The Milky Way disk

TL;DR

This review discusses current debates on the structure of the Milky Way disk, including the nature of the thick disk, spiral structure inference challenges, and the properties of the outer disk, highlighting unresolved issues and conflicting interpretations.

Contribution

It provides a comprehensive overview of recent research topics on the Galactic disk, emphasizing the need for clearer models and better data interpretation.

Findings

Evidence supports a dual Galactic disk with thin and thick components.

Spiral structure inference is highly dependent on tracers and models.

There is significant disagreement on the size and outer disk features.

Abstract

This review summarises the invited presentation I gave on the Milky Way disc. The idea underneath was to touch those topics that can be considered hot nowadays in the Galactic disk research: the reality of the thick disk, the spiral structure of the Milky Way, and the properties of the outer Galactic disk. A lot of work has been done in recent years on these topics, but a coherent and clear picture is still missing. Detailed studies with high quality spectroscopic data seem to support a dual Galactic disk, with a clear separation into a thin and a thick component. Much confusion and very discrepant ideas still exist concerning the spiral structure of the Milky Way. Our location in the disk makes it impossible to observe it, and we can only infer it. This process of inference is still far from being mature, and depends a lot on the selected tracers, the adopted models and their limitations, which in many cases are neither properly accounted for, nor pondered enough. Finally, there are very different opinions on the size (scale length, truncation radius) of the Galactic disk, and on the interpretation of the observed outer disk stellar populations in terms either of external entities (Monoceros, Triangulus-Andromeda, Canis Major), or as manifestations of genuine disk properties (e.g., warp and flare).