Vast Planes of Satellites in a High-Resolution Simulation of the Local Group: Comparison to Andromeda

Abstract

We search for vast planes of satellites (VPoS) in a high resolution simulation of the Local Group performed by the CLUES project, which improves significantly the resolution of former similar studies. We use a simple method for detecting planar configurations of satellites, and validate it on the known plane of M31. We implement a range of prescriptions for modelling the satellite populations, roughly reproducing the variety of recipes used in the literature, and investigate the occurence and properties of planar structures in these populations. The structure of the simulated satellite systems is strongly non-random and contains planes of satellites, predominantly co-rotating, with, in some cases, sizes comparable to the plane observed in M31 by Ibata et al.. However the latter is slightly richer in satellites, slightly thinner and has stronger co-rotation, which makes it stand out as overall more exceptional than the simulated planes, when compared to a random population. Although the simulated planes we find are generally dominated by one real structure, forming its backbone, they are also partly fortuitous and are thus not kinematically coherent structures as a whole. Provided that the simulated and observed planes of satellites are indeed of the same nature, our results suggest that the VPoS of M31 is not a coherent disc and that one third to one half of its satellites must have large proper motions perpendicular to the plane. arXiv:1412.3110v2 [astro-ph.GA] 5 Jan 2015 dwarf galaxies, similar to that observed in the Andromeda galaxy (M31), occur frequently in ΛCDM cosmology. Shortly afterwards, Ibata et al. (2014b); Pawlowski et al. (2014) re-examined this simulation, accounting for the observed plane's extent, thickness and abundance, and came to the opposite conclusion, that only 0.04% of galaxies possess planes as extreme as M31's. These studies were performed, "with the caveat that the Millennium-II simulation may not have sufficient mass resolution to identify confidently simulacra of low-luminosity dwarf galaxies", as duly noted by Ibata et al. (2014b): the semi-analytic modeling of Guo et al. (2013) differentiates normal galaxies from "orphans", the latter being systems whose parent sub-halo is no longer resolved. It is possible that many of these orphans are tidally disrupted, and hence that they are not directly comparable to the observed dwarf galaxies. In the present paper, we avoid this caveat by using a high resolution of the local group performed by the CLUES project, offering an improvement of a factor 15 in mass resolution with respect to the Millenium-II simulation, which allows us to resolve the satellites in the mass range of interest more consistently. This improvement comes however at the cost of volume, as we are left with only 2 host galaxies to study in the present paper. In Sec. 2 we present the simulation, the satellite population models used and the method for detecting planes of satellites. In Sec. 3 we present the results and the detected planes, followed by a short discussion an...

On the stability of satellite planes – I. Effects of mass, velocity, halo shape and alignment

“…However, the general conclusion is that planes of satellites are a transitory phenomenon. Similar conclusions are drawn by the Gillet et al (2015) analysis of CLUES (Constrained Local UniversE Simulations). The planes they find contained 11 satellite galaxies at most, indicating the likely possibility of M31's plane of 15 satellites being an extreme occurrence.…”

“…About a third of satellites have high velocities that are perpendicular to the planar formation, making their presence in the plane purely coincidental. Gillet et al (2015) conclude that the observed satellite planes appear to be in agreement with the ΛCDM but as a transitory feature, created with a few planar satellites and several timely interlopers. Such coincidental placement of galaxies may reduce the number of observed kinematically coherent planes.…”

The ISLAndS Project. II. The Lifetime Star Formation Histories of Six Andromeda dSphs*

“…The member satellites rotate about M31 in the same sense. The reality and probability of this plane has been discussed at length, especially with regard to whether such a structure is consistent with galaxy formation in a ΛCDM cosmology (see, e.g., Hammer et al 2013;Bahl & Baumgardt 2014;Ibata et al 2014;Pawlowski et al 2014;Buck et al 2015;Gillet et al 2015;Libeskind et al 2015;Phillips et al 2015). Collins et al (2015) have investigated the possibility that inplane galaxies and off-plane galaxies may have formed and evolved differently.…”

The Inhomogeneous Reionization Times of Present-day Galaxies

“…Both objects reionized in isolation relative to each other: their patches are easily identified and disconnected. Ocvirk & Aubert (2011) and Gillet et al (2015) predicted that this kind of reionization should lead to a more extended radial distribution of their satellites at z=0 compared to models without radiative transfer. The LG also reionized in isolation from the large-scale environment, as no ionization fronts appeared to sweep across them from outside.…”