  The observational features of the massive galaxy cluster "El Gordo" (ACT-CL
J0102-4915), such as the X-ray emission, the Sunyaev-Zel'dovich (SZ) effect,
and the surface mass density distribution, indicate that they are caused by an
exceptional ongoing high-speed collision of two galaxy clusters, similar to the
well-known Bullet Cluster. We perform a series of hydrodynamical simulations to
investigate the merging scenario and identify the initial conditions for the
collision in ACT-CL J0102-4915. By surveying the parameter space of the various
physical quantities that describe the two colliding clusters, including their
total mass (M), mass ratio (\xi), gas fractions (f_b), initial relative
velocity (V), and impact parameter (P), we find out an off-axis merger with
P~800h_{70}^{-1}kpc, V~2500km/s, M~3x10^{15}Msun, and \xi=3.6 that can lead to
most of the main observational features of ACT-CL J0102-4915. Those features
include the morphology of the X-ray emission with a remarkable wake-like
substructure trailing after the secondary cluster, the X-ray luminosity and the
temperature distributions, and also the SZ temperature decrement. The initial
relative velocity required for the merger is extremely high and rare compared
to that inferred from currently available Lambda cold dark matter (LCDM)
cosmological simulations, which raises a potential challenge to the LCDM model,
in addition to the case of the Bullet Cluster.

Cosmology and Nongalactic Astrophysics;Astrophysics of Galaxies