Pairing statistics for infrared-luminous galaxies

A knowledge of the pairing fraction of IR-selected galaxies is important in understanding the role of interactions in star formation and nuclear activity. It is clear that interactions are extremely common among the most IR-luminous systems, but results have been inconsistent for less luminous systems; this may result from using peculiar morphology as an indicator of an interaction. Since features associated with milder interactions (e.g., tidal tails) tend to be faint, their detection necessarily depends on the depth of the survey used to identify interacting pairs. We present a sample of IR- and optically-selected galaxies for which we have computed pairing fractions using strictly geometrical, quantitative criteria. For the analysis we used a likelihood ratio technique developed by Keel and van Soest (1992). We find that the pairing fraction for IR-selected galaxies is roughly 30 percent, comparable to that found by Keel and van Soest for optically-selected starbursts. We also detect a strong correlation (r above 0.99) between the 60 micron luminosity of a galaxy sample and its pairing fraction. We conclude that although tidal interactions do not guarantee enhanced activity (AGN or starburst), they can amplify internal properties that would feed such activity.