Cross-population radio galaxies as a test of the jet-accretion relationship

Radio galaxies (RGs) are extremely relevant in addressing important unknowns concerning the interaction among black hole accretion, radio jets, and the environment. In the classical scheme, their accretion rate and ejection of relativistic jets are directly linked: efficient accretion (HERG) is associated with powerful edge-brightened jets (FRIIs); inefficient accretion (LERG) is associated with weak edge-darkened jets (FRIs). The observation of RGs with an inefficient engine associated with edge-brightened radio emission (FRII-LERGs) broke this scheme. FRII-LERGs constitute a suitable population to explore how accretion and ejection are linked and evaluate the environment's role in shaping jets. To this aim, we performed a multiwavelength study of different RGs catalogs spanning from Jy to mJy flux densities. At first, we investigated the X-ray properties of a sample of 51 FRIIs belonging to the 3CR catalog at z<0.3. Two hypotheses were invoked to explain FRII-LERGs behavior: evolution from classical FRIIs; the role of the environment. Next, we explored the mJy sky by studying the optical-radio properties of hundreds of RGs at z<0.15 (Best & Heckman 2012 sample). FRII-LERGs appear more similar to the old FRI-LERGs than to the young FRII-HERGs. These results point towards an evolutive scenario, however, nuclear time scale changes, star population aging, and kpc-Mpc radio structure modification do not agree. The role of the Mpc environment was then investigated. The Wen et al. 2015 galaxy clusters sample, built exploiting the SDSS survey, allowed us to explore the habitat of 7219 RGs at z<0.3. Most RGs are found to live in outside clusters. For these sources, differences among RG classes are still present. Thus, the environment is not the key parameter, and the possibility of intrinsic differences was reconsidered: we speculated that different black hole properties (spin and magnetic field at its horizon) could determine the observed spread in jet luminosity.

A comprehensive study of the AGN feedback cycle in galaxy clusters from high resolution X-ray and radio observations

At the center of galaxy clusters, a dramatic interplay known as feedback cycle occurs between the hot intracluster medium (ICM) and the active galactic nucleus (AGN) of the central galaxy. The footprints of this interplay are evident from X-ray observations of the ICM, where X-ray cavities and shock fronts are associated with radio lobe emission tracing energetic AGN outbursts. While such jet activity reduces the efficiency of the hot gas to cool to lower temperatures, residual cooling can generate warm and cold gas clouds around the central galaxy. The condensed gas parcels can ultimately reach the core of the galaxy and be accreted by the AGN. This picture is the result of tremendous advances over the last three decades. Yet, a deeper understanding of the details of how the heating–cooling regulation is achieved and maintained is still missing. In this Thesis, we delve into key aspects of the feedback cycle. To this end, we leverage high-resolution (sub-arcsecond), multifrequency observations (mainly X-ray and radio) of several top-level facilities (e.g., Chandra, JVLA, VLBA, LOFAR). First, we investigate which conditions trigger a feedback response to gas cooling, by studying the properties of clusters where feedback is just about to start. Then, we focus on the details of how the AGN–ICM interaction progresses by examining cavity and shock heating in the cluster RBS797, an exemplary case of the jet feedback paradigm. Furthermore, we explore the importance of shock heating and the coupling of distinct jet power regimes (i.e., FRII, FRI and FR0 radio galaxies) to the environment. Ultimately, as heating models rely on the connection between the direct evidence (the jets) and the smoking gun (the X-ray cavities) of feedback, we examine the cases in which these two are dramatically misaligned.