The Dragonfly Galaxy

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

¹ Centro de Astrobiología (CSIC-INTA), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
e-mail: bjornemonts@gmail.com
² European Southern Observatory, Karl Schwarzschild Strasse 2, 85748 Garching, Germany
³ CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France
⁴ Sorbonne Universités, UPMC Université Paris VI, Institut d’Astrophysique de Paris, 75014 Paris, France
⁵ Astro-UAM, UAM, Unidad Asociada CSIC, Facultad de Ciencias, Campus de Cantoblanco, 28049 Madrid, Spain
⁶ Institut d’Astrophysique Spatiale, CNRS (UMR8617), Université Paris-Sud 11, Batiment 121, 91400 Orsay, France
⁷ Department of Earth and Space Science, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
⁸ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, UK
⁹ International Centre for Radio Astronomy Research, Curtin University, Perth, Bentley WA 6102, Australia
¹⁰ Johns Hopkins University, Department of Physics & Astronomy, 3400 N. Charles Street, Baltimore, MD, 21218, USA
¹¹ Kapteyn Astronomical Institute, University of Groningen, 9747 AD Groningen, The Netherlands

Received: 13 March 2015
Accepted: 5 October 2015

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 ~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 [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.