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 × 10 15 M ⊙ . 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.