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

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.

On the absence of backsplash analogues to NGC 3109 in the ΛCDM framework

“…The arguments raised by Metz et al (2009) and Pawlowski et al (2014) against the group infall and filamentary accretion scenarios were later independently confirmed by Shao et al (2018) using the EAGLE hydrodynamical cosmological simulation (Crain et al 2015;Schaye et al 2015). For a recent review of the satellite plane problem, we refer the reader to Pawlowski (2018), who considered both LG satellite planes and the recently discovered one around Centaurus A (Müller et al 2018(Müller et al , 2021.…”

“…Moreover, only around the MW, M31, and Centaurus A do we have information on the 3D distribution of satellite galaxies and at least one component of their velocity. A satellite plane is also evident around Centaurus A, with properties that are likely to arise in ΛCDM only 0.2% of the time (Müller et al 2021). As a result, it would be difficult to repeat the above-mentioned analyses further afield.…”

Phase-Space Correlations among Systems of Satellite Galaxies

“…Additional velocity measurements have since been obtained for another 12 satellites with VLT MUSE [73]. These confirm a strong kinematic coherence, with a total of 21 out of 28 satellites following a coherent velocity trend [74]. A recent major merger was proposed to explain numerous features of Centaurus A and its stellar halo [75].…”

“…Figure 6 provides comparisons to the three best-studied planes of satellite galaxies with one common, current state-of-the art hydrodynamical cosmological simulation, Illustris TNG-100 [88]. Hosts were selected to reproduce the mass, isolation, and absence of an ongoing major merger in the three hosts, and satellites were selected to mimic ob-servational constraints-see the respective publications that these data are based on for details [58,74,89]. The figures illustrate that there is strong tension between cosmological expectations for typical satellite galaxy systems and the observed flattening and kinematic coherence of the planes of satellites.…”

“…For Centaurus A (bottom panel), only the on-sky minor-to-major axis ratio b/a is used to determine the flattening of the observed system (to ensure independence of the more considerable distance uncertainties), while the kinematic coherence is quantified as in the case of Andromeda, with the number of satellites following a coherent line-of-sight velocity trend. This comparison is based on the analysis presented in Müller et al[74]. All three comparisons are based on the same cosmological simulation, the highest-resolution, hydrodynamical Illustris TNG-100 run[88].…”