Optical colors of early-type galaxies in compact groups

Abstract

Compact groups of galaxies with high spatial densities and low velocity dispersions are very unstable to merging. The mergers of spiral galaxies shoud be common in these environments, and if elliptical galaxies are the product of the merger of two spirals, then a population of compact group ellipticals with younger stellar populations than normally found in elliptical galaxies is expected. We have found one candidate elliptical galaxy which has the blue optical colors expected of an elliptical galaxy formed from the recent merger of two gas rich systems. We also have tentative evidence for similar signatures in several other compact group ellipticals. Models of the evolution of the number and color of early-type galaxies in compact groups do not fit the data very well unless it is assumed that the compact groups begin with a mix of galaxy types which has a greater fraction of old, red ellipticals than is observed in the loose groups from which compact groups are thought to originate.