Abstract

One of the most remarkable discoveries of the last two decades is that all massive galaxies host a supermassive black hole (SMBH) at their center, which has gone through an active phase of growth known as active galactic nucleus (AGN). Although galaxy evolution models need AGN feedback to reproduce the observed properties of galaxies, direct observational evidence for the mechanisms through which SMBHs and galaxies co-evolve has still to be proven. The peak epoch of galaxy assembly and SMBH growth, the so-called "cosmic noon" (1<z<3), represents a key laboratory to understand how the connection between AGN and their host galaxies was established. In this thesis, I presented observational studies of the interstellar medium (ISM) of X-ray selected AGN at cosmic noon. The gas content is directly linked to the evolution of the AGN and its host galaxy. Indeed, it feeds both the formation of new stars in the galaxy and the growth of the central SMBH. The gas in the host along our line of sight may also have a pivotal role in obscuring the central nuclear source. Finally, the kinematics and composition of the gas could be affected by the energy released by the central AGN, through the so-called AGN feedback. To achieve a comprehensive study of the ISM, I adopted a multi-wavelength approach and exploited a variety of data, from the X-ray to the radio regime, by using techniques such as broad-band spectral energy distribution (SED) fitting, X-ray spectral analysis and submm spectroscopy. I first investigated whether the obscuration observed in the X-ray spectra of AGN can be produced by the ISM of the host galaxy. For a sample of far-IR detected AGN at z>2.5 I found that the total hydrogen column density along the line of sight, measured in the X-ray band, is comparable to the column density associated with the ISM of the host, derived through SED-fitting analysis and assuming galaxy sizes. Therefore, the ISM of the host appears to be capable of providing significant absorption on kpc scales. Such absorption adds to (or even replaces) that produced on pc scales by any circumnuclear material, challenging the view of the obscured/unobscured AGN dichotomy as due to inclination effects only. Then, I conducted two complementary studies to probe the ionized and molecular phases of the ISM in a blindly-selected sample of AGN at z~2 that covers a wide range in luminosities. Such studies are necessary to properly constrain the impact of AGN feedback on galaxy evolution. The first, called SUPER, traces ionized outflows through the [OIII] line by using high-resolution spatially-resolved integral field spectroscopic observations (SINFONI). In this work I laid the foundations of the survey, by performing a multi-wavelength characterization of AGN and host galaxy properties, and showed how comparing insights from different techniques/observations is crucial to confirm and provide extra confidence in SED-fitting as well as spectroscopic results. These multi-wavelength properties will then be connected with those of the outflows as traced by SINFONI. The second study used a set of ALMA observations of the CO(J=3-2) transition, to carry out a comparison of the CO properties of AGN with those of a control sample of inactive galaxies matched in redshift, stellar mass and star-formation rate. I found that the AGN sample appears to be underluminous in CO with respect to the control sample and the difference is especially significant at high masses, log(Mstar) > 11. These observations demonstrated that the AGN may have an effect on the ISM of the hosts, although the exact mechanisms in place require further observations to be understood. These projects set the scene for future investigations, which will significantly improve our understanding of the role of AGN in galaxy evolution.