Dynamics of merging: Post-merger mixing and relaxation of an Illustris galaxy

TL;DR

This study uses the Illustris simulation to analyze the processes of mixing and relaxation during galaxy mergers, revealing that mixing and relaxation occur over similar timescales and that galaxies may not be fully relaxed even after reaching equilibrium.

Contribution

It provides the first direct measurement of the timescales for mixing and relaxation in galaxy mergers, distinguishing between Jeans equilibrium and full dynamical relaxation.

Findings

Mixing reduces differences in energy and angular momentum distributions by ~80% in 1 Gyr.

Relaxation towards DARKexp distribution occurs over approximately 8.5 Gyrs.

Stars and tightly bound particles are less affected by mixing due to their limited influence by fluctuating potentials.

Abstract

During the merger of two galaxies, the resulting system undergoes violent relaxation and seeks stable equilibrium. However, the details of this evolution are not fully understood. Using Illustris simulation, we probe two physically related processes, mixing and relaxation. Though the two are driven by the same dynamics---global time-varying potential for the energy, and torques caused by asymmetries for angular momentum---we measure them differently. We define mixing as the redistribution of energy and angular momentum between particles of the two merging galaxies. We assess the degree of mixing as the difference between the shapes of their N(E)s, and their N(L^2)s. We find that the difference is decreasing with time, indicating mixing. To measure relaxation, we compare N(E) of the newly merged system to N(E) of a theoretical prediction for relaxed collisionless systems, DARKexp, and witness the system becoming more relaxed, in the sense that N(E) approaches DARKexp N(E). Because the dynamics driving mixing and relaxation are the same, the timescale is similar for both. We measure two sequential timescales: a rapid, 1 Gyr phase after the initial merger, during which the difference in N(E) of the two merging halos decreases by ~80%, followed by a slow phase, when the difference decreases by ~50% over ~8.5 Gyrs. This is a direct measurement of the relaxation timescale. Our work also draws attention to the fact that when a galaxy has reached Jeans equilibrium it may not yet have reached a fully relaxed state given by DARKexp, in that it retains information about its past history. This manifests itself most strongly in stars being centrally concentrated. We argue that it is particularly difficult for stars, and other tightly bound particles, to mix because they have less time to be influenced by the fluctuating potential, even across multiple merger events.