The Orbital Poles of Milky Way Satellite Galaxies: A Rotationally Supported Disk of Satellites

Abstract

Available proper motion measurements of Milky Way (MW) satellite galaxies are used to calculate their orbital poles and projected uncertainties. These are compared to a set of recent cold dark-matter (CDM) simulations, tailored specifically to solve the MW satellite problem. We show that the CDM satellite orbital poles are fully consistent with being drawn from a random distribution, while the MW satellite orbital poles indicate that the disc-of-satellites of the Milky Way is rotationally supported. Furthermore, the bootstrapping analysis of the spatial distribution of theoretical CDM satellites also shows that they are consistent with being randomly drawn. The theoretical CDM satellite population thus shows a significantly different orbital and spatial distribution than the MW satellites, most probably indicating that the majority of the latter are of tidal origin rather than being DM dominated sub-structures. A statistic is presented that can be used to test a possible correlation of satellite galaxy orbits with their spatial distribution.

Dissipative phenomena in extended-body interactions

“…Over this external field, only the family of orbits in the neighbourhood of the phase-space point x = {22, 89, −34} kpc, v = {−136, 13, −45} in a reference frame S 0 collinear with the MW potential are investigated and extracted from the family of proper motions in Table 1 of Metz et al (2008) with the minimization action technique developed in Pasetto et al (2011). We adopt an initial potential-density pair for Carina from the sub-family (1, 4, γ) of the Zhao-models (Zhao 1996), widely known also as the γ-models.…”

A new spin on discs of satellite galaxies

“…Many members of this 'disc of satellites' have a common rotation direction and it has been claimed that the plane is a rotationally stabilized structure (Metz, Kroupa & Libeskind 2008;. Similarly, the spatial distribution of satellites around Andromeda is also thought to be anisotropic (Koch & Grebel 2006;McConnachie & Irwin 2006), with 15 out of 27 satellites observed by the Pan-Andromeda Archaeological Survey (PAndAS; McConnachie et al 2009) located in a very thin plane in which 13 out of the 15 satellites share the same sense of rotation (Ibata et al 2013).…”

The Future of Stellar Populations Studies in the Milky Way and the Local Group

“…Analysis of the proper motions of these MW satellites has more or less supported the notion of dynamical kinship of at least some satellites, at least in that a number of these systems seem to share similar orbital poles, showing common directions of angular momentum , Palma et al 2002, Metz et al 2008. The meaning of such alignments and whether they are at odds with, or consistent with, expectations of typical satellite configurations around MW-like galaxies in CDM models has been much debated of late.…”

“…Such spatial alignments could be a signature of a dynamical association of the satellites, but must be checked with full 3-dimensional velocity measurements for the satellites. Through painstaking work, the proper motions, μ, of MW satellites are slowly being accumulated, though in most cases with still only marginally significant μ/ μ , at least for ground-based data (see summary in Metz et al 2008). More recently, proper motions measured using Hubble Space Telescope imaging of the Magellanic Clouds ("MCs" hereafter; Kallivayalil et al 2006a,b, Piatek et al 2008, other dSph satellites (Piatek et al 2003(Piatek et al , 2005(Piatek et al , 2006(Piatek et al , 2007 and globular clusters (e.g., Bedin et al 2006, Kalirai et al 2007) have shown significantly reduced proper motion uncertainties, despite very small time baselines -with improvements to be expected if more epochs can be accumulated with the newly-installed WFC3.…”