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 6.2σ 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. (M200 = 3.2 × 1015 M ⊙ and Vinfall = 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 x 1015M ⊙. Here we explore the level of tension between El Gordo and ΛCDM for the new mass estimate, assuming several Vinfall values. We find that in order to reduce the tension below the 5σ level, the El Gordo subclusters should have Vinfall < 2300 km s−1 (Vinfall < 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 Vinfall ≥ 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 Vinfall should be tested with new hydrodynamical simulations that explore different configurations of the interaction.