Boxy/peanut/X bulges, barlenses and the thick part of galactic bars: What are they and how did they form?

TL;DR

This paper reviews the formation, evolution, and dynamics of three-dimensional structures in galactic bars, such as boxy/peanut/X bulges and barlenses, using simulations, orbital theory, and observations.

Contribution

It provides a comprehensive overview of the origins and development of thick bar components in galaxies, integrating theoretical and observational insights.

Findings

Thick bar components result from disc and bar instabilities.

Simulations and orbital theory explain the formation of boxy/peanut/X bulges and barlenses.

Observations support the connection between these structures and galaxy dynamics.

Abstract

Bars have a complex three-dimensional shape. In particular their inner part is vertically much thicker than the parts further out. Viewed edge-on, the thick part of the bar is what is commonly known as a boxy-, peanut- or X- bulge and viewed face-on it is referred to as a barlens. These components are due to disc and bar instabilities and are composed of disc material. I review here their formation, evolution and dynamics, using simulations, orbital structure theory and comparisons to observations.