学位論文 Observational Study on the Central Subparsec-Scale Structure of Active Galactic Nucleus with Very Long Baseline Interferometer

日浦, 皓一朗

2016-03-24
内容記述
Galaxies are fundamental elements of the universe, and consist of stars, gas, dark matter and supermassive black holes (SMBHs). It is important to understand galaxy formation and evolution when clarifying the evolution of universe. It is believed that stars are monotonically formed from gas in galaxies, and that galaxies evolve. However, from recent researches on galaxy formation and evolution, it is suggested that feedback effect that constrains star formation is a key process of galaxy evolution. One of important feedback effects is feedback from active galactic nucleus (AGN). AGN is a bright source at broad wavelength bands from radio to -ray in the galactic center. Typical size of AGN is 10􀀀3 less than that of the host galaxy, but AGN shines as brightly as the whole galaxy. This huge energy is come from gravitational energy released by mass accretion onto SMBH residing in the innermost region of AGN. The enormous released energy forms structures such as accretion disk, obscuring torus and jet in the center of AGN. Jets in radio-loud AGNs are relativistic plasma ow, and the size of jet is comparable to that of host galaxy, sometimes beyond host galaxy. Thus, jets play an important role in releasing energy of host galaxy. Radio galaxies are misaligned radio-loud AGNs, and ideal sources to explore general properties of jets since the misalignment of the jet axis with the line of sight provides a detailed view of the structure in the jet. Radio galaxies have compact radio source, radio core, in the central region, and relatively collimated radio jets extend from radio core. At the edge of jets, shock wave regions called hotspots are formed, and radio lobes extend beyond hotspots. Generally, these structures can be seen beyond 1,000 pc from the galactic center, and a big problem is how such relatively collimated and large structures can be formed and evolved. Observationally, it is known that radio-loud AGNs make up about 10% of the whole AGNs, and most of AGNs do not have radio jets. There is a problem why a part of galaxies show such phenomena. It is necessary to unveil the physics of the vicinity of jet nozzle in order to understand the formation process of radio jet. This jet base can be spatially resolved by very long baseline interferometer (VLBI) in the radio band. Against this background, I tried to unveil the subparsec-scale radio jet of the radio galaxy 3C 84/NGC 1275 by monitoring time variability of radio jet with the VLBI Ex- ploration of Radio Astrometry (VERA) array. Radio galaxy 3C 84 shows intermittent jet activity, and the radio brightness has increased since 2005. Thus, 3C 84 is one of the best source to study jet properties. Using VERA, Nagai et al. (2010) found that this 3 4 activity was ascribed to the central subparsec-scale core, accompanying the ejection of a new bright component. According to the Very Long Baseline Array (VLBA) observation at 43 GHz, Suzuki et al. (2012) found that the new bright component had emerged from a radio core before 2005, and traveled southward following a parabolic trajectory on the celestial sphere. In this study, I present the detailed kinematics of new bright component to reveal its true nature. I investigate kinematics of new bright component in detail from 2007 to 2013 by monitoring the subsequent motion for non-linear trajectory found by Suzuki et al. (2012). One of results is the apparent speed of new bright component relative to the radio core is almost constant and sub-relativistic (0:27 0:02c) from 2007 October to 2013 December. This property suggests that new bright component may be the head of mini- radio lobe including hotspots, rather than a relativistic knotty component formed as internal shock in underlying continuous jet ow. This result implies that the radio lobe in radio-loud AGNs might be already formed in subparsec-scale jets in the vicinity of SMBHs. Another result is that new bright component might follow a helical path with a period of about ve years thanks to highly-frequent observations. Although I cannot reliably identify the origin causing the wobbling motion because of the insufficient time span of our dataset and the lack of the information of absolute reference position, the motion might re ect the accretion disk precession induced by a spinning SMBH. In order to obtain the robust result, we continue to monitor the subparsec-scale jet of 3C 84 with high resolution phase-referencing VLBI. As mentioned above, I found that hotspots in radio lobes in radio galaxies might be already formed in subparsec-scale jets close to the central SMBHs. I also found that hotspots in radio lobe may be precessed by a spinning SMBH. These results are achieved by unprecedented highly-frequent observations. It is important that the fact that hotspots might be formed in subparsec-scale jets near the central SMBH can constrains the physical state such as density and velocity in jet base when understanding the formation and evolution of hotspots. Those ndings will contribute to constructing more sophisticated theoretical models in the future.
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Hokkaido University(北海道大学). 博士(理学)
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http://eprints.lib.hokudai.ac.jp/dspace/bitstream/2115/61697/1/Koichiro_Hiura.pdf

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