The Dragonfly Galaxy: II. ALMA unveils a triple merger and gas exchange in a hyper-luminous radio galaxy at z = 2

B. H.C. Emonts, C. De Breuck, M. D. Lehnert, J. Vernet, B. Gullberg, M. Villar-Martin, N. Nesvadba, G. Drouart, R. Ivison, N. Seymour, D. Wylezalek, P. Barthel

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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 ~1010 M 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 M ~ 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.

Original languageEnglish
Article numberA99
JournalAstronomy and Astrophysics
Publication statusPublished - 30 Nov 2015


  • Galaxies: active
  • Galaxies: high-redshift
  • Galaxies: interactions
  • Infrared: galaxies
  • Submillimeter: galaxies


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