PAIRING STATISTICS FOR INFRARED-LUMINOUS GALAXIES

A knowledge of the pairing fraction of IR-selected galaxies is importa nt in understanding the role of interactions in star formation and nuc lear activity. It is clear that interactions are extremely common amon g the most IR-luminous systems [Sanders et al., ApJ, 325, 74 (1988)], but results have been inconsistent for less luminous systems. It is li kely that these inconsistencies are the result of using peculiar morph ology as an indicator of an interaction. Since features associated wit h milder interactions (e.g., tidal tails) tend to be faint, their dete ction necessarily depends on the depth of the survey used to identify interacting pairs and, more importantly, the ''eye of the beholder.'' Here we present a sample of IR- and optically-selected galaxies for wh ich we have computed pairing fractions using strictly geometrical, qua ntitative criteria. For the analysis we used a likelihood ratio techni que developed by Keel & van Soest [A&AS, 94, 553 (1992)]. We find that the pairing fraction for IR-selected galaxies is roughly 30%, compara ble to that found by Keel and van Soest for optically-selected starbur sts. We also detect a strong correlation (r > 0. 99) between the 60 mu m luminosity of a galaxy sample and its pairing fraction. We conclude that although tidal interactions do not guarantee enhanced activity (A GN or starburst), they can amplify internal properties that would feed such activity.