Evolution of Galaxies in Compact Groups

We investigate the role of mergers and interactions in the evolution of galaxies by studying galaxies in compact groups. Compact groups of galaxies have high spatial densities and low velocity dispersions making these regions ideal laboratories in which to study the effect of interactions and mergers on galaxies. We present a large multicolor survey of the elliptical galaxies in compact groups and show that several of the elliptical galaxies in these groups have unusually blue colors indicative of recent interactions or mergers. We also find that a small fraction of galaxies in these groups are currently merging, based on their disturbed optical morphologies, warm far-infrared colors, and unusual dynamics. However, unusually blue elliptical galaxies and currently merging systems are relatively rare, indicating that the time scale for a compact group to evolve through merging into a single object is at least 3 Gyr. Although merging is not occurring rapidly, the elliptical galaxies appear to be affected by tidal interactions within the group. We find that the compact group elliptical galaxies are less likely than similar galaxies in other environments to have boxy isophotes and more likely to have irregular isophotal shapes indicative of a recent tidal encounter. The most striking difference between galaxies in compact groups and those in other regions is that the elliptical galaxies in compact groups appear to have systematically low velocity dispersions compared to other elliptical galaxies of the same effective radii, average surface brightness, absolute magnitudes, or colors. Therefore, they do not lie on the fundamental plane defined by elliptical galaxies in other environments.