The Role of Bars for Secular Bulge Formation

Summary

The purpose of this paper is to describe the dynamics in bars as predicted by classical mechanics, and its relationship with bulge structure and evolution. From physical ground the tight dynamical relationship between small bulges (spheroidal looking structures about as large or smaller than the disk scale-length) and bars leads to doubt that they are decoupled and dynamically independent structures, as often assumed in bulge-disk decompositions or bulge modeling. For big bulges (spheroidal structures much larger than the disk scale-length) the link with bars is looser or indirect, such as a common origin through a merger. Simulations show that big bulges may result from mergers any time after disk formation. Therefore, dynamics indicates that the age of bulges is not a generally well defined concept for bulges, because the formation of a large fraction of bulges can occur much later than the formation of their present material content, the stars. More likely, if bulges are made for a substantial part of a mixture of disk material older than 1 Gyr (otherwise they would not be called bulges) the age spread of the stars is comparable to the age of the oldest stars, thus the ill-defined definition of bulge ages. In turn, the better understanding of all the possible origins of bulges contributes to a revision of the interpretation of the Hubble sequence, which appears as a broad aging sequence of individual galaxies, from late to early types. […]