A Hydrodynamical Solution for the "Twin-Tailed" Colliding Galaxy Cluster "El Gordo"

TL;DR

This paper presents a hydrodynamical model of the galaxy cluster 'El Gordo', explaining its distinctive X-ray and SZ features as a high-speed collision of two massive clusters, constrained by multi-wavelength data.

Contribution

The study introduces a detailed N-body/hydrodynamic simulation that accurately reproduces El Gordo's morphology and dynamics, providing insights into its collision parameters and gas behavior.

Findings

Collision occurred about 480 million years ago.

Impact parameter estimated at ~300 kpc.

Initial infall velocity around 2250 km/sec.

Abstract

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 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, Sunyaev-Zel'dovich (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/sec when separated by the sum of the two virial radii assuming an initial total mass of 2.15x10^(15) Msun 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 massive component plunges through the main cluster. The challenge for LCDM 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 the new SZ based surveys.