[Paper Review] Wrinkles in the Gaia data unveil a dynamically young and perturbed Milky Way disk

T. Antoja, A. Helmi+11 more|arXiv (Cornell University)|Jan 1, 2017

Stellar, planetary, and galactic studiesPhysics and Astronomy50 references18 citations

TL;DR

Using Gaia DR2 data, this study reveals extensive phase-space substructures—such as arches, shells, snail shells, and ridges—in the Milky Way's disk stars, indicating the disk is dynamically young and currently out of equilibrium. The findings imply a recent perturbation (300–900 Myr ago), likely from the Sagittarius dwarf galaxy, challenging the long-held assumption of dynamical equilibrium and axisymmetry in the Galactic disk.

ABSTRACT

Most of the stars in our Galaxy including our Sun, move in a disk-like component and give the Milky Way its characteristic appearance on the night sky. As in all fields in science, motions can be used to reveal the underlying forces, and in the case of disk stars they provide important diagnostics on the structure and history of the Galaxy. But because of the challenges involved in measuring stellar motions, samples have so far remained limited in their number of stars, precision and spatial extent. This has changed dramatically with the second Data Release of the Gaia mission which has just become available. Here we report that the phase space distribution of stars in the disk of the Milky Way is full of substructure with a variety of morphologies, most of which have never been observed before. This includes shapes such as arches and shells in velocity space, and snail shells and ridges when spatial and velocity coordinates are combined. The nature of these substructures implies that the disk is phase mixing from an out of equilibrium state, and that the Galactic bar and/or spiral structure are strongly affecting the orbits of disk stars. Our analysis of the features leads us to infer that the disk was perturbed between 300 and 900 Myr ago, which matches current estimations of the previous pericentric passage of the Sagittarius dwarf galaxy. The Gaia data challenge the most basic premise of stellar dynamics of dynamical equilibrium, and show that modelling the Galactic disk as a time-independent axisymmetric component is definitively incorrect. These findings mark the start of a new era when, by modelling the richness of phase space substructures, we can determine the gravitational potential of the Galaxy, its time evolution and the characteristics of the perturbers that have most influenced our home in the Universe.

Motivation & Objective

• To investigate the phase-space distribution of disk stars using high-precision Gaia data to uncover hidden dynamical substructures.
• To determine whether the Milky Way disk is in dynamical equilibrium or has undergone recent perturbations.
• To assess the role of the Galactic bar and spiral structure in shaping stellar orbits.
• To link observed substructures to past gravitational interactions, particularly with the Sagittarius dwarf galaxy.
• To challenge the assumption of time-independent, axisymmetric potential models in Galactic dynamics.

Proposed method

• Analysis of the full phase-space distribution (spatial and velocity coordinates) of stars from Gaia Data Release 2.
• Identification of non-uniform morphological features in velocity space, such as arches, shells, ridges, and snail-shell-like patterns.
• Use of phase-space substructures as diagnostics for ongoing dynamical processes and non-equilibrium states.
• Comparison of observed substructure morphology with theoretical expectations from secular perturbations by the Galactic bar and spiral arms.
• Temporal inference of the perturbation epoch by matching substructure features to known events, such as the Sagittarius dwarf galaxy's pericentric passage.
• Rejection of the assumption of dynamical equilibrium and axisymmetry in modeling the Galactic disk.

Experimental results

Research questions

• RQ1 What types of phase-space substructures are present in the Milky Way disk based on Gaia DR2 data?
• RQ2 What do the morphologies of these substructures reveal about the current dynamical state of the Galactic disk?
• RQ3 When was the last major perturbation of the Milky Way disk, and what could have caused it?
• RQ4 How do the Galactic bar and spiral structure influence the orbital evolution of disk stars?
• RQ5 To what extent does the presence of substructures invalidate the standard assumption of a time-independent, axisymmetric potential in Galactic dynamics?

Key findings

• The Milky Way disk exhibits a rich variety of previously unseen phase-space substructures, including arches, shells, snail shells, and ridges.
• These substructures indicate that the disk is not in dynamical equilibrium and is currently undergoing phase mixing from a non-equilibrium state.
• The morphology of the substructures points to strong perturbations from the Galactic bar and/or spiral structure.
• The disk was likely perturbed between 300 and 900 million years ago, consistent with the timing of the Sagittarius dwarf galaxy's previous pericentric passage.
• The findings directly challenge the foundational assumption in stellar dynamics that the Galactic disk is in a time-independent, axisymmetric equilibrium.
• The observed substructures open a new pathway to reconstruct the gravitational potential of the Galaxy and its time-evolving perturbers.