On the nature of bulges in general and of box/peanut bulges in particular: input from N ‐body simulations

Objects designated as bulges in disc galaxies do not form a homogeneous class. I distinguish three types: the classical bulges, the properties of which 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; and, finally, disc‐like bulges, which result from the inflow of (mainly) gas to the centre‐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 to distinguish between the three types of bulges and avoid considerable confusion.