Hidden supermassive black holes in the deep universe

The existence of a large population of hidden active galactic nuclei (AGN), especially at high redshift, raises fundamental questions about their origins, their co-evolution with host galaxies, and their influence on the growth of supermassive black holes (SMBH) across cosmic time. This PhD thesis explores three key aspects of obscured AGN, with particular emphasis on high redshift: selection methods, demographic characterization, and their physical properties. I investigated these topics using a multiwavelength approach, incorporating radio, X-ray, optical, and infrared data, as well as using both photometry and spectroscopy, which is crucial for identifying and understanding these elusive sources. In the first part of this thesis, I focus on how radio observations can be used to identify obscured AGN, especially the highly obscured Compton-thick (CTK) population. I test the effectiveness of radio emission in selecting these sources both from an analytical and observational point of view, finding that with deep continuum radio observations it is possible to unveil much more heavily obscured AGN than what it is possible to do relying only on the X-rays. The thesis further investigates new spectroscopic methods for identifying obscured AGN at high redshift, where the James Webb Space Telescope (JWST) is opening new incredible opportunities for the investigation of these sources. I present three new diagnostic diagrams based on the [OIII]λ4363 auroral line, enabling the identification of AGN even in low-metallicity environments typical of the high-redshift universe, and providing a more effective tool for characterizing AGN in the JWST era. Finally, I describe the selection and the detailed multiwavelength analysis of narrow-line AGN (NLAGN) selected from the JWST CEERS survey, covering the redshifts range 2 < z < 9. I will explore their spectral properties, the effectiveness of the different emission line diagnostics diagrams, and I will provide insights on their X-ray and radio characteristics.