Bulge components of disc galaxies are the high-density centers interior to their outer discs. Once thought to be equivalent to elliptical galaxies, their observed properties and formation histories turn out to be richer and more varied than those of ellipticals. This book reviews progress in many areas of bulge studies. Two advances deserve emphasis: (1) Observations divide bulges into "classical bulges" that look indistinguishable from ellipticals and "pseudobulges" that are discier and (except in S0s) more actively star-forming than are ellipticals. Classical bulges and ellipticals are thought to form by major galaxy mergers. Discy pseudobulges are a product of the slow ("secular") evolution of galaxy discs. Nonaxisymmetries such as bars and oval distortions transport some disc gas toward the center, where it starbursts and builds a dense central component that is discier in structure than are classical bulges. Secular evolution explains many regular structures (e.g., rings) seen in galaxy discs. It is a new area of galaxy evolution work that complements hierarchical clustering. (2) Studies of high-redshift galaxies reveal that their discs are so gas-rich that they are violently unstable to the formation of mass clumps that sink to the center and merge. This is an alternative channel for the formation of classical bulges.This chapter summarizes big-picture successes and unsolved problems in the formation of bulges and ellipticals and their coevolution (or not) with supermassive black holes. I present an observer's perspective on simulations of cold dark matter galaxy formation including baryonic physics. Our picture of the quenching of star formation is becoming general and secure at redshifts z < 1. I conclude with a list of major uncertainties and problems. The biggest challenge is to produce realistic bulges + ellipticals and realistic discs that overlap over a factor of > 1000 in mass but that differ from each other as we observe over that whole range. A related difficulty is how hierarchical clustering makes so many giant, bulgeless galaxies in field but not cluster environments. I present arguments that we rely too much on star-formation feedback and AGN feedback to solve these challenges.
We retrace the development of a kinematic scaling relation, referred to as the Fundamental Manifold (FM), that addresses shortcomings of the commonly used fundamental plane in certain contexts. We then examine whether bulges separately satisfy the FM relation and discuss what the success or failure of such a match implies for the nature of classical- and pseudo-bulges. On the basis of this preliminary analysis we suggest that while classical bulges appear to be independent, dynamically complete subsystems within their host galaxies that satisfy the scaling relation, pseudobulges do not satisfy the scaling relation and so probably consist of an unrepresentative subset of disc stars. This is currently not a unique explanation of the results, but the use of kinematic scaling relations with larger samples, done in a more systematic manner, could lead to a more definitive resolution on the nature of bulges.
Most every graduate student in astronomy today knows what the "bulge" of a spiral galaxy is by name and what it looks like. A century ago few professional astronomers knew, or even cared. We trace the early and quite casual usage of the term (and its competitors) and then follow the later and gradually more strict adoption of the term "bulge", needed to call out a major component of galaxy morphology. In the specific context of the Milky Way "The Galactic Bulge" was a proper name. Only later, as they were seen and measured in other galaxies, were these same structural features to become known generically as "bulges". That term finally won out over its more unwieldy competitors such as the "amorphous central region", the "unresolved nuclear region" or the highly ambiguous term "central nucleus".
