Cosmic Cannibalism (3)

The simulation of galaxy mergers to produce a giant elliptical galaxy near the center of a cluster that was shown on the previous page is investigated further in the two animations shown on this page. These animations illustrate the structure of the central region of the cluster at two different times. The time is measured in terms of a redshift parameter z that will be defined in Chapter 24, but for our purposes now we only need to know that z = 0 corresponds to the present time and z = 0.35 corresponds to a time of about 2 billion years ago. The animations rotate the cluster through 360 degrees to permit a detailed view of its structure at the two times.

Evolution of the Cluster
In the top right animation the structure of the cluster about 2 billion years ago is illustrated (z = 0.35). As you rotate the cluster to view it from all directions, notice that the giant elliptical galaxy in the center is already taking shape. However, the cluster has a rather ragged appearance. The central blob has more than one nucleus, and tidal tails can be seen for several of the smaller galaxies. Contrast this with the lower left animation, which shows the simulation after 10 billion years (z = 0, which corresponds to the present time). Now the giant elliptical galaxy is completely formed and the structure of the cluster is much more orderly. In particular, there appears to be only a single nucleus for the giant elliptical, and there is less evidence of tidal interactions for the smaller galaxies that have survived the collisions and mergers of the previous 10 billion years.
Symmetry and Ages of Galaxy Clusters
The trends illustrated in the two animations on this page and the one on the preceding page are valid generally for structures such as clusters and superclusters. The younger the cluster, the more ragged the appearance, because interactions over time tend to make clusters more symmetric. We shall use this observation in Chapter 27 to argue that larger structures like clusters and superclusters presently appear to be younger than the galaxies that they contain because many are not very symmetric. Therefore, we shall conclude that large-scale structure was assembled from the "bottom up", by collisions and mergers of smaller structures such as those illustrated in this module.