The constant-density region of the dark haloes of spiral galaxies

We determine a crucial feature of the dark halo density distribution from t he fact that the luminous matter dominates the gravitational potential at a bout one disc scalelength R-d, but at the optical edge R(opt)similar or equ al to 3R(d) the dark matter has already become the main component of the ga laxy density. From the kinematics of 137 spirals we find that the dark matt er halo density profiles are self-similar at least out to R-opt and show co re radii much larger than the corresponding disc scalelengths. The luminous regions of spirals consist of stellar discs embedded in dark haloes with r oughly constant density. This invariant dark matter profile is very difficu lt to reconcile with the fundamental properties of the density distribution of cold dark matter haloes. With respect to previous work, the present evi dence is obtained by means of a robust method and for a large and complete sample of normal spirals.