Galaxy Formation and Evolution

Though the formation, and to some extent the evolution of galaxies still is one of the most active research areas in Astrophysics, some ideas are now widely accepted.

After the Big Bang the universe had a period when it was remarkably homogeneous, as can be observed in the Cosmic Background Radiation, the fluctuations of which are less than one part in one hundred thousand.

The questions of galaxy formation and evolution are How, from such a homogeneous universe, did we obtain the very inhomogeneous one we live in?, How did galaxies form?, How have galaxies changed in the past?

The most accepted view is that all the structure we observe today was formed as a consequence of the growing of primordial fluctuations by gravitational instability. Galaxies have formed in a "bottom up" process in which smaller units merge and form larger units. In our epoch, large concentrations of galaxies ("clusters") are still assembling.

There are some problems with this model. For example, recent data strongly suggests that the first galaxies formed as early as 600 million years after the big bang, hardly enough time for the tiny primordial instabilities to grow sufficiently. Also, spiral galaxies cannot be built up by mergers, since events as violent as a merger would completely mix up the fragile structure of the disk.

One way to escape the latter problem is the observation of [High Velocity Clouds]?, clouds of neutral hydrogen that are still ``raining'' down on the Galaxy's disk. With some fiddling models can be constructed in which the bulge? of spiral galaxies forms by mergers, whereas the disk can be regenerated by later infall of gas.

Amend: Giant Ellipticals, Role of Interaction, Cluster vs. Field Galaxies, Heating.

/Talk