| Abstract: | Isolated self-gravitating systems evolve by spreading -- the inner parts shrink while the outer parts expand -- provided that some physical process can transport energy or angular momentum efficiently. The underlying reason is that self-gravitating systems have negative specific heats. As a result, the evolution of stars, protoplanetary disks, star clusters, and galaxy disks are fundamentally similar. I begin with this general evolution context and then concentrate on the consequent, slow evolution of isolated galaxy disks. In particular, I focus on the distinction between dense central parts of galaxies ("bulges") that are made quickly and violently by galaxy mergers and "pseudobulges" that are grown slowly and gently out of disks. I summarize observational and numerical evidence that isolated disks evolve by outward transport of angular momentum. The consequences explain virtually all of the regular structure seen in disk galaxies, including pseudobulges. Thus a large variety of observational and theoretical results contribute to a new picture of galaxy evolution that complements our well developed theory of gravitational clustering and merging. However, these results challenge that theory, because it cannot explain galaxies that are completely bulgeless. Galaxy mergers are expected to happen so often that every big galaxy should have a bulge. But we observe many bulgeless galaxies. Now we realize that many dense centers of galaxies that we used to think are bulges were not made by mergers. Rather, they were grown out of disks. So the challenge gets more difficult. Giant pure-disk galaxies like our Milky Way are especially hard to understand, because dark matter halos grow by merging, and our Galaxy contains a very big halo. Yet such systems dominate over merger remnants in the local Universe by about 2:1. This is the biggest problem currently faced by our theory of galaxy formation. |
