The changing impact of radio jets as they evolve: the view from the cold gas

We present ALMA CO(1-0) and CO(3-2) observations of a powerful young radio galaxy, PKS 0023−26, that is hosted by a far-infrared bright galaxy. The galaxy has a luminous optical active galactic nucleus (AGN) and a very extended distribution of molecular gas. We used these observations (together with available ALMA CO(2-1) data) to trace the impact of the active nucleus across the extent of the radio emission and beyond on scales of a few kiloparsec (kpc). Despite the strength of the optical AGN, the kinematics of the cold molecular gas is strongly affected only in the central kpc, and it is more weakly affected around the northern lobe. We found other signatures of the substantial impact of the radio AGN, however. Most notably, the extreme line ratios of the CO transitions in a region that is aligned with the radio axis indicate conditions that are very different from those observed in the undisturbed gas at large radii. The non-detection of CO(1-0) at the location of the core of the radio source implies extreme conditions at this location. Furthermore, on the scale of a few kpc, the cold molecular gas appears to be wrapped around the northern radio lobe. This suggests that a strong jet-cloud interaction has depleted the northern lobe of molecular gas, perhaps as a result of the hot wind behind the jet-induced shock that shreds the clouds via hydrodynamic instabilities. The higher gas velocity dispersion and molecular excitation that we observed close to this location may then be the result of a milder interaction in which the expanding jet cocoon induces turbulence in the surrounding interstellar medium. These results highlight that the impact of an AGN can manifest itself not only in the kinematics of the gas, but also in molecular line ratios and in the distribution of the gas. The work also highlights that it is important to spatially resolve the gas throughout the radio source to trace different modes of AGN feedback that can coexist. Although the radio plasma and the cold molecular gas are clearly coupled, the kinetic energy that is transferred to the interstellar medium is only a small fraction of the energy available from the AGN.