Chapter 6 - The Effect of the Cluster Environment on Galaxies

Summary

Abstract. Various observations indicate that the cluster environment can affect the structure and dynamics of galaxies. This review concentrates on the effect the environment can have on three of the most basic properties of a galaxy; the morphological type, the size, and the distribution of mass. A reexamination of the morphology - density relation suggests that the fundamental driver may be related to some global property of the cluster, such as the distance from the cluster center, rather than some local property, such as membership in a local subclump within the cluster. While there is good evidence that the size of a galaxy can be increased (i.e., cD galaxies) or decreased (i.e., early type galaxies near the centers of clusters) by the cluster environment, it is not clear what physical mechanism is responsible. There is tentative evidence that rotation curves of spiral galaxies near the centers of clusters are falling, perhaps indicating that the dark halo has been stripped off. Rotation curves for spiral galaxies in compact groups are even more bizarre, providing strong evidence that the group environment has affected the kinematics of these galaxies.

INTRODUCTION

Perhaps the three most basic questions an extragalactic astronomer might be asked are:

1. Why are some galaxies flattened into disks while others are elliptical in shape?

2. How big are galaxies?

3. How massive are galaxies?

Although we can fill journals with details about galaxies, an astronomer cannot really answer these three basic questions with any confidence.