[Paper Review] Gaia DR2 Proper Motions of Dwarf Galaxies within 420 kpc: Orbits, Milky Way Mass, Tidal Influences, Planar Alignments, and Group Infall

T. K. Fritz, G. Battaglia+6 more|arXiv (Cornell University)|May 2, 2018

Stellar, planetary, and galactic studiesPhysics and Astronomy130 references95 citations

TL;DR

This work derives systemic proper motions for 39 Milky Way dwarf galaxies using Gaia DR2 and spectroscopic memberships, computes their 3D velocities and orbits in two Milky Way halo mass models, and discusses implications for Milky Way mass, tidal effects, planar alignments, and group infall.

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" ($0.8 imes 10^{12} M_\odot$) and "high" mass ($1.6 imes 10^{12} M_\odot$). 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.

Motivation & Objective

• Quantify systemic proper motions for a large sample of Milky Way dwarf galaxies using Gaia DR2 and spectroscopy.
• Derive Galactocentric velocities and orbital properties under different Milky Way halo mass assumptions.
• Assess tidal influences, planar alignment incidence, and potential group infall scenarios for satellites.

Proposed method

• Select probable members using spectroscopic data with a 40% membership probability threshold.
• Cross-match spectroscopic members with Gaia DR2 and apply parallax and proper motion consistency checks against modeled escape speeds.
• Compute error-weighted mean proper motions and add systematic error components to total uncertainties.
• Convert proper motions to heliocentric and then Galactocentric velocities; perform Monte Carlo simulations (forward and backward) to estimate uncertainties for derived quantities.
• Integrate orbits in Milky Way potentials (MWPotential14) with two halo masses (0.8 and 1.6 x 10^12 Msun) using galpy to obtain orbital parameters (pericenter, apocenter, eccentricity).
• Compare results with independent measurements and discuss implications for MW mass, tidal effects, and missing satellites.

Experimental results

Research questions

• RQ1 What are the systemic proper motions of MW dwarf galaxies within 420 kpc as measured by Gaia DR2 and vetted with spectroscopic membership?
• RQ2 What are the derived Galactocentric velocities and orbital properties of these satellites under different MW halo mass assumptions (low vs high mass)?
• RQ3 Do the orbital distributions and poles support a planar satellite structure or suggest a more isotropic distribution?
• RQ4 Are there signs of significant tidal influence or recent group infall among the satellites?
• RQ5 How do the results inform the missing satellite problem and the presence of distant satellites yet to be discovered?

Key findings

• Systemic proper motions are derived for 39 dwarf galaxies and candidates within 420 kpc, with generally good consistency against other studies for the classical dwarfs.
• Under high-mass MW halo (1.6 x 10^12 Msun) the satellite velocity and apocenter distributions better match expectations for bound orbits to the Milky Way, compared to the low-mass halo.
• Tuc III, Crater II, and other candidates have pericenters small enough for significant tidal influences.
• Fewer satellites are observed near apocenter than near pericenter, implying a population of distant dwarfs yet to be discovered, relevant to the missing satellite problem.
• Among 39 dwarfs, 12 have orbital poles not aligned with the MW plane of satellites, 10 have insufficient PM accuracy to assess alignment, and 17 align (with 11 co-orbiting and 6 counter-orbiting).
• Group infall may contribute to the observed alignments, though no definitive association is found for Crater-Leo members.