An N-body/hydrodynamical simulation study of the merging cluster El Gordo: A compelling case for self-interacting dark matter?

Abstract

We used a large set N-body/hydrodynamical simulations to study the physical properties of the merging cluster El Gordo. We find that the observed X-ray structures, along with other data, can be matched fairly well by simulations with collision velocities 2000 km s−1 ≲ V ≲ 2500 km s−1 and impact parameters 600 kpc ≲ P ≲ 800 kpc. The mass of the primary is constrained to be between ∼1015 M⊙ and ∼1.6 × 1015 M⊙, in accordance with recent lensing-based mass measurements. Moreover, a returning, post-apocenter, scenario is not supported by our head-on simulations. We also considered merger models that incorporate dark matter self-interactions. The simulation results show that the observed spatial offsets between the different mass components are well reproduced in self-interacting dark matter models with an elastic cross-section in the range σDM/mX ∼ 4 − 5 cm2 gr−1. In addition, the mean relative line-of-sight radial velocity between the two brightest cluster galaxies is found to be on the order of several hundred km s−1. We argue that these findings provide an unambiguous signature of a dark matter behavior that exhibits collisional properties in a very energetic high-redshift cluster collision. The range of allowed values we find for σDM/mX is, however, inconsistent with present upper limits. To resolve this tension, we suggest the possibility that the self-interacting dark matter model used here be considered as only a low-order approximation, and that the underlying physical processes that describe the interaction of dark matter in major cluster mergers are more complex than can be adequately represented by the commonly assumed approach based on the scattering of dark matter particles.