Phase Wrapping of Epicyclic Perturbations in the Wobbly Galaxy

TL;DR

This study demonstrates that phase wrapping of epicyclic motions, excited by a past passage of a massive dwarf galaxy, can explain observed vertical velocity streaming in the Milky Way's stellar disc.

Contribution

It isolates phase wrapping effects from other wave modes using test-particle simulations, highlighting the impact of a recent dwarf galaxy passage on stellar motions.

Findings

Massive Sagittarius dwarf galaxy can produce observed streaming motions.

Recent passage (~1 Gyr ago) is more effective than older passages.

Velocity gradients should vary on kiloparsec scales near the Sun.

Abstract

We use test-particle integrations to show that epicyclic motions excited by a pericentre passage of a dwarf galaxy could account for bulk vertical velocity streaming motions recently observed in the Galactic stellar disc near the Sun. We use fixed potential test-particle integrations to isolate the role of phase wrapping of epicyclic perturbations from bending and breathing waves or modes, which require self-gravity to oscillate. Perturbations from a fairly massive Sagittarius dwarf galaxy, $M_d \sim 2.5 \times 10^{10} M_\odot$, are required to account for the size of the observed streaming motions from its orbital pericentre approximately a Gyr ago. A previous passage of the dwarf through the Galactic disc approximately 2.2 Gyr ago (with a then more massive dwarf galaxy) is less effective. If phase wrapping of epicyclic perturbations is responsible for stellar streaming motions in the Galactic disc, then there should be variations in velocity gradients on scales of a few kpc in the vicinity of the Sun.