Phase spirals across galactic disks I: Exploring dynamical influences on winding

TL;DR

This study investigates how various dynamical processes influence the winding of vertical phase-space spirals in galactic disks, revealing that real galaxy conditions cause delays and oscillations in spiral winding compared to idealized models.

Contribution

It demonstrates how self-consistent galaxy simulations show deviations from pure phase mixing theory, highlighting the impact of evolving potentials on phase spiral winding times.

Findings

Winding in test particle simulations matches pure phase mixing theory.

In N-body simulations, winding is delayed and oscillates over time.

Winding rates depend on azimuthal action and disk evolution.

Abstract

The vertical phase-space spirals in the Milky Way are clear evidence of disequilibrium. However, they are challenging to study because phase mixing signals evolve under the influence of many different dynamical processes and can be driven by many sources of disequilibrium. We characterize phase spirals in two simulations -- one test particle and one N-body -- with basis function expansions, using these to derive winding times ($T_{\rm fit}$). We find that phase spirals in the test particle simulation wind up as expected from pure phase mixing theory while those in the self-consistent simulation do not. Specifically, in the N-body simulation we find that (i) the onset of winding is delayed, (ii) the winding rate is slowed, and (iii) the rate of winding oscillates with time. The extent of these effects depends on the azimuthal action $J_\phi$ of the phase spiral region. We build some physical intuition for these effects through 1-D toy models which follow a group of co-moving stars traveling through several different evolving potentials. We find that phase spiral winding can be delayed until the group no longer moves coherently with the midplane of the (perturbed) potential and oscillates with time as the group experiences (e.g.) a breathing mode traveling through the disk. Rates of winding change as the vertical structure of the disk evolves. The modifications to winding are strongest in the inner galaxy where the disk potential dominates. We conclude that in the Milky Way, all calculations of the winding time should be interpreted as lower limits and that the most trustworthy winding times are likely in the outer disk.