The El Gordo Galaxy Cluster Challenges ΛCDM for Any Plausible Collision Velocity

Abstract

El Gordo (ACT-CL J0102-4915) is an extraordinarily large and bright galaxy cluster collision. In a previous study, we found that El Gordo is in 6.2 σ tension with the Λ cold dark matter (ΛCDM) standard model when assuming the nominal mass and infall velocity values from the hydrodynamical simulations of Zhang et al. ( M _200 = 3.2 × 10 ^15 M _⊙ and V _infall = 2500 km s ^−1 , respectively). The recent weak-lensing study of Kim et al. showed that the mass of El Gordo is actually ${2.13}_{-0.23}^{+0.25}\times {10}^{15}{M}_{\odot }$ . Here we explore the level of tension between El Gordo and ΛCDM for the new mass estimate, assuming several V _infall values. We find that in order to reduce the tension below the 5 σ level, the El Gordo subclusters should have V _infall < 2300 km s ^−1 ( V _infall < 1800 km s ^−1 when considering the combined tension with the Bullet Cluster). To the best of our knowledge, the El Gordo hydrodynamical simulations conducted so far require V _infall ≥ 2500 km s ^−1 to simultaneously reproduce its morphology and its high X-ray luminosity and temperature. We therefore conclude that El Gordo still poses a significant challenge to ΛCDM cosmology. Whether the properties of El Gordo can be reconciled with a lower V _infall should be tested with new hydrodynamical simulations that explore different configurations of the interaction.

Keywords

• Galaxy clusters
• Large-scale structure of the universe
• Gravitation
• Dark matter