paper

Elliptical Galaxies and Bulges of Disk Galaxies: Summary of Progress and Outstanding Issues

arXiv:1504.03330 · doi:10.1007/978-3-319-19378-6_16

Abstract

This is the summary chapter of a review book on galaxy bulges. Bulge properties and formation histories are more varied than those of ellipticals. I emphasize two advances: 1 - "Classical bulges" are observationally indistinguishable from ellipticals, and like them, are thought to form by major galaxy mergers. "Disky pseudobulges" are diskier and more actively star-forming (except in S0s) than are ellipticals. Theys are products of the slow ("secular") evolution of galaxy disks: bars and other nonaxisymmetries move disk gas toward the center, where it starbursts and builds relatively flat, rapidly rotating components. This secular evolution is a new area of galaxy evolution work that complements hierarchical clustering. 2 - Disks of high-redshift galaxies are unstable to the formation of mass clumps that sink to the center and merge - an alternative channel for the formation of classical bulges. I review successes and unsolved problems in the formation of bulges+ellipticals and their coevolution (or not) with supermassive black holes. I present an observer's perspective on simulations of dark matter galaxy formation including baryons. I review how our picture of the quenching of star formation is becoming general and secure at redshifts z < 1. The biggest challenge is to produce realistic bulges+ellipticals and disks that overlap over a factor of 10**3 in mass but that differ from each other as observed over that whole range. Second, how does hierarchical clustering make so many giant, bulgeless galaxies in field but not cluster environments? I argue that we rely too much on AGN and star-formation feedback to solve these challenges.

46 pages, 10 postscript figures, accepted for publication in Galactic Bulges, ed. E. Laurikainen, R. F. Peletier, & D. A. Gadotti (New York: Springer), in press (2015)

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