Affiliation: INAF – OATs

Contribution: Oral

Title: Witnessing the assembly of massive galaxies in the early Universe

Abstract: The coexistence between the outstanding mass growth of the Supermassive Black Holes (SMBHs; M_BH~10^8-10 M_sun) powering luminous quasars at the Epoch of Reionization (EoR, z > 6), and the concurrent growth of their host galaxies, still challenges theoretical models aimed at explaining how these systems formed in less than 1 Gyr. While the characterisation of high-z quasars has been achieved thanks to large surveys in the rest-frame UV-optical bands, the knowledge of how quasar host-galaxies build up and evolve can only be achieved with far-IR/sub-mm observations. Furthermore, sub-mm observation have proved to be a reliable probe of the obscured star formation, which constitutes a significant, or even dominant, fraction of star formation at early epochs. In this context, several surveys in the sub-mm regime dedicated at revealing the properties of dusty objects at high-z. These observations reveal compact dust sizes for both quasar hosts and star forming galaxies, suggesting that efficient accretion processes may favor the formation of massive systems in a relatively short period of time. In this context, I will present new and archival mm observations used to characterise the molecular gas reservoirs, star formation rates, and dust properties in a sample of ~20 z>6 quasars. Indeed, copious amounts of dust (up to few 10⁸ Msun) and gas (up to 10¹¹ Msun) trace the assembly of large galaxies at early epochs, with rates of star formation (SFRs) up to 1000-3000 M_sun/yr. The resulting star formation efficiencies are among the highest observed among massive galaxies, exceeding that of galaxies without active nuclei at similar epochs by a factor of 10 or more. There are evidence for the accreting SMBH to heat/remove the cold molecular gas from the host galaxy, halting the formation of stars in few tens of Myr. Thanks to ALMA fine sensitivity, we will also reveal the presence of several companion galaxies surrounding the massive quasar, invisible at rest-frame UV/optical bands, suggesting that mergers may play a crucial role in sustaining the star formation in high-density sites like this hosting quasars. I will also investigate the evolution of these properties with redshift, by comparing z>6 quasars with comparison sample of quasars, AGN and star forming galaxies at different epochs, down to the local Universe.

This contribution can be found here (pdf).