Search and X spectroscopy of Compton Thick Active Galactic Nuclei in the 4 Ms Chandra observation of the CDFS

Although the selection work performed until z ≤ 3 allows to know the obscured AGN population properties, to carry out a complete study of the cosmological history for these objects it is necessary to push the census toward higher redshifts.
The 4Ms Chandra survey permits to observe the Universe in the Xrays starting from its dawn, allowing the observation of extremely faint and/or far objects. For the present work, the observation of CT AGN candidates at z> 3 has not been made as much with a real preselection, but rather with a so-called “brute force” approach. This means that all galaxies at z> 3 in the CDFS have been analysed.

More precisely, all the analysed galaxies at z> 3 comes from the GOODS field surveys, described in the following. This kind of approach, “brutal-like”, where every galaxy in there is analysed in X-ray band, is justified, in part, by the low number of galaxies, in the restricted redshift and field ranges considered. On the other hand, the lacking of effective selection methods at these distances (just some Myr after the Big Bang) justifies the above-said procedure.
A fundamental assist comes from the efficient campaigns designed to study the CDFS: a number of surveys have been made in this piece of sky to provide as much as possible multiwavelength counterparts for each source.
Of course, some surveys are more complete than others (more multiwavelengths counterparts, more accurate, etc.) and the same is for some catalogs built starting from these surveys. However, despite of efforts to characterize the surveys in the best possible way, a number of sources extracted from the following samples have not a reliable statistic in the X-ray band, preventing a good fit. As a consequence only the sources with ≥ 40 photon counts will be discussed in detail.

For the other sources, just the main properties derived from the fit will be showed, integrated with analysis through stacking technique. In the next section the surveys and the samples used for the characterization of the high redshift sources analysed in the X-ray band will be showed.