Letter to the Editor

The coherent motion of Cen A dwarf satellite galaxies remains a challenge for ΛCDM cosmology^⋆

¹ Observatoire Astronomique de Strasbourg (ObAS), Universite de Strasbourg – CNRS, UMR 7550, Strasbourg, France
e-mail: oliver.muller@astro.unistra.fr
² Leibniz-Institut fur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
³ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁴ European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
⁵ Department of Physics, University of Surrey, Guildford GU2 7XH, UK
⁶ University of Lyon, UCB Lyon-1/CNRS/IN2P3, I2PI, France
⁷ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia

Received: 23 November 2020
Accepted: 14 December 2020

Abstract

The plane-of-satellites problem is one of the most severe small-scale challenges for the standard Λ cold dark matter (ΛCDM) cosmological model: Several dwarf galaxies around the Milky Way and Andromeda co-orbit in thin, planar structures. A similar case has been identified around the nearby elliptical galaxy Centaurus A (Cen A). In this Letter, we study the satellite system of Cen A, adding twelve new galaxies with line-of-sight velocities from VLT/MUSE observations. We find that 21 out of 28 dwarf galaxies with measured velocities share a coherent motion. Similarly, flattened and coherently moving structures are found only in 0.2% of Cen A analogs in the Illustris-TNG100 cosmological simulation, independently of whether we use its dark-matter-only or hydrodynamical run. These analogs are not co-orbiting, and they arise only by chance projection, thus they are short-lived structures in such simulations. Our findings indicate that the observed co-rotating planes of satellites are a persistent challenge for ΛCDM, which is largely independent from baryon physics.