Accreting Supermassive Black Holes in the First Billion Years: Impact on their Environments from Parsecs to Megaparsecs.

Studying the environments of quasars in the first Gyr of the universe, or at z > 5.5, is crucial to understand their growth and evolution from such early times. These massive quasars with ∼ 10^8 − 10^9 Solar mass black holes are predicted to be born in the most massive halos of the underlying dark matter distribution and thus would be immersed in protoclusters of galaxies. However, the impact of a quasar’s powerful radiation on the formation and growth of galaxies in its Mpc-scale environment is still debated observationally. Additionally, the different components contributing to the quasar activity from subpc- to kpc-scales can affect the gas and dust used to form stars in its host galaxy, thus impacting the black hole – host galaxy co-evolution. This thesis aims at providing more understanding of the different scales of quasar environments by investigating two exemplary quasars. We investigated the luminous quasar ULAS J1342+0928 at z = 7.54 and looked for galaxies in its ∼ 1 pMpc^2 environment. We found one UV-bright Lyman-break galaxy candidate in addition to a [CII]-emitter associated with the quasar environment and clustered within a projected distance of ≲ 220 pkpc from the quasar. Future observations in the near-IR and mm would be necessary to confirm the redshift and physical properties of these galaxies and assess the number density of one of the earliest quasar large-scale environments yet explored. We also studied extensively the radio-loud quasar P352–15 at z = 5.832; the only source found thus far with evidence of a kpc-scale extended jet. Thus, this quasar is the ideal laboratory to investigate the first stages of black hole–jet–host galaxy co-evolution. We first studied the cold dust and [CII] gas of the host galaxy of P352–15 to explore whether the jet presence affects the host galaxy properties. The results on the inferred star formation rate and far-infrared luminosity were found to be comparable to studies on the radio-quiet quasar population. However, we found evidence of a spectral break ...