2015ApJ...800...37M

The distinctive cometary X-ray morphology of the recently discovered massive galaxy cluster "El Gordo" (ACT-CT J0102-4915; z = 0.87) indicates that an unusually high-speed collision is ongoing between two massive galaxy clusters. A bright X-ray "bullet" leads a "twin-tailed" wake, with the Sunyaev-Zel'dovich (SZ) centroid at the end of the northern tail. We show how the physical properties of this system can be determined using our FLASH-based, N-body/hydrodynamic model, constrained by detailed X-ray, SZ, and Hubble lensing and dynamical data. The X-ray morphology and the location of the two dark matter components and the SZ peak are accurately described by a simple binary collision viewed about 480 million years after the first core passage. We derive an impact parameter of ≃300 kpc, and a relative initial infall velocity of ≃2250 km/s when separated by the sum of the two virial radii assuming an initial total mass of 2.15x10¹⁵ M_☉ and a mass ratio of 1.9. Our model demonstrates that tidally stretched gas accounts for the northern X-ray tail along the collision axis between the mass peaks, and that the southern tail lies off axis, comprising compressed and shock heated gas generated as the less massive component plunges through the main cluster. The challenge for ΛCDM will be to find out if this physically extreme event can be plausibly accommodated when combined with the similarly massive, high-infall-velocity case of the Bullet cluster and other such cases being uncovered in new SZ based surveys.