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

Understanding the way galaxies formed and the time evolution of structures in the Universe is one of the principal and most fascinating goals of modern cosmology.
Observations confirm what the theoretical models predict: Active Galactic Nuclei (AGN ) and their host galaxies are intimately connected, and show such correlations to support a co-evolutionary scenario. In this context, a key result is the discovery of correlations between the mass of the Supermassive Black Hole residing in the nucleus of the host galaxy and some properties of the spheroidal component such as the luminosity, the mass, the velocity stellar dispersion. Therefore, to understand the way galaxies evolves it’s necessary to understand how their nucleus evolves, and its interaction with the galaxy. Then, a census of the AGN population is mandatory.
However, a part of the X-ray background is still unresolved, despite of improvements made since the Cosmic X-ray Background discovery, that goes back to half century ago. There is a large AGN population, indeed, whose X-ray emission is obscured by the matter along the line of sight, when it has column densities greater than, or equal to, the inverse of Thompson cross section (N_H ≥ σ_T = 1.5 × 10^24 cm^−2 ). These AGN are defined as "Compton Thick" (CT ). These sources emit really weak fluxes below 10 keV, easily eluding the observations, due to actual on-orbit instruments for X-ray astronomy that reach the needed flux limit hardly. As a consequence, the
highly obscured (or even CT) and/or high redshift population is still unresolved. To recover these sources, it’s possible resorting to a “brute force” approach, that is the observation of a sky area for very long times. The deepest X survey has been realized to this end: the 4 Ms Chandra observation of the Chandra Deep Field-South (CDFS). The X-ray analysis, indeed, is essential because it’s the only one capable to detect the CT AGN distinctive features: the obscuring material column density and the iron line at 6.4 keV.
In this thesis work, a systematic X-ray analysis has been performed for sources coming from the iperdeep Chandra survey. These sources have been selected in 4 different ways:
1. highly star-forming galaxies at z∼2 selected through their colors in the B, z, K bands;
2. highly star-forming galaxies at z∼2 selected through their colors in the 24 μm, I, K ;
3. z > 3 galaxies in the GOODS-MUSIC survey;
4. z > 3 galaxies in the GOODS-ERS survey.
The resulting study allowed the identification of a substantial high redshift CT AGN population, and to characterize their X-ray spectra, the column densities, the spectral indices, the fluxes, the luminosities and the iron line intensities.
A ΛCDM cosmology (H_0 = 70 km s^−1 Mpc−1 ; Ω_M = 0.3; Ω_Λ = 0.7) is adopted throughout this thesis.