Determining orbits for the Milky Way's dwarfs

TL;DR

This paper calculates the orbits of Milky Way dwarf galaxies using proper motions, compares them with cosmological simulations, and assesses the accuracy of orbit recovery under various measurement and modeling uncertainties.

Contribution

It introduces a method to recover dwarf galaxy orbits from proper motions and evaluates its accuracy with simulations, highlighting the impact of measurement errors and potential complexities.

Findings

Orbit recovery improves from ~40% to ~14% with Gaia data.

Most dwarfs formed before reionisation, consistent with simulation data.

Non-sphericity and satellite interactions significantly affect orbit determination.

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 the simulation data to assess how well orbits may be recovered in the face of measurement errors, a time varying triaxial gravitational potential, and satellite-satellite interactions. For present measurement uncertainties, we recover the apocentre r_a and pericentre r_p to ~40%. With improved data from the Gaia satellite we should be able to recover r_a and r_p to ~14%, respectively. However, recovering the 3D positions and orbital phase of satellites over several orbits is more challenging. This owes primarily to the non-sphericity of the potential and satellite interactions during group infall. Dynamical friction, satellite mass loss and the mass evolution of the main halo play a more minor role in the uncertainties. We apply our technique to nine Milky Way dwarfs with observed proper motions. We show that their mean apocentre is lower than the mean of the most massive subhalos in our cosmological simulation, but consistent with the most massive subhalos that form before z=10. This lends further support to the idea that the Milky Way's dwarfs formed before reionisation.