GALAXIES II

Groups and Clusters

Galaxies reside in groups, poor clusters, and superclusters. There are also rich clusters.

Our Local Group is a group. It contains some 40 galaxies.

- 2 large spirals
- 1 moderate sized spiral
- 30 odd dwarf ellipticals
- few dwarf irregulars

In rich clusters, the center of the cluster has mainly elliptical galaxies and the outer regions of the rich clusters is comprised of spiral galaxies. The sizes of these clusters are 10 million light years across. X-ray observations show lots of gas between the galaxies in these clusters. This gas is heavily enriched with metals. Furthermore, that this gas stay confined to the clusters means that there is a lot more gravity in the cluster than would be inferred from just counting up the galaxy masses. This suggest a mysterious form of matter we call dark matter.

We watched some movies of galaxies colliding and how super clusters of galaxies are distributed throughout the universe.

That some of these giant elliptical galaxies in the centers of rich clusters have multiple "cores", suggests that they were built up by cannibalizing other smaller galaxies in the cluster.

This has lead to the notion of hierarchical clustering. That is, we believe the biggest structures in the universe were build by the combining together of smaller structures. Gravity pulled them together.