De Cicco, Demetra (2017) Characterization of active galactic nuclei through photometric and spectroscopic variability. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Lingua: English
Title: Characterization of active galactic nuclei through photometric and spectroscopic variability
De Cicco,
Date: 10 April 2017
Number of Pages: 152
Institution: Università degli Studi di Napoli Federico II
Department: Fisica
Dottorato: Fisica
Ciclo di dottorato: 29
Coordinatore del Corso di dottorato:
Paolillo, MaurizioUNSPECIFIED
Longo, GiuseppeUNSPECIFIED
Brandt, William N.UNSPECIFIED
Date: 10 April 2017
Number of Pages: 152
Uncontrolled Keywords: active galactic nuclei; AGN; variability; quasar; QSO; BALQSO; broad absorption line quasar
Settori scientifico-disciplinari del MIUR: Area 02 - Scienze fisiche > FIS/05 - Astronomia e astrofisica
Date Deposited: 03 May 2017 15:26
Last Modified: 08 Mar 2018 10:58
DOI: 10.6093/UNINA/FEDOA/11816


Nowadays we generally accept the idea that the emission from active galactic nuclei (AGNs) is powered by an accreting supermassive black hole (SMBH) at the center of a galaxy. It is also well known that most -probably all- galaxies go through an active phase at some point in their lives, and that several empirical relations connect black hole and host galaxy properties: this suggests that a tight feedback between the evolution of the black hole and the host galaxy exists; hence, in order to go deeper into galaxy evolution, it is crucial for us to learn more about the formation and evolution of the black holes residing in their centers. Variability is a defining property of AGN emission at all wavebands, and concerns both continuum and broad-line emission. It is generally attributed to instabilities in the AGN accretion disk, together with changes in the accretion rate. Since the extent of variations in different wavelength ranges is not the same, variability measurements can help understand the underlying emission mechanism, constraining the size and structure of the emitting region. The present work investigates AGN variability from two different perspectives. The first part of the project tests the efficiency of optical variability as a tool to select AGNs, since optical continuum variability seems to be a universal feature of broad-line AGNs on timescales from months to years, with variations generally ranging from 1% to 10% of the magnitude, but also much larger (50% of the magnitude) in some cases. Testing techniques for AGN identification based on data from ground-based telescopes is of great relevance in the framework of current and future wide-field surveys (e.g., Dark Energy Survey, Large Synoptic Survey Telescope), since we will need reliable methods to detect and classify the wealth of sources they will provide. The second part of the project investigates the variability of broad absorption lines (BALs) in quasi-stellar object (QSO) spectra, in order to deeply understand the physics and structure of AGNs. BALs originate from outflowing winds along our line of sight; winds are thought to originate from the accretion disk, in the very proximity of the central SMBH; we generally think that they are responsible for a triggering of the accretion mechanism onto the SMBH, as they remove angular momentum from the disk and, since they evacuate gas from the host galaxy, they also play a leading role into galaxy evolution. Several works show that BAL equivalent widths can change on typical timescales from months to years. Such variability is generally attributed to changes in the covering factor (due to rotation and/or changes in the wind structure) and/or in the ionization level. We investigate BAL variability, focusing on BAL disappearance, in a sample of more than 1500 QSOs -the largest sample ever used for such an analysis- to gain insight into the structure and co-evolution of the SMBH and the host galaxy.


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