2012MNRAS.425.1320I

We report new, sensitive observations of two z ∼ 3–3.5 far-infrared-luminous radio galaxies, 6C1909+72 and B3J2330+3927, in the ¹²CO J = 1-0 transition with the Karl Jansky Very Large Array and at 100–500m using Herschel, alongside new and archival ¹²CO J = 4-3 observations from the Plateau de Bure Interferometer. 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 active galactic nuclei (AGN), but it is not detected in ¹²CO despite ∼ 20000km/s of velocity coverage. Also in the 6C1909+72 field, a large, red dust feature spanning ≈ 500kpc 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 ¹²CO 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 ¹²CO brightness temperature 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 \mi L\mi CO\mn 4\mo –\mn 3\mo '\mo/\mi L\mi CO\mn 1\mo –\mn 0\mo '\mo >\mn 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 time-scale commensurate with that of the jet activity, ≲ 10Myr.