Outflowing active galactic nuclei and where to find them

Active Galactic Nuclei (AGN) represent a key stage of galaxies’ life cycle, powered by matter accretion onto a central supermassive black hole (BH). Featured by high luminosity and strong variability, AGN leave imprints over the entire electromagnetic spectrum (from radio to γ-rays). In the last three decades, AGN have increasingly become protagonists of galaxy evolution studies, because of the profound effects accreting BHs can have on their host galaxy - the so-called AGN feedback. In particular, AGN-driven outflows are invoked by galaxy evolutionary models to explain the origin of the observed BH-host coevolution, and to quench star formation in galaxies by either expelling or heating the host gas reservoir. Although extensively observed from low to high redshift, we still miss definitive observational proof of the real impact of outflows on their host galaxy. This PhD thesis employs ground-based Integral Field Unit (IFU) observations to investigate the multi-phase nature of outflows in the local galaxy merger NGC 6240, and their acceleration mechanisms as well as physical properties in two distinct samples of AGN at the Cosmic Noon (z~2, the golden epoch of AGN feedback). In addition, this thesis investigates the use of HeII line emission as powerful tracer of AGN hidden in local star-forming galaxies; also addresses the limits of the standard AGN unified model by reporting the discovery of a new changing-look AGN (NGC 4156), and an observed discrepancy in outflow kinematics at z~2, hardly explainable in terms of pure inclination effects. With a view to exploring the high-redshift Universe with the new generation of IFU facilities (e.g. JWST/NIRSpec, VLT/ERIS), developing novel emission-line diagnostics is fundamental to get complete AGN samples, including also elusive AGN residing in highly star-forming and/or metal-poor galaxies at high redshift, where standard methods are seen to fail. Thanks to the unprecedented spatial resolution and sensitivity of the new available facilities, we will finally take the next step to fully understand AGN feedback and galaxy evolution.