Observed properties of boxy/peanut/barlens bulges

TL;DR

This review discusses the properties, detection methods, and structural significance of boxy/peanut-shaped bulges and barlenses in galaxies, highlighting their prevalence and role in galaxy evolution.

Contribution

It synthesizes observational evidence and structural analyses to clarify the nature and importance of B/P bulges and barlenses in galaxy morphology.

Findings

Nearly half of nearby edge-on galaxies have B/P bulges.

Barlenses are face-on counterparts of B/P bulges.

B/P/barlens structures account for most bulge light in galaxies.

Abstract

We review the observed morphological, photometric, and kinematic properties of boxy/peanut (B/P) shape bulges. Nearly half of the bulges in the nearby edge-on galaxies have these characteristics, which fraction is similar to the observed bar fraction in Hubble types earlier than Scd. B/P bulges are generally detected in the edge-on view, but it has been recently demonstrated that barlenses, which are lens-like structures embedded in bars, are the more face-on counterparts of the B/P bulges. Multi-component structural decompositions have shown that B/P/barlens structures are likely to account for most of the bulge light, including the early-type disks harboring most of the bulge mass in galaxies. Cool central disks are often embedded in the B/P/barlens bulges. Barred galaxies contain also dynamically hot classical bulges, but it is not yet clear to what extent they are really dynamically distinct structure components, and to what extent stars wrapped into the central regions of the galaxies during the formation and evolution of bars. If most of the bulge mass in the Milky Way mass galaxies in the nearby universe in- deed resides in the B/P-shape bulges, and not in the classical bulges, that idea needs to be integrated into the paradigm of galaxy formation.