2021A&A...656A..55M

We present high angular resolution (0.13-0.4 arcsec) ALMA CO(2-1) and 1.7 mm continuum observations of the far-infrared-bright galaxy PKS 0023-26 (z = 0.32), which hosts a young radio source as well as a luminous optical active galactic nucleus (AGN). Although young, the powerful radio source has already grown to a size of a few kiloparsec, making it potentially capable of affecting the interstellar medium (ISM) of the host galaxy. We detect a very extended distribution of molecular gas with a mass between 0.3 and 3 x 10¹⁰ M_☉, depending on the X_CO conversion factor. The gas has a maximum radial extent of ∼5 arcsec (24 kpc) from the nucleus and is distributed in an asymmetric structure offset from the radio galaxy and with a fairly smooth velocity gradient. At large radii, tails of gas are observed in the direction of companion galaxies, suggesting that tidal interactions may be responsible for the origin of the gas. Overall, the observed properties are reminiscent of the molecular structures observed in some galaxy clusters. However, in the inner few kiloparsec, across the entire extent of the radio continuum, the kinematics of the gas appears to be affected by the radio source. In the central, sub-kiloparsec region, we observe the brightest emission from the molecular gas and the broadest velocity profiles with a full width at zero intensity (FWZI) of ∼500 km s^–1, which indicate that in this region a direct interaction of the jet with dense clouds and outflowing molecular gas is happening. On larger, kiloparsec-scales, the molecular gas appears to avoid the radio lobes, while gas with a somewhat smaller velocity dispersion (FWZI of ∼350 km s^–1) is observed around the radio lobes. Thus, in these regions, the gas appears to be affected by the expanding cocoon surrounding the radio source, likely dispersing and heating preexisting molecular clouds. The observations suggest that the mode of coupling between radio jets and the ISM changes from an outflowing phase limited to the sub-kiloparsec region to a maintenance phase, excavating cavities devoid of dense gas, at larger radii. This reveals that, already on galaxy scales, the impact of the AGN is not limited to outflows. This is in accordance with predictions from numerical simulations. With a star-formation rate of 25 M_☉ yr^–1, PKS 0023-26 is located on the SFR-M_* relation for star forming galaxies. Thus, the AGN does not appear to have, at present, a major impact on the host galaxy in terms of the overall level of star-formation activity. However, as the jet and lobes expand throughout the galaxy in the coming few x10⁷ yr, they will carry enough energy to be able to prevent further gas cooling and/or to inject turbulence and thus affect future star formation.