Gaia DR2 proper motions of dwarf galaxies within 420 kpc

Abstract

A proper understanding of the Milky Way (MW) dwarf galaxies in a cosmological context requires knowledge of their 3D velocities and orbits. However, proper motion (PM) measurements have generally been of limited accuracy and are available only for more massive dwarfs. We therefore present a new study of the kinematics of the MW dwarf galaxies. We use the Gaia DR2 for those dwarfs that have been spectroscopically observed in the literature. We derive systemic PMs for 39 galaxies and galaxy candidates out to 420 kpc, and generally find good consistency for the subset with measurements available from other studies. We derive the implied Galactocentric velocities, and calculate orbits in canonical MW halo potentials of low (0.8 × 10 12 M ) and high mass (1.6 × 10 12 M ). Comparison of the distributions of orbital apocenters and 3D velocities to the halo virial radius and escape velocity, respectively, suggests that the satellite kinematics are best explained in the high-mass halo. Tuc III, Crater II, and additional candidates have orbital pericenters small enough to imply significant tidal influences. Relevant to the missing satellite problem, the fact that fewer galaxies are observed to be near apocenter than near pericenter implies that there must be a population of distant dwarf galaxies yet to be discovered. Of the 39 dwarfs: 12 have orbital poles that do not align with the MW plane of satellites (given reasonable assumptions about its intrinsic thickness); 10 have insufficient PM accuracy to establish whether they align; and 17 satellites align, of which 11 are co-orbiting and (somewhat surprisingly, in view of prior knowledge) 6 are counter-orbiting. Group infall might have contributed to this, but no definitive association is found for the members of the Crater-Leo group.

The Milky Way’s stellar streams and globular clusters do not align in a Vast Polar Structure

“…The classical satellite galaxies of the Milky Way have been shown to have a highly-clustered distribution of orbital poles (Pawlowski & Kroupa 2013), a trend that has become more significant as higher-precision proper motion measurements become available (Pawlowski & Kroupa 2020) and that extends to Milky Way ultrafaint satellites (Fritz et al 2018). This observed clustering of satellite orbital poles is not consistent with expectations from cosmological simulations, regardless of whether the simulations considered are dark-matter-only (Pawlowski & McGaugh 2014;Ibata et al 2014b) or include baryonic effects (Müller et al 2018;Pawlowski et al 2019).…”

“…This is the direction along a great circle oriented perpendicular to the position of the cluster (as seen from the center of the Milky Way) which minimizes the angle to the assumed VPOS normal. With this optimal orbital pole, we can calculate the same summary statistics as in Fritz et al (2018) to evaluate if a cluster is a member of the VPOS:…”

“…We consider systems with p inVPOS > 0.5 to be VPOS members, p inVPOS < 0.05 to firmly and conclusively rule out VPOS membership, and intermediate values as too weakly constrained to be conclusive. This scheme is simple and reproduces most of the classifications that Fritz et al (2018) arrived at for satellite galaxies (they do not state their classification criteria explicitly). Table 1 provides an overview of the statistics used to determine VPOS membership.…”

“…As more (and fainter) satellite galaxies have been discovered in the Sloan Digital Sky Survey, the Dark Energy Survey, and smaller community efforts (Simon 2019), these new satellites continue to preferentially reside in the same spatial plane (Metz et al 2009a;Pawlowski & Kroupa 2014;Pawlowski et al 2015a). Proper motion measurements using data from Gaia Data Release 2 (DR2; Gaia Collaboration et al 2018a) show that the orbital poles for non-classical satellites also preferentially cluster in this plane (Fritz et al 2018).…”

“…We also show the normal vector of a disk fitted to the spatial distribution of young halo globular clusters (Pawlowski et al 2012, blue diamond) and the minor axis direction of a spatial plane fitted to all Milky Way satellites (Pawlowski et al 2015a, purple hexagon). The green circles each contain 10% of the sky around the assumed VPOS pole (Pawlowski & Kroupa 2013;Fritz et al 2018), given as an "x" for the co-orbiting direction and a "+" for the counter-orbiting direction. The previously observed preferential alignment of Milky Way satellite galaxies, stellar streams, and young halo globular clusters has been dubbed the Vast Polar Structure (VPOS). ture has usually been referred to as the Vast Polar Structure (VPOS), comprised of satellite galaxies, globular clusters, and stellar streams.…”