The origin of the Gaia phase-plane spiral

TL;DR

This paper presents a simple model explaining the formation of spiral patterns in the vertical phase plane of stars, caused by interactions with massive substructures like the Sagittarius dwarf galaxy, matching Gaia observations.

Contribution

It introduces a model linking vertical oscillation frequencies to angular momentum, explaining the Gaia phase-plane spiral formation due to galactic substructure interactions.

Findings

Spirals form in <v_phi> and <v_R> after substructure passages.

The model reproduces observed spiral features.

Tidal forces in the galactic plane drive spiral formation.

Abstract

A simple model is presented of the formation of the spiral the (z,v_z) phase plane of solar-neighbourhood stars that was recently discovered in Gaia data. The key is that the frequency Omega_z at which stars oscillate vertically depends on angular momentum about the z axis in addition to the amplitude of the star's vertical oscillations. Spirals should form in both <v_phi> and <v_R> whenever a massive substructure, such as the Sgr dwarf galaxy, passes through the Galactic plane. The model yields similar spirals to those observed in both <v_phi> and <v_R>. The primary driver is the component of the tidal force that lies in the plane. We investigate the longevity of the spirals and the mass of the substructure, but the approximations inherent in the model make quantitative results unreliable. The work relies heavily on a self-consistent, multi-component model of our Galaxy produced by the AGAMA package for f(J) modelling.