The cored distribution of dark matter in spiral galaxies

We present, for five spiral galaxies, H i data which, along with the Ha rotation curves, are used to derive the distribution of dark matter within these objects. A new method for extracting rotation curves from H i data cubes is presented, which takes into account the existence of a warp and minimizes projection effects. The rotation curves obtained are tested by taking them as input in the construction of model data cubes that are compared with the observed ones: the agreement is excellent. Model data cubes built using rotation curves obtained with standard methods, such as the first-moment analysis, however, fail the test. The H i rotation curves agree well with the Ha data, where they coexist. Moreover, the combined Ha+ H i rotation curves are smooth, symmetric and extend to large radii. The rotation curves are decomposed into stellar, gaseous and dark matter contributions, and the inferred density distribution is compared with various mass distributions: dark haloes with a central density core,cold dark matter (CDM) haloes (NFW and Moore profiles), H i scaling and MOND. The observations point to haloes with constant-density cores of size rcore∼ropt and central densities scaling approximately as 0r-2/3core. CDM models (which predict a central cusp in the density profile) are in clear conflict with the data. H i scaling and MOND cannot account for the observed kinematics: we find some counter-examples.