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

TL;DR

This review summarizes recent advances in understanding galaxy bulges, highlighting the distinction between classical bulges formed by mergers and disky pseudobulges formed through secular evolution, and discusses ongoing challenges in galaxy formation modeling.

Contribution

It emphasizes the observational similarity between classical bulges and ellipticals, and introduces secular evolution as a key process in bulge formation, contrasting it with hierarchical clustering.

Findings

Classical bulges are indistinguishable from ellipticals and form via major mergers.

Disky pseudobulges are formed through secular evolution involving bars and gas inflows.

High-redshift disk instabilities can lead to classical bulge formation.

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.