This final chapter summarizes areas of major progress in understanding galaxy bulges and tries to distill the important unresolved issues that need further work.
I do not revisit the subjects covered by all chapters – Madore (2015: historical review), Méndez-Abreu (2015: intrinsic shapes), Falcón-Barroso (2015: kinematic observations), Sánchez-Blázquez (2015: stellar populations), Laurikainen & Salo (2015: observations of boxy bulges), Athanassoula 2015: modeling of boxy bulges), Gonzalez & Gadotti (2015: observations of the Milky Way boxy bulge), Shen & Li (2015: modeling of the Milky Way boxy bulge), Cole & Debattista (2015: nuclear star clusters), and Combes (2015: bulge formation within MOND). I comment briefly on Zaritsky's (2015) chapter on scaling relations.
I concentrate in this summary chapter on three main areas of progress and on two main areas where there are unresolved difficulties:
Two additions to our picture of bulge formation are (1) formation by massive clump instabilities in high-z disks; Bournaud (2015) develops this story, but it deserves emphasis here, too, and (2) our picture of secular evolution of galaxy disks that produces two distinct kinds of dense central components in galaxies, disky pseudobulges (reviewed here by Fisher & Drory 2015) and boxy pseudobulges (discussed in four chapters listed above). Both deserve emphasis here, too.
The main areas with unresolved issues come in two varieties:
Probably the most important chapter in this book is Brooks & Christensen (2015) on the modeling of galaxy–and thus also bulge–formation. These models that add baryonic physics to giant n-body simulations of the hierarchical clustering of cold dark matter (CDM) in a ΛCDM universe define the state of the art in the most general version of galaxy formation theory. Much has been accomplished, and progress is rapid. Brooks & Christensen (2015) is an excellent review of the state of the art as seen by its practitioners. In this chapter, I would like to add the viewpoint of an observer of galaxy archaeology. I suggest a slightly different emphasis on the successes and shortcomings of present models. My main purpose is to promote a dialog between theorists and observers that may help to refine the observational constraints that are most telling and the modeling exercises that may be most profitable. Baryonic galaxy formation is an extraordinarily rich and difficult problem. Many groups struggle honorably and carefully with different aspects of it. In this subject, besides a strong push on remaining limitations such as resolution, the main need seems to me to be a broader use of observational constraints and a consequent refinement of the physics that may succeed in explaining them.
A second issue involves Graham's (2015) chapter on supermassive black holes. It is inconsistent with all other work that I am aware of on this subject, including McConnell & Ma (2013) and Kormendy & Ho (2013). Section 6 summarizes this subject using results from Kormendy & Ho (2013, hereafter KH13).
Section 7 reviews the quenching of star formation in galaxies. Many different lines of research are converging on a consistent picture of how quenching happens.
Finally, I conclude with a personal view of the most important, big-picture issues that are still unsolved by our developing picture of galaxy evolution.