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 cus p in the density profiles of virialized dark haloes is predicted. This pred iction disagrees with the soft inner halo mass distribution inferred from o bservations 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 ga laxy cluster scales, with an average value of around 0.02 M. pc(-3). We sho w 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 ther mal energies during halo formation. However, the scale invariance of the ha lo central density is not reproduced in these cases. The introduction of se lf-interaction in the CDM particles offers the most attractive alternative to the core problem. We propose gravothermal expansion as a possible mechan ism to produce soft cores in the CDM haloes with self-interacting particles . A global thermodynamical equilibrium can explain the central density scal e invariance. We find a minimum cross-section capable of establishing isoth ermal cores in agreement with the observed shallow cores. If sigma is the c ross-section, m(x) the mass of the dark matter particle and v the halo velo city dispersion, then sigma m(x) approximate to 4 x 10(-25)(100 km s(-1) v( -1)) cm(2) GeV-1.