Secular evolution in disk galaxies

Abstract

Self-gravitating systems evolve toward the most tightly bound configuration that is reachable via available evolution processes. The inner parts shrink and the outer parts expand, provided that some physical process transports energy or angular momentum outward. The evolution of stars, star clusters, protostellar and protoplanetary disks, black hole accretion disks, and galaxy disks are all fundamentally similar. These processes for galaxy disks are the subjects of my lectures and of this Canary Islands Winter School. Part 1 discusses formation, growth, and death of bars. Part 2 details the slow (secular) rearrangement of angular momentum that results from interactions between stars or gas and nonaxisymmetries such as bars. We have a heuristic understanding of how this forms outer rings, inner rings, and stuff dumped into the center. Observations show that barred galaxies have central concentrations of gas and star formation. Timescales imply that they grow central pseudobulges that get mistaken for elliptical-galaxy-like classical bulges but that formed gently out of disks. Part 3 shows how we distinguish between classical and pseudo bulges. Part 4 reviews how environmental secular evolution transforms gas-rich, star-forming spirals and irregulars into gas-poor, red and dead S0 and spheroidal (Sph) galaxies. Sphs are not ellipticals; they have structural parameters like those of low-luminosity S+Im galaxies. So Sphs are bulgeless S0s. Part 5 combines hierarchical clustering and secular evolution into a comprehensive picture.