Simulating the Galaxy Cluster “El Gordo” and Identifying the Merger Configuration

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 (ξ), gas fractions (f b ), initial relative velocity (V ), and impact parameter (P ), we find out an off-axis merger with P ∼ 800 h −1 70 kpc, V ∼ 2500 km s −1 , M ∼ 3 × 10 15 M ⊙ , and ξ = 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 Λ cold dark matter (ΛCDM) cosmological simulations, which raises a potential challenge to the ΛCDM model, in addition to the case of the Bullet Cluster.Throughout the paper, we assume a flat ΛCDM cosmology model with Ω m = 0.30, Ω Λ = 0.70, and the Hubble constant H 0 = 70 h 70 km s −1 Mpc −1 .

A massive blow for ΛCDM – the high redshift, mass, and collision velocity of the interacting galaxy cluster El Gordo contradicts concordance cosmology

“…If finding two objects this massive sufficiently close to each other is already rare, finding three objects like this is extremely unlikely. Moreover, the mass ratio between the two components of El Gordo is almost certainly in the range 2 − 5 (Zhang et al 2015), with their best guess being 3.6 while other studies suggest ≈ 2 (Jee et al 2014;Molnar & Broadhurst 2015). This makes El Gordo a major merger.…”

“…The overall logic of the project is summarized in Figure 1. For the second step, we make use of the results obtained by Zhang et al (2015) in their hydrodynamical simulations of El Gordo. The main purpose of our work is to check how often the initial conditions of their plausible models arise in a very large ΛCDM cosmological simulation with side length 6 h −1 cGpc developed by the Juropa Hubble Volume Simulation Project (Watson et al 2013).…”

A $\mathcal {M}\gtrsim 3$ shock in ‘El Gordo’ cluster and the origin of the radio relic

“…Optical and X-ray observations revealed that 'El Gordo' is in a merging state (Menanteau et al 2010(Menanteau et al , 2012. Recent numerical simulations were able to reproduce the overall system properties assuming a nearly head on major merger (Donnert 2014;Molnar & Broadhurst 2015;Zhang et al 2015;Ng et al 2015).…”

Multi-phenomena Modeling of the New Bullet-like Cluster ZwCl 008.8+52 Using N-body/Hydrodynamical Simulations

“…There are also very bright gas features corresponding compressed gas that, although subsonic, are relatively dense so that the X-ray emission is enhanced showing a large scale cometary morphology, such as El Gordo (Menanteau et al 2012). This morphology has been readily explained as an off axis binary collision in self-consistent N-body/hydrodynamical numerical simulations assuming zero selfinteracting cross section for the dark matter (Zhang et al 2015;Molnar & Broadhurst 2015). Radio relics in El Gordo delineate a bow shock and a back shock.…”