Large-Scale Structure and cD Galaxies:

If galaxy cannibalism occurs after cluster virialization, then the shape and orientation of the supergiant cD galaxy will be random. But, it cannibalism occurs very early in a clusters lifetime, we expect to see a `memory' of the large-scale structure of the Universe on the shape of the cD galaxy.

Work in progress (Schombert and West 1998) has shown just that. The shape of the supergiant cD galaxy aligns with its nearest neighbors, the cluster shape, and the filaments of large-scale structure.

The fact that this memory of large-scale structure still exists in the cores of clusters indicates that the Universe is `dynamically young'. That clusters are still in the process of evolving.

A similar, but surprising result, is that small scale structure within the supergiant galaxy is also aligned with large-scale structure.

For example, the radio lobes from the core of cD galaxies is aligned with the shape of the outer potential isophotes. Since the radio jets are expelled from the top and bottom of the accretion disk surrounding a galactic mass black hole, then this implies that incoming fuel for the accretion disk is also aligned with the isophotes, i.e. cannibalized galaxies.

Summary:

  • Galaxy collisions and mergers, once thought to be a curiosity for N-body simulations, now appears to be a significant component to the evolution of galaxies.

  • Aside from being probes into the internal kinematics of galaxies, dynamical evolution is also a peek into the process that forms clusters and defines the Hubble sequence in rich environments (most of what we know about the past is from studies of rich environments at high redshift).

  • A large fraction of dynamical evolution occurs at very early epochs, such as the formation of supergiant cD galaxies. Memory of that era is still visible in the shape and orientation of cD galaxies.

  • Our understanding of galaxy evolution is incomplete if these early mergers are not accounted for in our galaxy models. Particularly the fact that the Hubble sequence may be the consequence of early galaxy interactions which then reflect into star formation rates, which are the underlying physical basis for the Hubble sequence.