The Universe contains billions of galaxies, which come in may different sizes and colours. The main building blocks of these galaxies are stars, a central supermassive black hole (SMBH), dark matter and the interstellar medium. At present-day there are two main types of galaxies: spiral galaxies, which have both young and old stars and massive ellipticals, which have mainly old stars. In addition, observations of elliptical galaxies and the bulges of spiral galaxies revealed tight correlations between some properties of the host galaxy and the mass of its SMBH, which suggest co-evolution of the galaxies with their SMBHs.
In order to understand how galaxies formed and evolved into different types, and to reveal the processes that shaped their evolution, we need to look back in time to find their progenitors. Active galactic nuclei (AGNs) at cosmological distances are the prefect targets for this, as these are bright sources, where the central SMBH is actively growing. Moreover, such distant, powerful AGNs are hosted by massive galaxies and likely trace over-dense environments or protoclusters.
The aim of this thesis is to study the environment of cosmologically distant quasars and radio galaxies, the possible progenitors of local massive galaxies. The thesis presents three case studies of AGN-companion galaxy systems found at 1.7-2.5 billion years after the Big Bang, and which were selected based on the AGN host galaxy appearing gas-poor, while the companion galaxy being very gas-rich.
Much our previous knowledge of the gas and dust in distant AGNs and their host galaxies has been derived from low-resolution observations making it difficult to disentangle the contribution from companion galaxies. In my thesis we have challenged previous results using higher quality data at mm and radio wavelengths.These observations revealed that a significant amount of molecular gas and dust emission is associated with the host galaxies of the AGNs. My results highlight the importance of sensitive and high-resolution observations and demonstrate that high-z AGN systems are more complex and diverse, than it is implied by theoretical studies.