First direct measurement of dust temperature in quasars at Cosmic Dawn

Quasars at the dawn of cosmic time (redshift z > 6 , age of the universe < 1 Gyr) are among the most active and luminous sources in the early universe. In these systems, large amounts of gas (M_H2 > 10^10 M⊙) fuel the fast growth of the central supermassive black hole (SMBH, with masses M_BH > 10^8 M⊙) as well as intense bursts of star formation (SF, with star formation rates SFR > 100 M⊙/yr). The advent of new generation mm/submm telescopes, such as ALMA or NOEMA, revolutionized the study of high-z quasar hosts dust properties. At z > 6, however, they remain largely unconstrained due to the still limited data. In this thesis project, we aim to enlarge the sample of high-z quasars where the host galaxy dust has been characterized. To achieve this, we analyzed new ALMA ACA Band 8 observations (∼ 400 GHz), and combined them with archival and literature data to pin down the dust spectral energy distribution (SED) in 11 type 1 radio quiet quasar host galaxies located at z between 6 and 7. This allowed for precise estimates of their infrared luminosities (L_IR) and of the associated SFRs. This analysis yielded mean values for the dust mass, emissivity index and temperature that align well with the specific literature. Regarding L_IR and SFR, the large average values derived in this work (L_IR = 9 ± 2 · 10^12 L⊙, SFR = 1340 ± 292 M⊙/yr) confirm that quasar host galaxies are systems experiencing a rapid evolution which, coupled to the intense accretion onto the central SMBH, can possibly set the scaling relation observed in the local universe. We also exploit the result obtained on T_d to confirm the dust temperature evolution scenario, linking it to the increased star formation efficiency (SFE) that simulations predict for galaxies populating the early universe. Finally, we tested our observations in order to investigate the potential influence of the AGN, finding no evidence of the quasar significantly contributing to dust heating on a galaxy-wide scale.