Halo properties in cosmological simulations of self-interacting cold dark matter

We present a comparison of halo properties in cosmological simulations of c ollisionless cold dark matter (CDM) and self-interacting dark matter (SIDM) for a range of dark matter cross sections. We find, in agreement with vari ous authors, that CDM yields cuspy halos that are too centrally concentrate d as compared to observations. Conversely, SIDM simulations using a Monte C arlo N-body technique produce halos with significantly reduced central dens ities and flatter cores. We introduce a concentration parameter based on en closed mass that we expect will be straightforward to determine observation ally and present predictions for SIDM and CDM. SIDM also produces more sphe rical halos than CDM, providing possibly the strongest observational constr aint on SIDM. In our simulations with SIDM cross sections sigma DM approxim ate to 10(-23) to 10(-25) cm(2) GeV-1, collisions reduce the central densit y in galaxy halos, with larger cross sections producing lower densities. Si mulations in the upper end of this range best match current observations of galaxy properties but are disfavored by cluster simulations of Yoshida and colleagues. This problem would be alleviated in models in which the cross section is velocity dependent.