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.