| Abstract
| - The Dragonfly Galaxy (MRC 0152-209), at redshift z~ 2, is one of the most vigorously star-forming radio galaxies in the Universe. What triggered its activity? We present ALMA Cycle 2 observations of cold molecular CO(6 −5) gas and dust, which reveal that this is likely a gas-rich triple merger. It consists of a close double nucleus (separation ~4 kpc) and a weak CO-emitter at ~10 kpc distance, all of which have counterparts in HST/NICMOS imagery. The hyper-luminous starburst and powerful radio-AGN were triggered at this precoalescent stage of the merger. The CO(6 −5) traces dense molecular gas in the central region, and complements existing CO(1 −0) data, which reveal more widespread tidal debris of cold gas. We also find ~10 10M⊙ of molecular gas with enhanced excitation at the highest velocities. At least 20 −50 % of this high-excitation, high-velocity gas shows kinematics that suggests it is being displaced and redistributed within the merger, although with line-of-sight velocities of | v| < 500 km s -1, this gas will probably not escape the system. The processes that drive the redistribution of cold gas are likely related to either the gravitational interaction between two kpc-scale discs, or starburst/AGN-driven outflows. We estimate that the rate at which the molecular gas is redistributed is at least [Ṁentity!#x2009!] ~ 1200 ± 500 M⊙ yr -1, and could perhaps even approach the star formation rate of ~3000 ± 800 M⊙ yr -1. The fact that the gas depletion and gas redistribution timescales are similar implies that dynamical processes can be important in the evolution of massive high- z galaxies.
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