Kim, Dongjin ORCID: 0000-0002-7038-2118 (2022). Circumnuclear gas in radio AGN revealed by molecular absorption. PhD thesis, Universität zu Köln.

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Active galactic nuclei (AGN) are among the most powerful energy sources in the Universe. The radio jet outflows triggered by AGN activity are supposed to play an important role in galaxy formation and evolution. However, our knowledge about the different types of radio jets and their influence on their host galaxies (i.e., radio jet feedback) is still limited. As a matter reservoir in the vicinity of the central engine, the circumnuclear gas is the key to study different accretion environments and the process of radio jet feedback in radio AGN. Although radio AGN tend to be gas-poor and distant, observations of molecular absorption lines against bright continuum radio jets facilitate the detection of this circumnuclear gas, independent from their cosmological distance. Moreover, spectral line VLBI observations towards molecular absorption lines provide a unique tool to study the small-scale (<pc) structure of the circumnuclear gas, which might be related to AGN fueling and feedback processes. By taking such advantages, this thesis aims to shed more light on the physical connection between the central portions of radio jets and their surrounding medium in radio AGN. The first chapter of this thesis briefly introduces the current understanding of AGN and touches on some remaining open questions. Chapter 2 briefly describes relevant observing tools for absorption line studies and principles of radio observations. Chapter 3 presents attempts to find molecular absorption lines towards 41 radio AGN with the 100-m Effelsberg and 30-m IRAM single dish telescopes. Although strong radio frequency interference and standing waves in the observed spectra resulted in a rather low detection rate, we detected molecular absorption in 3 AGN, and the 22 GHz H2O maser line in 2 AGN. Diagnostics using the molecular absorption and maser lines show different kinematics of the circumnuclear gas in the Seyfert galaxy NGC3079 and in the radio-loud AGN NGC1052. As a new approach to overcome technical hindrances from single-dish absorption studies, an interferometric absorption line study was conducted using the NOEMA interferometer. These results are presented in Chapter 4. The copious molecular absorption lines detected in 3C 293 and 4C 31.04 demonstrate the excellence of the interferometric array in absorption line observations towards radio AGN with bright continuum jets. In particular, the physical and chemical properties constrained by the detected molecular absorption lines better clarify the physical origin of the obscuring gas, and provide a diagnostics of radio jet feedback on the circumnuclear gas in the restarted (3C 293) and young (4C 31.04) radio AGN. In order to reach even smaller angular and spatial scales of the circumnuclear gas in radio AGN, high-resolution Very Long Baseline Interferometry (VLBI) observations were conducted towards OH absorption lines. Chapter 5 presents VLBI observations towards OH absorption lines at 6 GHz in the Seyfert galaxy NGC 3079 using the European VLBI Network (EVN). The still putative radio jet feedback was confirmed by the detection of spatially resolved OH absorption lines on the continuum jets. The blue-shifted OH absorption on the propagating jet component indicates jet-ISM interaction and a resultant molecular outflow at a distance of a few pc from the central engine. An increasing jet luminosity and decelerated jet motion show the impact of jet-ISM interaction on jet evolution. Chapter 6 presents High Sensitivity Array (HSA) and EVN observations towards two distinct radio-loud AGN Cygnus A and NGC 4261. The 6 GHz and 13 GHz excited OH absorption lines were searched to test the unified scheme and different accretion environments in Cygnus A (HERG and FR II) and NGC 4261 (LERG and FR I). The 13 GHz OH absorption lines detected in front of the jet, but not on the counter-jet in CygnusA contradict the classical scenario of an obscuring torus, which covers the counter-jet, but not the jet. The red-shifted and compact OH cloud in front of the jet might indicate fueling gas falling into the central engine. In contrast to this, no OH absorption was seen in NGC 4261 despite previous detection of 6 GHz OH absorption. Several scenarios are discussed to explain the lack of OH absorption in NGC 4261. Chapter 7 summarizes the implications of molecular absorption line studies towards radio AGN from the ongoing work and addresses some remaining questions. The absorption studies presented in this thesis provide new insights on the circumnuclear gas in the observed radio AGN, constraining their physical and chemical properties down to sub-parsec scales and near the jet-launching region. With planned imaging arrays of higher sensitivity and extended spectral coverage, future absorption line VLBI studies have a great potential to reveal even more details on the physical and geometrical properties of the circumnuclear gas and on the interaction between inflow and outflow of matter within the central regions of radio galaxies.

Item Type: Thesis (PhD thesis)
CreatorsEmailORCIDORCID Put Code
Thesis advisorKrichbaum, Thomas P.UNSPECIFIED
URN: urn:nbn:de:hbz:38-547997
Date: January 2022
Language: English
Faculty: Faculty of Mathematics and Natural Sciences
Divisions: Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics I
Subjects: Natural sciences and mathematics
Uncontrolled Keywords:
AGN, active galactic nuclei, VLBI, interferometry, circumnuclear gas, CND, torus, jet, black hole, SMBH, molecular gas, spectral line, absorption, ISM, accretion, jet, jet-ISM, AGN feedback, CygnusA, NGC3079English
Date of oral exam: 8 December 2021
NameAcademic Title
Jensus, J. AntonProf. Dr.
Eckart, AndreasProf. Dr.
Fiedler, StephanieProf. Dr.
Refereed: Yes


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