Tuesday, July 26th, at 15:00, in Sala Seminari:
(The Pennsylvania State University, US)
The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion
We exploit the 7 Ms Chandra observations in the Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at 3.5<z<6.5. This aim is achieved by stacking the Chandra data at the positions of optically selected galaxies. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as constraining the SFRD at high redshift. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion onto SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function (logL_X~42) at z>4, with evidence for fairly flat slopes. This is in contrast with recent findings which implied a significant contribution of AGN to the cosmic Reionization.