Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall (1805.00908v4)

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 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" ($0.8 \times 10^{12} M_\odot$) and "high" mass ($1.6 \times 10^{12} M_\odot$). 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 paper presents new Gaia DR2 proper motion measurements for 39 dwarf galaxies to refine their orbital dynamics around the Milky Way.
• It employs dual halo models to compute 3D velocities, finding that a high-mass Milky Way model best explains the observed satellite motions.
• Significant tidal influences and planar alignments highlight gravitational interactions and suggest a hidden population of distant dwarf galaxies.

Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Insight into Their Orbits and the Milky Way's Halo

This paper presents a rigorous paper employing the Gaia Data Release 2 (DR2) to estimate and analyze the proper motions of 39 dwarf galaxies within a distance of 420 kpc from the Milky Way (MW). Given the critical role of 3D velocities in understanding dwarf galaxies' evolution and interaction dynamics with the MW, this research significantly enhances kinematic data in this domain by providing measurements for systems that have previously lacked comprehensive observation due to the limitations of earlier astrometric releases.

Methodology

The authors relied on Gaia DR2 data, complemented by spectroscopic observations culled from the literature to refine membership and align systemic proper motions. By derivation and cross-comparison, they ascertain systemic proper motions, which were further utilized to compute Galactocentric velocities of the dwarf galaxies. They employed two models of the MW's halo potential—designated as low-mass (0.8 × 10¹² M_☉) and high-mass (1.6 × 10¹² M_☉)—to simulate and interpret the orbits of these satellites around the MW.

Results

Key results center around the systemic proper motions for 39 galaxies and candidates. Here, the distribution of orbital apocenters and 3D velocities were juxtaposed with the MW's virial radius and escape velocity. Notably, the dynamics observed are more congruent with a high-mass halo, challenging lighter halo models of the MW. Tucana III, Crater II, and others exhibit orbital pericenters suggesting significant tidal influences, emphasizing the gravitational interplay at smaller radii.

Implications for the Milky Way's Halo and Dwarf Galaxy Population

The paper's findings underscore several crucial aspects of MW's satellite dynamics. The evidence that a more substantial MW halo model best fits the observed velocity distributions could imply that previous mass estimates require revaluation. Crucially, this paper infers a necessity to discover a latent population of distant dwarf galaxies; these systems likely reside near their orbital apocenters, thus eluding current detection thresholds and potentially addressing the "missing satellite" problem — an enduring discrepancy between observed and predicted satellite counts from cosmological simulations.

Polar Alignments and Group Infall Dynamics

The analysis of orbital poles shows notable planar alignments for a subset of satellites, accentuating systemic links between satellite orbits and the MW's disk — a dynamical feature often attributed to group infall processes. Interestingly, no definitive association for the Crater-Leo group's members was observed, suggesting that further data might be required to uncover these group dynamics convincingly.

Concluding Remarks

This research enriches the precision and scope of dwarf galaxy proper motion data within our local cosmic neighborhood. Future Gaia data releases, with reduced astrometric uncertainties, promise further insights into smaller and more distant systems, potentially uncovering new aspects of the MW's gravitational landscape and satellite population. The paper's implications for the mass and scale of the MW halo contribute essential granularity to our understanding of the structural dynamics and evolutionary trajectory of our galaxy and its complement of satellite systems. As the field advances, incorporating factors such as the LMC's perturbative influence on satellite orbits represents a productive trajectory for furthering the domain.