Dissertation committee:
Εμμανουήλ Δανέζης Επικ. Καθηγητής ΕΚΠΑ (επιβλέπων), Απόστολος Μαστιχιάδης Καθηγητής ΕΚΠΑ, Νεκτάριος Βλαχάκης Αναπλ. Καθηγητής ΕΚΠΑ
Summary:
The current thesis focuses on the study of Broad absorption Lines (BALs) of Si IV and C IV in the UV spectra of Broad Absorption line Quasars (BALQSOs). BALs identify high-velocity outflows launched from a rotating accretion disc that surrounds and feeds a central supermassive black hole. The properties of BAL material in phase space are uncertain and their origin and extreme variability remain open questions. The main problem concerning the origin of BALs is whether BALs are the product of a smooth and homogeneous outflow or the product of independent absorption components formed in individual absorbing clouds in the line of sight.
In order to answer these questions the following are required: a) a model able to deblend the members of resonance doublets like Si IV and C IV and provide multicomponent fitting of the corresponding BALs, b) a method which guarantees that the number of components each BAL is analyzed into is uniquely determined, c) for each absorption component the values of the measured physical parameters (radial velocity, FWHM, optical depth at line center) must be uniquely determined, d) the development of a software able to process and incorporate all the above, e) the investigation of Si IV and C IV BAL variability which can provide useful insights about the structure of BAL outflows, i.e. whether BALs are the product of a smooth and homogeneous outflow or the product of multiple and independent components formed in clumpy gas clouds.
In our study all the above requirements were met and our results indicate strongly that BALs are the product of independent components formed in clumpy gas clouds inside the turbulent and unstable quasar outflow.
In the first part of the thesis we applied the proposed fitting criteria and performed multicomponent fits to Si IV and C IV BALs in a sample of 20 BALQSOs. For every absorption component we calculated the radial velocity, the FWHM and optical depth at line center. In the second part of the thesis we studied the variability of Si IV and C IV BAL components in 10 quasars of our sample.
In more detail, in this work we used GR model (Danezis et al. 2003, Lyratzi et al. 2007, 2009) which can deblend the members of Si IV and C IV resonance doublets and provide the best fit of BALs through multiple components. Furthermore, the model provides the line function of every absorption component as well as the interpolation function that fits the whole BAL profile as a synthesis of multiple components. Finally, the fitting criteria of Si IV and C IV BALs, proposed in the current thesis (Stathopoulos et al. 2015, 2019) guarantee that the final fit, the number of components and the values of the measured physical parameters are uniquely determined. Finally, in order to apply all the above we developed a new software called ASTA (Tzimeas, Stathopoulos et al. 2019).
Our results strongly indicate that BALs are the product of individual absorption components originating in clumpy gas clouds in the line of sight. Therefore, BAL outflows are far from being smooth and homogeneous but are rather clumpy and unstable.
Based on our analysis we conclude that in each quasar Si IV and C IV BALs consist of the same number of absorption components. Each BAL trough arises from clumpy gas clouds which have similar locations, kinematic structure and physical conditions. Furthermore, Si IV and C IV originate in the same outflowing clouds which are clusters of smaller structures (called clumps).
The BAL trough formation mechanism is as follows: Each clump produces an absorption line the width of which depends on the thermal and microturbulent motion of ions inside each clump. The clump absorption lines are very close in velocity space thus overlapping and producing broad components which correspond to clouds. The broad cloud components blend among themselves producing the broad absorption troughs called BALs.
The main cause of BAL variability is due to changes in the optical depths of individual Si IV and C IV components. Variability occurs only in individual components of Si IV and C IV BAL troughs. None of the ten quasars exhibits variability in all of its components i.e. in its entire trough. Si IV and C IV absorption components present independent variations which is an indication for the distinction and independence of the clouds from each other. Concerning the responsible mechanism for the observed variability, our results favour the ionization change scenario to be more dominant, as we did not observe any changes in the radial velocities of clouds.
An extended Summary of this work, in English, is provided in pages 5-31 of the current thesis. Furthermore, references appearing in the above text can also be found in page 201.
Keywords:
Active Galaxies, Quasars, Absorption lines, UV spectroscopy
