||Observational Study on the Central Subparsec-Scale Structure of Active Galactic Nucleus with Very Long Baseline Interferometer
Galaxies are fundamental elements of the universe, and consist of stars, gas, dark matterand supermassive black holes (SMBHs). It is important to understand galaxy formationand evolution when clarifying the evolution of universe. It is believed that stars aremonotonically formed from gas in galaxies, and that galaxies evolve. However, fromrecent researches on galaxy formation and evolution, it is suggested that feedback effectthat constrains star formation is a key process of galaxy evolution. One of importantfeedback effects is feedback from active galactic nucleus (AGN).AGN is a bright source at broad wavelength bands from radio to -ray in the galacticcenter. Typical size of AGN is 103 less than that of the host galaxy, but AGN shines asbrightly as the whole galaxy. This huge energy is come from gravitational energy releasedby mass accretion onto SMBH residing in the innermost region of AGN. The enormousreleased energy forms structures such as accretion disk, obscuring torus and jet in thecenter of AGN. Jets in radio-loud AGNs are relativistic plasma ow, and the size of jetis comparable to that of host galaxy, sometimes beyond host galaxy. Thus, jets play animportant role in releasing energy of host galaxy.Radio galaxies are misaligned radio-loud AGNs, and ideal sources to explore generalproperties of jets since the misalignment of the jet axis with the line of sight providesa 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 radiocore. At the edge of jets, shock wave regions called hotspots are formed, and radiolobes extend beyond hotspots. Generally, these structures can be seen beyond 1,000 pcfrom the galactic center, and a big problem is how such relatively collimated and largestructures can be formed and evolved. Observationally, it is known that radio-loud AGNsmake up about 10% of the whole AGNs, and most of AGNs do not have radio jets. Thereis a problem why a part of galaxies show such phenomena. It is necessary to unveil thephysics of the vicinity of jet nozzle in order to understand the formation process of radiojet. 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 radiogalaxy 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 intermittentjet activity, and the radio brightness has increased since 2005. Thus, 3C 84 is one of thebest source to study jet properties. Using VERA, Nagai et al. (2010) found that this34activity was ascribed to the central subparsec-scale core, accompanying the ejection of anew bright component. According to the Very Long Baseline Array (VLBA) observationat 43 GHz, Suzuki et al. (2012) found that the new bright component had emerged froma radio core before 2005, and traveled southward following a parabolic trajectory on thecelestial sphere. In this study, I present the detailed kinematics of new bright componentto reveal its true nature. I investigate kinematics of new bright component in detail from2007 to 2013 by monitoring the subsequent motion for non-linear trajectory found bySuzuki et al. (2012).One of results is the apparent speed of new bright component relative to the radiocore is almost constant and sub-relativistic (0:27 0:02c) from 2007 October to 2013December. This property suggests that new bright component may be the head of mini-radio lobe including hotspots, rather than a relativistic knotty component formed asinternal shock in underlying continuous jet ow. This result implies that the radio lobein radio-loud AGNs might be already formed in subparsec-scale jets in the vicinity ofSMBHs.Another result is that new bright component might follow a helical path with a periodof about ve years thanks to highly-frequent observations. Although I cannot reliablyidentify the origin causing the wobbling motion because of the insufficient time span ofour dataset and the lack of the information of absolute reference position, the motionmight reect the accretion disk precession induced by a spinning SMBH. In order toobtain the robust result, we continue to monitor the subparsec-scale jet of 3C 84 withhigh resolution phase-referencing VLBI.As mentioned above, I found that hotspots in radio lobes in radio galaxies might bealready formed in subparsec-scale jets close to the central SMBHs. I also found thathotspots in radio lobe may be precessed by a spinning SMBH. These results are achievedby unprecedented highly-frequent observations. It is important that the fact that hotspotsmight be formed in subparsec-scale jets near the central SMBH can constrains the physicalstate such as density and velocity in jet base when understanding the formation andevolution of hotspots. Those ndings will contribute to constructing more sophisticatedtheoretical models in the future.
Hokkaido University（北海道大学）. 博士(理学)