A CO-rich merger shaping a powerful and hyper-luminous infrared radio
  galaxy at z=2: the Dragonfly Galaxy
  In the low-redshift Universe, the most powerful radio sources are often
associated with gas-rich galaxy mergers or interactions. We here present
evidence for an advanced, gas-rich (`wet') merger associated with a powerful
radio galaxy at a redshift of z~2. This radio galaxy, MRC 0152-209, is the most
infrared-luminous high-redshift radio galaxy known in the southern hemisphere.
Using the Australia Telescope Compact Array, we obtained high-resolution
CO(1-0) data of cold molecular gas, which we complement with HST/WFPC2 imaging
and WHT long-slit spectroscopy. We find that, while roughly M(H2) ~ 2 x
10$^{10}$ M$_{\odot}$ of molecular gas coincides with the central host galaxy,
another M(H2) ~ 3 x 10$^{10}$ M$_{\odot}$ is spread across a total extent of
~60 kpc. Most of this widespread CO(1-0) appears to follow prominent tidal
features visible in the rest-frame near-UV HST/WFPC2 imaging. Ly$\alpha$
emission shows an excess over HeII, but a deficiency over L(IR), which is
likely the result of photo-ionisation by enhanced but very obscured star
formation that was triggered by the merger. In terms of feedback, the radio
source is aligned with widespread CO(1-0) emission, which suggests that there
is a physical link between the propagating radio jets and the presence of cold
molecular gas on scales of the galaxy's halo. Its optical appearance, combined
with the transformational stage at which we witness the evolution of MRC
0152-209, leads us to adopt the name `Dragonfly Galaxy'.
