The AGN fuelling/feedback cycle in LERGs: the molecular gas component and its interplay with radio jets

This Thesis has been carried out as part of an international collaboration overall aiming at a multi-component (stars, warm/cold gas, dust, radio jets) investigation of the AGN fuelling/feedback loop in the cores of a volume-limited (z<0.03) sample of eleven low excitation radio galaxies (LERGs). In this general context, this PhD Thesis mainly aimed at i) shedding light on the properties, kinematics and origin of the cold gas using ALMA CO(2-1) observations of nine sample members; ii) investigating the radio jets/cold ISM interplay on (sub-)kpc scales, coupling ALMA CO(2-1) and 230~GHz continuum observations with newly-acquired radio JVLA data at 10 GHz of five sample sources. Our work shows that rotating (sub-)kpc CO discs are very common in LERGs (six out of nine sources detected in CO). The 3D modelling of these discs demonstrates that the bulk of the gas is in ordered rotation (at least at the resolution of our ALMA observations). This may explain the relatively low accretion rate of LERGs. Nevertheless, low-level perturbations and/or non-circular motions are ubiquitous, indicating that the gas is not fully relaxed into the host galaxy potential. Whenever possible, we demonstrated that the observed CO discs are co-spatial with dust discs/lanes. In a couple of cases multi-wavelength constraints point towards an external origin of the gas. These evidence are difficult to reconcile with the chaotic cold accretion scenario, with implications for the powering mechanism of LERGs. The relative jet/CO disc inclination angles are found to span a wide range (27°-64°), with a marginal preference for angles around 45°. This is consistent with previous statistical 3D studies of the relative jet/dust disc orientation in radio galaxies. Hints of the presence of a jet/disc interaction are also observed in one case and tentatively found in the kinematics of other two sources.