Evidence of self-interacting cold dark matter from galactic to galaxy cluster scales

Within the framework of the cold dark matter (CDM) cosmogony, a central cusp in the density profiles of virialized dark haloes is predicted. This prediction disagrees with the soft inner halo mass distribution inferred from observations of dwarf and low surface brightness galaxies, and some clusters of galaxies. By analysing data for some of these objects, we find that the halo central density is nearly independent of the mass from galactic to galaxy cluster scales, with an average value of around 0.02 M. pc(-3). We show that soft cores can be produced in the CDM haloes by introducing a lower cut-off in the power spectra of fluctuations and assuming high orbital thermal energies during halo formation. However, the scale invariance of the halo central density is not reproduced in these cases. The introduction of self-interaction in the CDM particles offers the most attractive alternative to the core problem. We propose gravothermal expansion as a possible mechanism to produce soft cores in the CDM haloes with self-interacting particles. A global thermodynamical equilibrium can explain the central density scale invariance. We find a minimum cross-section capable of establishing isothermal cores in agreement with the observed shallow cores. If sigma is the cross-section, m(x) the mass of the dark matter particle and v the halo velocity dispersion, then sigma m(x) approximate to 4 x 10(-25)(100 km s(-1) v(-1)) cm(2) GeV-1.