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We use a large set N-body/hydrodynamical simulations to study the physicalproperties of the merging cluster El Gordo. We found that the observed X-raystructures, along with other data, can be fairly matched by simulations withcollision velocities 2,000 kms <= V <= 2,500 kms and impact parameters 600 kpc<= P <= 800 kpc. The mass of the primary is constrained to be between 10^{15}M_sun and ~ 1.6 10^{15} M_sun, in accord with recent lensing-based massmeasurements. Moreover, a returning, post-apocenter, scenario is not supportedby our head-on simulations. We also consider merger models that incorporatedark matter self-interactions. The simulation results show that the observedspatial offsets between the different mass components are well reproduced inself-interacting dark matter models with an elastic cross-section in the range\sigma_DM/m_X ~ 4 -5 cm^2/gr. In addition, the mean relative line-of-sightradial velocity between the two brightest cluster galaxies is found to be ofthe order of several hundreds of km/s. We argue that these findings provide anunambiguous signature of a dark matter behavior that exhibits collisionalproperties in a very energetic high-redshift cluster collision. The range ofallowed values we found for sigma_DM/m_X is however inconsistent with presentupper limits. To resolve this tension we suggest the possibility that theself-interacting dark matter model used here should be considered as only a loworder approximation, and that the underlying physical processes that describethe interaction of dark matter in major cluster mergers are more complex thancan be adequately represented by the commonly assumed approach based onscattering of dark matter particles.

An N-body/hydrodynamical simulation study of the merging cluster El Gordo: a compelling case for SIDM ?