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The Intricate Connection Between Major Mergers and AGN with the Highest Eddington Ratios

Marian, Victor

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Abstract

Over the past decade, research has revealed contradictory results on whether mergers of galaxies of similar mass, so-called major mergers, are the dominant or at least an essential mechanism for the formation of active galactic nuclei (AGN) and the growth of supermassive black holes (SMBH) in galaxies. In this work, it is investigated whether such a connection exists for the ‘most plausible’ objects – broad line AGN with the highest Eddington ratios. The first sample is based on HST/WFC3 observations and focuses on these objects that possess an Eddington Ratio >0.7 at z=2, the peak epoch of black hole activity. To validate the results and to identify possible discrepancies between observations and simulations, a comparison study is performed on similar objects modeled by the large-scale cosmological simulation ILLUSTRIS-TNG. An additional sample consists of local z=0.2 AGN with an Eddington ratio >0.3, observed with the VLT/FORS. All AGN samples are compared to inactive galaxies of similar stellar mass and redshift. For all three individual studies, the applied methodology is identical: sources are visually identified and classified into merging and unperturbed galaxies. An excess for the respective AGN host galaxies in the subsequently derived major merger fractions would then indicate that AGN are predominantly triggered by such galaxy mergers. The analysis of the observed galaxies at z=2 yields merger fractions of 0.24 ± 0.09 and 0.19 ± 0.04 for the AGN hosts and inactive galaxies, respectively. In addition to the visual assessment, the ILLUSTRIS-TNG data set is analyzed by evaluating the galaxies’ merger histories and accounting for typical observational uncertainties. The resulting best estimates give 0.27 ± 0.07 and 0.22 ± 0.01, indicating an excellent agreement between the two studies at this redshift. In contrast, with 0.41 ± 0.12 and 0.08 ± 0.06, the fractions at z=0.2 show a clear excess in the incidence of major mergers for the AGN host galaxies. The results are analyzed extensively, including an investigation of possible influences due to selection effects, a time lag between the visibility of galaxy mergers and the AGN, as well as the methodology used. While there is no significant evidence that such major mergers are the dominant process for the existence of the studied AGN at z=2, such gravitational encounters appear to be an important mechanism for comparable AGN at z=0.2. This may be due to the fact that the gas required for SMBH to achieve such high accretion rates is less abundant in galaxies at low redshifts, leading to major mergers playing a more dominant role in the local Universe. Nevertheless, regardless of redshift, for a minimum of 50% of this particular AGN population, the origin remains unclear.

Document type: Dissertation
Supervisor: Jahnke, Dr. Knud
Place of Publication: Heidelberg
Date of thesis defense: 28 April 2021
Date Deposited: 11 May 2021 07:13
Date: 2021
Faculties / Institutes: The Faculty of Physics and Astronomy > Dekanat der Fakultät für Physik und Astronomie
DDC-classification: 520 Astronomy and allied sciences
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