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IMPACT OF THE COMPACT GROUP ENVIRONMENT ON GALAXY EVOLUTION

Compact groups of galaxies (CGs) are extremely dense groups of galaxies, where the galaxy separation is approximately the size of a galaxy. These groups provide a local analog to galaxy interaction in the early universe. The frequent and prolonged gravitational encounters in CGs affect the evolution of member galaxies in myriad ways. To probe this, we have investigated the mid-infrared (MIR) colors of galaxies in CGs, and find evidence for a bimodal distribution of galaxies, with a dearth between quiescent and active galaxy colors. We speculate that the CG environment fosters accelerated evolution of galaxies from star-forming and neutral gasrich to quiescent and neutral gas-poor, yielding a gap in MIR properties. We also analyze the optical colors of CG galaxies and find a significant population of optically red galaxies and relatively few galaxies with blue optical colors. Comparison with the MIR colors indicates that some of these optically red galaxies are old, while others are active but suffer from extinction. Comparison with the optical colors of field galaxies indicates that the stellar populations in compact group galaxies tend to be older than in field galaxies. Importance of Compact Groups Understanding how gas is processed during galaxy interactions and mergers is critical to understanding galaxy evolution in general. Compact groups (CGs) are an incomparable opportunity to study the effects of constant interactions and mergers. Compared to other relatively nearby environments, local CGs most closely reproduce the conditions in the earlier universe when galaxies were assembled (e.g., Baron and White 1987; Ouchi et al. 2005; Stiavelli et al. 2005). Thus, they are a crucial piece of hierarchical structure formation. Moreover, with their resemblance to the early universe, CGs can provide laboratories to study a variety of processes, including the formation of active galactic nuclei (AGN), super star clusters, dwarf galaxies, and gas processing. These groups are among the densest concentrations of galaxies known, comparable to the centers of rich clusters (Hickson 1982). However, CGs have relatively low velocity dispersions, increasing the duration of gravitational interactions and redistributing the gas and dust throughout the system (Hickson et al. 1992). Although these groups are not nearly as massive or luminous as galaxy clusters, they embody an important stage of galaxy evolution, and the lack of detailed understanding of CGs represents a ‘missing link’ in hierarchical galaxy assembly. Evolution within Compact Groups Galaxies evolve differently in different environments. Field galaxies, with no large, nearby neighbors, evolve secularly. Cluster galaxies are quickly stripped of their gas and then evolve without it, mainly affected by the global potential