On the nature of bulges in general and of box/peanut bulges in particular. Input from $N$-body simulations

Abstract

Objects designated as bulges in disc galaxies do not form a homogeneous class. I distinguish three types. The classical bulges, whose properties are similar to those of ellipticals and which form by collapse or merging. Boxy and peanut bulges, which are seen in near edge-on galaxies and which are in fact just a part of the bar seen edge-on. Finally disc-like bulges, which result from the inflow of (mainly) gas to the center-most parts, and subsequent star formation. I make a detailed comparison of the properties of boxy and peanut bulges with those of $N$-body bars seen edge-on and answer previously voiced objections about the links between the two. I also present and analyse simulations where a boxy /peanut feature is present at the same time as a classical spheroidal bulge and compare them with observations. Finally, I propose a nomenclature that can help distinguish between the three types of bulges and avoid considerable confusion.