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