[Paper Review] Milky Way dynamics in light of Gaia

John Douglas Hunt, Eugene Vasiliev|arXiv (Cornell University)|Jan 7, 2025

Geomagnetism and Paleomagnetism StudiesBiochemistry, Genetics and Molecular Biology5 citations

TL;DR

A Gaia-centric review of Milky Way dynamics highlighting disc, bar/bulge, and halo structure and kinematics, disequilibrium processes, and ongoing challenges in constraining the Galactic potential.

ABSTRACT

The Gaia mission has triggered major developments in the field of Galactic dynamics in recent years, which we discuss in this review. The structure and kinematics of all Galactic components - disc, bar/bulge and halo - are now mapped in great detail not only in the Solar neighbourhood, but across a large part of the Milky Way. The dramatic improvements in the coverage and precision of observations revealed various disequilibrium processes, such as perturbations in the Galactic disc and the deformations of the outer halo, which are partly attributed to the interaction with satellite galaxies. The knowledge of the gravitational potential at all scales has also advanced considerably, but we are still far from having a consistent view on the key properties of the Galaxy, such as the bar pattern speed or the mass profile and shape of the dark halo. The complexity and interplay of several dynamical processes makes the interpretation of observational data challenging, and it is fair to say that more theoretical effort is needed to fully reap the fruit of the Gaia revolution.

Motivation & Objective

• Summarize how Gaia and complementary surveys map the Milky Way's structure and kinematics across components (disc, bar/bulge, halo).
• Highlight disequilibrium processes revealed by Gaia data and their implications for dynamics.
• review methods and challenges in constraining the Galactic gravitational potential and mass distribution.

Proposed method

• Survey-driven synthesis of Gaia data products (positions, parallaxes, proper motions, line-of-sight velocities) and complementary photometric/spectroscopic surveys.
• Discussion of distance estimation methods and their priors (parallax-based, photogeometric, and spectrophotometric distances).
• Assessment of selection functions and their impact on kinematic inferences and dynamical modelling.
• Integration of structural/component-focused analyses (disc, bar/spiral arms, halo) with dynamical mechanisms (equilibrium and disequilibrium processes).
• Outline of mass-modelling techniques and current constraints on Milky Way mass distribution and shape.

Figure 1: Major photometric surveys of the Milky Way. Sky coverage of four optical surveys (SDSS, PanSTARRS, Legacy surveys comprising DECaLS, DES and a few others, and DECaPS) is shown as shaded regions in four panels. Footprints of near-IR surveys of the bulge and disc region (VVV and its extensio

Experimental results

Research questions

• RQ1 What does Gaia reveal about the equilibrium structure and kinematics of the Galactic disc, including radial and vertical velocity gradients?
• RQ2 How do non-axisymmetric features (bar, spiral arms) influence local and remote dynamics, and what Gaia data show about these effects?
• RQ3 What are the characteristics and origins of vertical perturbations (warp, waves, Gaia phase spiral) in the disc?
• RQ4 How is the stellar halo characterized kinematically and spatially with Gaia, and what does this imply for halo formation?
• RQ5 What are the current constraints and uncertainties in the Milky Way's mass distribution and dark halo shape based on Gaia-era analyses?

Key findings

• Gaia enables detailed mapping of disc, bar/bulge, and halo kinematics across large Galactic volumes.
• Disequilibrium processes and perturbations, partly from satellite interactions, are now evident in Gaia data.
• Advances in modelling the Galactic potential are substantial, but key properties like bar pattern speed and dark halo mass profile/shape remain uncertain.
• Interplay of multiple dynamical processes complicates interpretation, necessitating further theoretical development.
• Selection functions and distance estimation methods critically affect kinematic inferences and dynamical modelling.

Figure 2: Major spectroscopic surveys of the Milky Way. Left panels show the spatial coverage of each dataset in galactic coordinates; top right panel shows the velocity uncertainty as a function of $G$ -band magnitude, and bottom right panel shows the distribution of magnitudes.