Dynamical invariants and diffusion of merger substructures in time-dependent gravitational potentials

TL;DR

This paper introduces a mathematical method to derive dynamical invariants in time-dependent gravitational potentials, enabling better understanding of the evolution of stellar structures and the dissolution of tidal substructures in galaxies.

Contribution

It presents a canonical transformation technique to find invariants in evolving potentials, with applications to galaxy evolution and tidal structure analysis.

Findings

Dynamical invariants can be derived via canonical transformations.

Growth of host potential erases tidal substructures over time.

Method allows tracking of microcanonical ensembles in dynamic systems.

Abstract

This paper explores a mathematical technique for deriving dynamical invariants (i.e. constants of motion) in time-dependent gravitational potentials. The method relies on the construction of a canonical transformation that removes the explicit time-dependence from the Hamiltonian of the system. By referring the phase-space locations of particles to a coordinate frame in which the potential remains `static' the dynamical effects introduced by the time evolution vanish. It follows that dynamical invariants correspond to the integrals of motion for the static potential expressed in the transformed coordinates. The main difficulty of the method reduces to solving the differential equations that define the canonical transformation, which are typically coupled with the equations of motion. We discuss a few examples where both sets of equations can be exactly de-coupled, and cases that require approximations. The construction of dynamical invariants has far-reaching applications. These quantities allow us, for example, to describe the evolution of (statistical) microcanonical ensembles in time-dependent gravitational potentials without relying on ergodicity or probability assumptions. As an illustration, we follow the evolution of dynamical fossils in galaxies that build up mass hierarchically. It is shown that the growth of the host potential tends to efface tidal substructures in the integral-of-motion space through an orbital diffusion process. The inexorable cycle of deposition, and progressive dissolution, of tidal clumps naturally leads to the formation of a `smooth' stellar halo.