The Role of Galaxy Interaction in the SFR-M _* Relation: Characterizing Morphological Properties of Herschel-selected Galaxies at 0.2 < z < 1.5

Abstract

Galaxy interactions/mergers have been shown to dominate the population of IR-luminous galaxies (L _IR >~ 10^11.6 L _⊙) in the local universe (z <~ 0.25). Recent studies based on the relation between galaxies' star formation rates and stellar mass (the SFR-M _* relation or the "galaxy main sequence") have suggested that galaxy interaction/mergers may only become significant when galaxies fall well above the galaxy main sequence. Since the typical SFR at a given M _* increases with redshift, the existence of the galaxy main sequence implies that massive, IR-luminous galaxies at high z may not necessarily be driven by galaxy interactions. We examine the role of galaxy interactions in the SFR-M _* relation by carrying out a morphological analysis of 2084 Herschel-selected galaxies at 0.2 < z < 1.5 in the COSMOS field. Using a detailed visual classification scheme, we show that the fraction of "disk galaxies" decreases and the fraction of "irregular" galaxies increases systematically with increasing L _IR out to z <~ 1.5 and z <~ 1.0, respectively. At L _IR >10^11.5 L _⊙, >~ 50% of the objects show evident features of strongly interacting/merger systems, where this percentage is similar to the studies of local IR-luminous galaxies. The fraction of interacting/merger systems also systematically increases with the deviation from the SFR-M _* relation, supporting the view that galaxies falling above the main sequence are more dominated by mergers than the main-sequence galaxies. Meanwhile, we find that >~ 18% of massive IR-luminous "main-sequence galaxies" are classified as interacting systems, where this population may not evolve through the evolutionary track predicted by a simple gas exhaustion model.