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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.

Mostra/Nascondi contenuto.
Chapter 1 Introduction 1.1 Active Galactic Nuclei: toward a definition The term “Active Galactic Nuclei” (AGN) is used to describe a wide variety of phenomena of non-stellar origin observed in the nuclei of many galaxies. About 1% of the known galaxies in the local universe shows some of the following properties: (a) nuclear luminosity greater than or equal to that of the host galaxy (L bol = 10 41 − 10 48 erg s −1 ) 1 ; (b) fast variability (Δt∼seconds-years) that entails compactness (R∼ 10 12 − 10 18 cm); (c) broad band emission (from the radio waves to the gamma rays); (d) strong emission lines in the optical/ultraviolet/X ranges; (e) powerful radio emission (P 5GHz > 10 25 W Hz −1 sr −1 ) for about 10% of the AGN. The reason for the reference to the galactic nucleus is that the non-stellar emission comes from the central galactic regions, point-like when observed 2 . Galaxies that shows at least some of these features are called "active galaxies". In addition to general features, the nuclear activity is shown in several ways. Different peculiarities are used to classify AGN. Every class is distinguished thanks to particular properties in different bands of the spectrum. The subclasses set up the so-called AGN "taxonomy". 1.1.1 Taxonomy AGN are classified according to the radio emission, the luminosity, the features of the optical (OPT) and/or the ultraviolet (UV) spectrum and the X spectrum morphology. The first AGN division was made by the intensity of radio emission, due to the first AGN identification as powerful radio-emitter (Baade & Minkowski 1 For comparison, standard galaxies have L bol < 10 42 erg s −1 . 2 Angular dimensions lesser than the more powerful available telescopes resolution: ∼ 0.5 00 for Chandra (X-Ray),∼ 0.1 00 for Hubble Space Telescope (Optical),∼ 0.001 00 for the Very Long Baseline Interferometer (Radio). 1

Laurea liv.II (specialistica)

Facoltà: Scienze Matematiche, Fisiche e Naturali

Autore: Christian Malacaria Contatta »

Composta da 153 pagine.

 

Questa tesi ha raggiunto 34 click dal 16/12/2011.

Disponibile in PDF, la consultazione è esclusivamente in formato digitale.