AGN as cosmological probes

Active Galactic Nuclei (AGN) are exceptionally powerful sources, characterised by the radiation coming from the accretion onto a Supermassive Black Hole (SMBH) at the centre of a galaxy. Their luminosity ranges from 1040 to 1048 erg s−1, and can exceed that of the hole stellar emission from the galaxy they are found in. They are observed at all cosmic epochs, from the local Universe to up to when it was less than a billion years old, and they are very numerous. These properties makes them very interesting objects for cosmology. AGN implementation in cosmology has two sides: first of all,, their emission can be used as a tool to measure cosmological distances and to determine the expansion rate of the Universe. At the same time, AGN emission is intricately linked to the SMBH at their core. Gaining a comprehensive understanding of SMBH formation and accretion history is pivotal to grasping the history of structure formation of the Universe, and the co-evolution of SMBH with galaxies. In this thesis, we investigate both of these two ways in which AGN can be used to enhance our comprehension of the Universe and its history. Additionally, we probe various aspects of the physical and emission properties of AGN. Despite our current knowledge, there is still much to be understood about these objects. A deeper insight into them not only broadens our understanding but also equips us to harness them more effectively in cosmo- logical studies.