Towards understanding the dynamics of the bar/bulge region in our Galaxy

TL;DR

This paper reviews the 3D structure of galactic bars, discusses the specific case of the Milky Way's bars, and explores bar formation and evolution through simulations including gas dynamics and cosmological context.

Contribution

It provides a comprehensive analysis of the vertical structure of bars, proposes that the Milky Way's two observed bars are parts of a single bar, and discusses bar formation in gas-rich and cosmological scenarios.

Findings

Bars have a vertically thick inner part and a thin outer part.

The Milky Way's two bars are likely parts of a single bar.

Bar formation is influenced by gas dynamics and cosmological evolution.

Abstract

I review some of the work on bars which is closely linked to the bar/bulge system in our Galaxy. Several independent studies, using totally independent methods, come to the same results about the 3D structure of a bar, i.e., that a bar is composed of a vertically thick inner part and a vertically thin outer part. I give examples of this from simulations and substantiate the discussion with input from orbital structure analysis and from observations. The thick part has a considerably shorter radial extent than the thin part. I then see how this applies to our Galaxy, where two bars have been reported, the COBE/DIRBE bar and the Long bar. Comparing their extents and making the reasonable and necessary assumption that our Galaxy has properties similar to those of other galaxies of similar type, leads to the conclusion that these two bars can not form a standard double bar system. I then discuss arguments in favour of the two bars being simply different parts of the same bar, the COBE/DIRBE bar being the thick inner part and the Long bar being the thin outer part of this bar. I also very briefly discuss some related new results. I first consider bar formation and evolution in disc galaxies with a gaseous component - including star formation, feedback and evolution - and a triaxial halo. Then I consider bar formation in a fully cosmological context using hydrodynamical LCDM simulations, where the host galaxies grow, accrete matter and significantly evolve during the formation and evolution of the bar.