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

TL;DR

This study uses detailed simulations of the merging galaxy cluster El Gordo to explore dark matter properties, finding that self-interacting dark matter models can explain observed features but conflict with existing upper limits, suggesting more complex physics.

Contribution

It provides the first detailed simulation-based analysis of El Gordo incorporating dark matter self-interactions, linking observed offsets to specific cross-section ranges.

Findings

Simulations match observed X-ray structures with specific collision velocities and impact parameters.

Self-interacting dark matter models reproduce observed spatial offsets between mass components.

The inferred dark matter cross-section range conflicts with current upper limits, indicating the need for more complex models.

Abstract

We use 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 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 accordance with recent lensing-based mass measurements. Moreover, a returning, post-apocenter, scenario is not supported by our head-on simulations. We 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 \sigma_DM/m_X ~ 4 -5 cm^2/gr. 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. 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 sigma_DM/m_X 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.