Most Submillimeter Galaxies Are Major Mergers

We analyze subarcsecond resolution interferometric CO line data for 12 submillimeter-luminous (S_ (850 μm) ≥ 5 mJy) galaxies with redshifts between 1 and 3, presenting new data for 4 of them. Morphologically and kinematically, most of the 12 systems appear to be major mergers. Five of them are well-resolved binary systems, and seven are compact or poorly resolved. Of the four binary systems for which mass measurements for both separate components can be made, all have mass ratios of 1: 3 or closer. Furthermore, comparison of the ratio of compact to binary systems with that observed in local ULIRGs indicates that at least a significant fraction of the compact submillimeter-luminous galaxies (SMGs) must also be late-stage mergers. In addition, the dynamical and gas masses we derive are most consistent with the lower end of the range of stellar masses published for these systems, favoring cosmological models in which SMGs result from mergers. These results all point to the same conclusion that most of the bright SMGs with L_ (IR) ≳ 5 × 10^ (12) L_☉ are likely major mergers.