Simulating the galaxy cluster "El Gordo" and identifying the merger configuration

TL;DR

This paper uses hydrodynamical simulations to identify the initial collision parameters of the galaxy cluster 'El Gordo', matching observed features and highlighting challenges to standard cosmological models due to the high collision velocity.

Contribution

It presents the first detailed simulation-based merger scenario for 'El Gordo', constraining initial conditions that reproduce observed features and discussing implications for cosmology.

Findings

Identified off-axis merger parameters matching observations

Revealed extremely high initial collision velocity challenging LCDM

Simulated X-ray and SZ features consistent with observations

Abstract

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.