The planes of satellite galaxies problem, suggested solutions, and open questions

Abstract

Satellite galaxies of the Milky Way and of the Andromeda galaxy have been found to preferentially align in significantly flattened planes of satellite galaxies, and available velocity measurements are indicative of a preference of satellites in those structures to co-orbit. There is increasing evidence that such kinematically correlated satellite planes are also present around more distant hosts. Detailed comparisons show that similarly anisotropic phase-space distributions of sub-halos are exceedingly rare in cosmological simulations based on the ΛCDM paradigm. Analogs to the observed systems have frequencies of ≤ 0.5 per cent in such simulations. In contrast to other small-scale problems, the satellite planes issue is not strongly affected by baryonic processes because the distribution of sub-halos on scales of hundreds of kpc is dominated by gravitational effects. This makes the satellite planes one of the most serious small-scale problem for ΛCDM. This review summarizes the observational evidence for planes of satellite galaxies in the Local Group and beyond, and provides an overview of how they compare to cosmological simulations. It also discusses scenarios which aim at explaining the coherence of satellite positions and orbits, and why they all are currently unable to satisfactorily resolve the issue. 1:17 WSPC/INSTRUCTION FILE review 2 Marcel S. Pawlowski successes do not constitute proof of correctness, 2, 3 and science strives to probe the limitations of its models by pushing to test them in novel ways. In particular when expanded beyond its initial scope of applicability, a model is most thoroughly tested and opportunities for refinement or revision can be identified. For ΛCDM -developed to fit the large-scale structure and evolution of the Universe (on scales of many Mpc to Gpc) -such a novel domain of applicability is the regime of galaxies and their satellite galaxy systems (on scales of hundreds of kpc and below). Due to the non-linearity of ΛCDM on those scales, model predictions are made via numerical simulations. These evolve an initially almost homogenious matter distribution forward in time, simulating the formation and evolution of structure in the cosmos until the present day. This implies an important caveat for tests in this regime: not the model itself is tested directly, but its realisation via numerical simulations.Our best observational knowledge of the smallest galaxies stems from observation of the ≈ 50 and 40 known dwarf Spheroidal (dSph) satellite galaxies of our Milky Way (MW) and the neighboring Andromeda galaxy (M31), respectively. Together with ≈ 15 more isolated dwarf galaxies, they make up the Local Group of galaxies, covering a volume of approximately 1 Mpc in radius. Comparisons of cosmological simulations of dark matter halos resembling those expected to host the MW and M31 with the observed systems have revealed a number of small-scale problems of ΛCDM. 4 Among the best known issues are the Missing Satellites Problem, the Core-Cusp Problem, and the Too-Big-to-Fail problem...

The Milky Way’s disc of classical satellite galaxies in light of Gaia DR2

“…9. This confirms the expectation that the satellite galaxy phase space distribution constitutes a robust test of the cosmological model (Kroupa et al 2005;Pawlowski 2018), indicates that previous studies using dark-matter-only simulations to compare to the VPOS remain valid, and confirms earlier indications that the modelling of baryonic processes in cosmological simulations does not alleviate the planes of satellite galaxies problem (Ahmed et al 2017;Pawlowski et al 2015bPawlowski et al , 2017Pawlowski et al , 2019Shao et al 2018).…”

“…This is at odds with the high mass-to-light ratios of many of the observed dwarf satellite galaxies, unless the dark matter hypothesis is abandoned in favor of a modified gravity approach (Bílek et al 2018;Banik et al 2018). For a review of the observed satellite planes, comparison of them with cosmological simulations, and proposed solutions to the planes of satellite galaxies problem, see Pawlowski (2018).…”

The Sheet of Giants: Unusual properties of the Milky Way’s immediate neighbourhood

“…This is an important point because similar approaches for estimating the incidence of systems with specific properties arise in other contexts, such as estimating the rarity of 'satellite planes' (see, e.g. Pawlowski 2018).…”

“…This should significantly increase the number of systems with well-resolved satellite populations which will allow for higher-precision measurements of satellite radial distributions and the variation in satellite luminosity function amplitudes (Bennet et al 2019). The expanded satellite samples should also result in better characterisation of the properties and incidence of the 'satellite planes' (Pawlowski 2018). The high abundance of bright galaxies discussed in Section 5 means that there are a large number of targets for such searches.…”

MOND vs. dark matter in light of historical parallels

“…6; instead, it has a single (wrong) slope, and thus shows no sign of the critical acceleration a 0 . And, see also Pawlowski, 2018, who discusses the planes of satellites not being reproduced in DM simulations.…”