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If dark matter has a large self-interaction scattering cross section, theninteractions among dark-matter particles will drive galaxy and cluster halos tobecome spherical in their centers. Work in the past has used this effect torule out velocity-independent, elastic cross sections larger than sigma/m ~0.02 cm^2/g based on comparisons to the shapes of galaxy cluster lensingpotentials and X-ray isophotes. In this paper, we use cosmological simulationsto show that these constraints were off by more than an order of magnitudebecause (a) they did not properly account for the fact that the observedellipticity gets contributions from the triaxial mass distribution outside thecore set by scatterings, (b) the scatter in axis ratios is large and (c) thecore region retains more of its triaxial nature than estimated before.Including these effects properly shows that the same observations now allowdark matter self-interaction cross sections at least as large as sigma/m = 0.1cm^2/g. We show that constraints on self-interacting dark matter fromstrong-lensing clusters are likely to improve significantly in the near future,but possibly more via central densities and core sizes than halo shapes.

Cosmological Simulations with Self-Interacting Dark Matter II: Halo Shapes vs. Observations