around galaxies, and points toward the existence of an intriguing halo scale-length of homogeneity. This work adds up to the results of Blais-Ouellette et al. (2002) and Bottema (2002) in suggesting that at galactic scales CDM theory should incorporate, as an intrinsic property, a "density core" feature.

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 r0 and core density ρ0 related by ρ0 = 4.5×10 −2 (r0/kpc) −2/3 M⊙pc −3 . At the highest masses ρ0 decreases exponentially, with r0 revealing a lack of objects with disk masses > 10 11 M⊙ and central densities > 1.5 × 10 −2 (r0/kpc) −3 M⊙pc −3 , which implies a maximum mass of ≈ 2 × 10 12 M⊙ 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.

Abstract. 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 r0 and core density ρ0 related by ρ0 = 4.5 × 10 −2 (r0/kpc) −2/3 M⊙pc −3 . At the highest masses ρ0 decreases exponentially with r0, revealing a lack of objects with disk masses > 10 11 M⊙ and central densities > 1.5 × 10 −2 (r0/kpc) −3 M⊙pc −3 implying a maximum mass of ≈ 2 × 10 12 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.