Determining orbits for the Milky Way's dwarfs

TL;DR

This paper calculates and assesses the accuracy of orbital reconstructions for Milky Way dwarf galaxies using proper motions, simulation comparisons, and error analysis, revealing limitations due to potential shape and satellite interactions.

Contribution

It introduces a method to recover dwarf galaxy orbits from proper motions and evaluates its accuracy considering realistic measurement errors and complex gravitational effects.

Findings

Orbit recovery is accurate to ~40% with current data.

Non-sphericity and satellite interactions complicate orbit determination.

Milky Way dwarfs likely formed before reionisation.

Abstract

We calculate orbits for the Milky Way dwarf galaxies with proper motions, and compare these to subhalo orbits in a high resolution cosmological simulation. We use this same simulation to assess how well are able to recover orbits in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. We find that, for present measurement uncertainties, we are able to recover the apocentre r_a and pericentre r_p to ~ 40%. However, even with better data the non-sphericity of the potential and satellite interactions during group infall make the orbital recovery more challenging. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is consistent with the most massive subhalos that form before z=10, lending support to the idea that the Milky Way dwarfs formed before reionisation.