Kinematics and dynamics of the Galactic disc

TL;DR

This paper reviews various hypotheses explaining the Milky Way disc's complex features and kinematic anomalies, emphasizing the role of intergalactic matter accretion and gravitational interactions in shaping the galaxy's structure.

Contribution

It provides a comprehensive review of the potential causes of Galactic disc anomalies, highlighting the significance of intergalactic matter accretion and gravitational effects.

Findings

Galactic features are sensitive to internal and external forces.

Accretion of intergalactic matter explains many observed anomalies.

Other hypotheses only partially account for the observations.

Abstract

The Milky Way disc presents a warp, a flare, lopsidedness and other deviations from a purely axisymmetrical double exponential density component, both for the stellar and the gas component. Moreover, recent large-scale extended kinematics maps of Gaia data have shown significant departures from circularity in the mean orbits with radial Galactocentric velocities between -20 and +20 km/s and vertical velocities between -10 and +10 km/s, asymmetries northern/southern and other anomalies. The observed features are sensitive to internal and external forces, especially at the largest Galactocentric distances. Here we review some of the Galactic dynamics hypotheses that explain these anomalies: either in terms of gravitational interaction, magnetic fields, accretion of intergalactic matter or others. The gravitational interaction may be among the different components of the Galaxy or between the Milky Way and another companion galaxy. The accretion of intergalactic matter may be either into the halo, with a later gravitational interaction between the misaligned halo and the disc, or directly onto the disc. Most of the relevant observations are explained in terms of the accretion of intergalactic matter onto a disc that is far from a simple stationary configuration in rotational equilibrium. Other hypotheses only partially explain some observations.