Quasars at z>6 (age of the Universe: <1 Gyr) are arguably the most active astrophysical objects in the early universe. They are powered by fast accretion on their central black holes (which already have masses of 0.1-10 billion Msun). Their galaxies form stars at rates of >100 Msun/yr, and, despite the young cosmic age, they appear chemically enriched. These humongous star formation and...
A tight non-linear relation between the X-ray and the optical-ultraviolet luminosity has been observed in AGN over 5 orders of magnitude and up to high redshift.
This suggests a coupling between the disk, emitting the primary radiation in the UV band, and the hot corona emitting in the X-ray.
In this work, we have studied the $L_X-L_{UV}$ relation for a sample of high-redshift ($z>4$) quasars,...
The cosmological concordance model (ΛCDM) well accounts for a wealth of observations, from the existence of Cosmic Microwave background (CMB) to the discovery of the accelerated expansion of the universe from Type Ia supernovae. Yet, it assumes a still unknown form of dark energy and matter and some tensions arose recently as, for instance, the discovery of a 3.4σ discrepancy between the local...
The first SMBHs and their host galaxies represent some of the most extreme astrophysical objects that we currently know at z>6. Their observed properties allow to constrain possible evolutionary models. Here we present some recent results on the nature of their black hole seeds, on their mass growth rate, and on the assembly history of their host galaxies.
I will review the arguments in favour of/against a substantial
contribution of AGNs and/or star-forming galaxies to the reionization
of the Universe at z>5, by using extrapolations of the most recent
determination of the AGN and LBG high-z luminosity functions (LFs) and
their redshift evolution. A galaxy driven reionization requires a
significant contribution of faint dwarf galaxies and a LyC...
The existence of extremely massive black holes at very high redshift is a true challenge to the commonly accepted black hole formation and evolution models. The quasars found at z>4 host extremely massive black holes, up to the case of a quasar found at z>6 with 11 billion solar masses. These objects are particularly problematic: there is not enough time to accrete such large masses in a...
Looking for Active Galactic Nuclei (AGNs) in the first Gyr of the Universe is crucial to understand when and how the first super massive black holes (SMBHs) formed, how they are related to galaxy formation and what is their role in the re-ionisation process. Explaining how such massive systems could be built up in the short (< 1Gyr) available cosmic time is still an open issue. Selecting...
More than 200 quasars (QSOs) with spectroscopic redshift z > 6 have been discovered so far.
Multi-wavelength observations showed that these QSOs are evolved systems with large black hole masses (10$^{8-10}$ M$_{\odot}$), and their broad-band spectral energy distributions (SEDs) and rest-frame NIR/optical/UV spectra have not significantly evolved over cosmic time.
The formation of their Super...
Deep X-ray surveys provide unprecedented access to the population of accreting super-massive black holes (SMBH) at high redshift. I will present our recent results (Vito et al. 2018) on the 3 < z < 6 AGN population in the 7 Ms CDF-S and 2 Ms CDF-N, the deepest X-ray surveys to date. We put tight constraints on quantities such as the obscured AGN fraction and the number density of z>3 AGN. In...
