Most Submillimetre Galaxies are Major Mergers

TL;DR

This study uses high-resolution CO line data to show that most bright submillimetre galaxies at redshifts 1-3 are major galaxy mergers, supporting models where such mergers drive their luminosity.

Contribution

It provides new high-resolution morphological and kinematic evidence that the majority of bright SMGs are major mergers, with detailed mass ratio analysis and comparison to local ULIRGs.

Findings

Most SMGs are major mergers based on morphology and kinematics.

Binary systems have mass ratios of 1:3 or closer.

A significant fraction of compact SMGs are late-stage mergers.

Abstract

We analyse subarcsecond resolution interferometric CO line data for twelve sub-millimetre-luminous (S850um > 5mJy) galaxies with redshifts between 1 and 3, presenting new data for four of them. Morphologically and kinematically most of the twelve 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 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, favouring cosmological models in which SMGs result from mergers. These results all point to the same conclusion, that likely most of the bright SMGs with L_IR > 5x10e12L_sun are major mergers.