On the stability of tidal streams in action space

TL;DR

This paper compares static and time-evolving gravitational models for the Milky Way, showing that time-evolving models better preserve the coherence of tidal streams in action space over time.

Contribution

It demonstrates that time-evolving potentials are essential for accurately tracking tidal streams over Gyr timescales, especially in systems with merger activity.

Findings

Time-evolving models preserve action space coherence longer than static models.

Static models can mischaracterize stellar orbits as early as 0.5-1 Gyr ago.

Merger history significantly affects the accuracy of static potential models.

Abstract

In the Gaia era it is increasingly apparent that traditional static, parameterized models are insufficient to describe the mass distribution of our complex, dynamically evolving Milky Way (MW). In this work, we compare different time-evolving and time-independent representations of the gravitational potentials of simulated MW-mass galaxies from the FIRE-2 suite of cosmological baryonic simulations. Using these potentials, we calculate actions for star particles in tidal streams around three galaxies with varying merger histories at each snapshot from 7 Gyr ago to the present day. We determine the action-space coherence preserved by each model using the Kullback-Leibler Divergence to gauge the degree of clustering in actions and the relative stability of the clusters over time. We find that all models produce a clustered action space for simulations with no significant mergers. However, a massive (mass ratio prior to infall more similar than 1:8) interacting galaxy not present in the model will result in mischaracterized orbits for stars most affected by the interaction. The locations of the action space clusters (i.e. the orbits of the stream stars) are only preserved by the time-evolving model, while the time-independent models can lose significant amounts of information as soon as 0.5--1 Gyr ago, even if the system does not undergo a significant merger. Our results imply that reverse-integration of stream orbits in the MW using a fixed potential is likely to give incorrect results if integrated longer than 0.5 Gyr into the past.