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15th of August 2015
The Dragonfly Galaxy: ATCA unravels the role of cold gas in a merging hyper-luminous radio galaxy at z=2
by Bjorn Emonts (Centro de Astrobiologia)
The Dragonfly Galaxy (MRC 0152-209) is the most infrared-luminous radio galaxy known in the southern hemisphere. It is forming new stars at an extreme rate of ~3000 solar masses per year and has an active black hole that is expelling extremely powerful radio jets. By combining high-resolution ATCA observations with new HST imaging, we found evidence that a gas-rich (`wet') merger of galaxies is driving the rapid evolution of the Dragonfly Galaxy. The ATCA observations trace the distribution of the cold molecular CO(1-0) gas, as detected in 40 hours with the 7mm system and baselines ranging from 31m to 1.3km. We detect that more than half of the cold gas detected in CO(1-0) appears to be associated with gaseous debris expelled by the merger. Part of the cold gas curiously aligns with the radio source, suggesting that shocks may trigger the formation of additional molecules as the radio jets propagate through the cold and presumably dusty medium. The Dragonfly Galaxy is likely an extreme high-z analogue to the most powerful radio galaxies at low-z, whose activity is also believed to be fueled by cold gas deposited after a galaxy merger. As such, the Dragonfly Galaxy offers us a unique opportunity to further investigate the most active stages of galaxy evolution in the Early Universe.

Technical note: we detect CO(1-0) emission across a total extent of ~60 kpc, which is significantly larger than almost any other CO-observations of high-z galaxies. We argue that this is likely the result of the relatively large number of short baselines in the data, which makes the ATCA an extremely suitable instrument for detecting low-surface-brightness emission at high redshifts. A recently completed ALMA cycle-2 study of CO(6-5) in the Dragonfly Galaxy seems to confirm this, and will show-case the complementarity of ATCA and ALMA for high-z CO work (Emonts et al, submitted).

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