Boxy/peanut and discy bulges: formation, evolution and properties

Abstract

AbstractThe class ‘bulges’ contains objects with very different formation and evolution paths and very different properties. I review two types of ‘bulges’, the boxy/peanut bulges (B/Ps) and the discy bulges. The former are parts of bars seen edge-on, have their origin in vertical instabilities of the disc and are somewhat shorter in extent than bars. Their stellar population is similar to that of the inner part of the disc from which they formed. Discy bulges have a disc-like outline, i.e., seen face-on they are circular or oval and seen edge-on they are thin. Their extent is of the order of 5 times smaller than that of the boxy/peanut bulges. They form from the inflow of mainly gaseous material to the centre of the galaxy and from subsequent star formation. They thus contain a lot of young stars and gas. Bulges of different types often coexist in the same galaxy. I review the main known results on these two types of bulges and present new simulation results.B/Ps form about 1Gyr after the bar, via a vertical buckling. At that time the bar strength decreases, its inner part becomes thicker – forming the peanut or boxy shape – and the ratio $\sigma_z^2/\sigma_r^2$ increases. A second buckling episode is seen in simulations with strong bars, also accompanied by a thickening of the peanut and a weakening of the bar. The properties of the B/Ps correlate strongly with those of the bar: stronger bars have stronger peanuts, a more flat-topped vertical density distribution and have experienced more bucklings.I also present simulations of disc galaxy formation, which include the formation of a discy bulge. Decomposition of their radial density profile into an exponential disc and a Sérsic bulge gives realistic values for the disc and bulge scale-lengths and mass ratios, and a Sérsic shape index of the order of 1.It is thus clear that classical bulges, B/P bulges and discy bulges are three distinct classes of objects and that lumping them together can lead to confusion. To avoid this, the two latter could be called B/P features and inner discs, respectively.