Published in "Galactic Bulges", Astrophysics and Space Science Library, Volume 418. ISBN 978-3-319-19377-9. Springer International Publishing Switzerland, 2016, p. 431.

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John Kormendy

Department of Astronomy, University of Texas at Austin, 2515 Speedway, Mail Stop C1400, Austin, Texas 78712-1205, USA

Abstract: Bulge components of disk galaxies are the high-density centers interior to their outer disks. 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 diskier and (except in S0s) more actively star-forming than are ellipticals. Classical bulges and ellipticals are thought to form by major galaxy mergers. Disky pseudobulges are a product of the slow (“secular”) evolution of galaxy disks. Nonaxisymmetries such as bars and oval distortions transport some disk gas toward the center, where it starbursts and builds a dense central component that is diskier in structure than are classical bulges. Secular evolution explains many regular structures (e.g., rings) seen in galaxy disks. It is a new area of galaxy evolution work that complements hierarchical clustering. (2) Studies of high-redshift galaxies reveal that their disks 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'ss 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 disks 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.

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