Gas-rich mergers and feedback are ubiqitous amongst starbursting radio
  galaxies, as revealed by JVLA, IRAM PdBI and Herschel
  We report new, sensitive observations of two z ~ 3-3.5 FIR-luminous radio
galaxies, 6C1909+72 and B3J2330+3927, in 12CO J=1-0 with the Karl Jansky VLA
and at 100-500um using Herschel, alongside new and archival 12CO J=4-3
observations from IRAM PdBI. We introduce a new colour-colour diagnostic plot
to constrain the redshifts of several distant, dusty galaxies in our target
fields. A bright SMG near 6C1909+72 likely shares the same node or filament as
the signpost AGN, but it is not detected in CO despite ~20,000 km/s of velocity
coverage. Also in the 6C1909+72 field, a large, red dust feature spanning ~500
kpc is aligned with the radio jet. We suggest several processes by which
metal-rich material may have been transported, favouring a collimated outflow
reminiscent of the jet-oriented metal enrichment seen in local cluster
environments. Our interferometric imaging reveals a gas-rich companion to
B3J2330+3927; indeed, all bar one of the eight z >~ 2 radio galaxies (or
companions) detected in CO provide some evidence that starburst activity in
radio-loud AGN at high redshift is driven by the interaction of two or more
gas-rich systems in which a significant mass of stars has already formed,
rather than via steady accretion of cold gas from the cosmic web. We find that
the CO Tb ratios in radio-loud AGN host galaxies are significantly higher than
those seen in similarly intense starbursts where AGN activity is less
pronounced. Our most extreme example, where L'(CO4-3)/L'(CO1-0) > 2.7, provides
evidence that significant energy is being deposited rapidly into the molecular
gas via X-rays and/or mechanical (`quasar-mode') feedback from the AGN, leading
to a high degree of turbulence globally and a low optical depth in 12CO -
feedback that may lead to the cessation of star formation on a timescale
commensurate with that of the jet activity, <~10 Myr.
