Obscured quasars at redshift z ~ 2

摘要

Completing the census of AGN in the Universe is the key to understanding the cosmic evolution of supermassive black holes (SMBH) and galaxies, and to resolving the spectrum of the X-ray background (XRB). However, a large population of AGN, especially the heavily obscured, Compton-thick AGN, are still missing from even the deepest X-ray surveys. The infrared spectra energy distribution (SED) of distant star-forming galaxies can reveal the presence of bright AGN activity. Using some of the deepest infrared, X-ray and radio data available in the GOODS fields, we identify a population of infrared bright quasars at redshift z~2, which are often missed in the X-ray band. Amongst these sources the number of obscured and heavily-obscured quasars is much higher than those previously found in several X-ray and optical selected samples. A unique view on these heavily-obscured quasars is now given at high energies by NuSTAR. I will present the first NuSTAR detection of a heavily obscured quasar at z 2. This source is a potential archetype of the heavily-obscured high-z AGN in which most of the black hole growth is happening, that can explain the mysterious missing fraction of the XRB.