Active Galactic Nuclei (AGNs) are known as one of the brightest object in the universe. The energy source of their whole enormous emission is supposed to be generated by central super massive black hole and accreting material onto it. In the context of the standard unified model, the structure of AGN is explained as below. First, the accreting gas around the super massive black hole form an accretion disk at 104 K or higher temperature, transferring its gravitational potential energy into thermal emission radiated in wavelength range from optical to X-ray. Next, the super massive black hole and accretion disk is surrounded by gas clouds which move at high velocity more than a few 103km/s. The gas clouds are ionized by UV flux from accretion disk and radiate emission lines when recombination occurs, which are broadened due to high velocity motion of the clouds. Then, at the outer region of the broad emission line region (BLR), there distribute optically thick dust. The dust obstruct line of sight from a certain direction to the accretion disk or the BLR. As a result of the obscuration, two types of spectrum of AGN with or without broad emission lines (BELs) could be observed (Antonucci 1993). Each type of AGN is called as type 1 and type 2 AGN respectively.

Now consider about relationship of accretion disk and surrounding dust. The dust around BLR not only obscure the central region of AGN but also radiate thermal emission, warmed up by UV/optical emission from accretion disk. The highest temperature of warmed dust is limited by sublimation temperature of dust grains which is about 1800K for graphite grains. Thus the spectrum of thermal emission by dust should have its peak at near-infrared (NIR) wavelength region.

Meanwhile, sublimation of dust by UV/optical flux from accretion disk leads to hollow distribution of dust, like a torus, because the dust grains at close region to accretion disk reach its sublimation temperature and could not survive. The sublimation radius of dust γ(sub) should be proportional to the square root of UV/optical luminosity L(UV/opt), or γ(sub) ∝ L(0.5)(UV/opt), because UV/optical flux is diluted with increasing the distance from accretion disk. In other words, the innermost radius of dust torus is determined by UV/optical luminosity of accretion disk.

With such torus-like structure of dust and thermal reprocessing of UV/optical emission into NIR emission, it is expected that variation of UV/optical flux is followed by NIR flux variation with a certain time delay corresponding to light travel time between accretion disk and innermost region of dust torus. Such phenomena called dust reverberation have been actually observed in a number of AGNs (e.g. Clavel et al. 1989; Glass 1992) and given a strong support to the unified model of AGN.

Inversely, using dust reverberation model describe above, intrinsic UV/optical luminosity of accretion disk could be obtained from observed time delay between optical and NIR variation with an appropriate dust sublimation model. The derived intrinsic luminosity could be converted into luminosity distance of AGNs without assuming any cosmological parameter, thus the time delay between UV/optical variation and NIR variation at AGNs could be available as a new distance indicator. Aiming at establishment of this new distance indicator, Multicolor AGN Monitoring project (MAGNUM, Yoshii 2002; Yoshii et al. 2003) have started photometric monitoring of AGNs at year 2000. They succeeded in obtaining the most intensive and accurate light curve of tens of AGNs in wide wavelength range from optical and NIR after continuing the photometric monitoring with a 2m telescope specialized to the purpose for 8 years. For a number of the target AGNs, they estimated the time delay between optical and NIR from the light curves employing the Cross Correlation function (CCF) analysis. The derived time delays were confirmed to strongly correlate with the optical luminosity as expected from the dust reverberation model (Minezaki et al. 2004). The also confirmed that the inner region of dust torus were located at just outer region of BLR, comparing the time delay between optical and NIR variation with that between optical continuum and broad emission line (Suganuma et al. 2006). These results observationally supported the dust torus structure in the unified model of AGN.With accurate and intensive light curves of MAGNUM project, Tomita (2005) also succeeded in extracting accretion disk flux in NIR band and conclude that the shape of spectral energy distribution (SED) of accretion disk flux could be represented as typically α=0.0-0.5 in a form of power law index of the spectrum.

In this dissertation, I derived the time delays between optical and near-infrared variation of 17 Nearby Seyfert 1 galaxies with photometric monitoring data of MAGNUM project. One of the aim of this study is to obtain the largest homogeneous sample of the inner radii of dust tori ever before. With this sample, it is possible to confirm the generality of the dust reverberation or basic structure of dust in AGN observationally. This is also necessary for establishment of new distance indicator by dust reverberation.

Besides the estimation of time delays in large number of AGNs, I also payed attention to longevity of the monitoring program of MAGNUM and estimated the time delay for each plural variation features on the light curves of an individual AGN. Although it has been showed that the time delays or inner radii of dust tori were correlated with the optical luminosity of AGN, it have not been confirmed that the radius of dust torus would be vary or not with the UV/optical variation in an individual AGN. By multi-epoch measurements of the time delay, the existence of variation of the time delay in individual AGN could be discussed. To investigate the response of time delay against the optical variation would give important suggestions on characteristics of the dust in AGN central region.

For these aims, I derived the light curves in UBVIJHK band for the 17 nearby Seyfert galaxies after data reduction and aperture photometry. In the photometric aperture, contaminating flux from host galaxies or narrow emission lines were included. These component disturb accurate estimation of the optical luminosity of accretion disk, so I subtract these component. Besides the contaminating flux above, the flux from outer and low temperature region of accretion disk is contained in NIR bands (Tomita 2005). The NIR flux from accretion disk is supposed to be synchronized rather to optical flux from accretion disk than thermal dust flux, so it also disturb accurate estimation of inner radius of dust torus. I subtracted the contamination from all Seyfert galaxies assuming the power law index of accretion disk flux α=0.0.

Then, to estimate inner radius of dust torus in nearby Seyfert galaxies,I employed CCF analysis. In CCF analysis, it is necessary to interpolate discrete observed light curve to obtain a flux pair at the same observational time. Suganuma et al. (2006) suggested two interpolation method of bi-directional (BI) interpolation method and equal sampling (ES) method, and I employed the BI method because the method weighted more actually observed data than simulated data. The error of the time delay were estimated by Monte-Carlo simulation. For the estimation of inner radius of dust torus, I selected light curves in V band as an indicator of UV/optical flux from accretion disk and that in K band as thermal dust flux.

As a result of CCF analysis for light curves obtained by MAGNUM project, 49 of the time delay could be derived with 17 nearby Seyfert 1 galaxies. Figure showed strong correlation of these time delays, which indicated the inner radii of dust tori, with absolute magnitude in V band, which indicated the UV/optical luminosity of accretion disk. This is the first time to show the correlation with such large and homogeneous sample. The correlation derived here showed good agreement with scaling law of the time delay Δt∝L(0.5)(UV/opt) predicted by dust sublimation model in the wide range of absolute magnitudes of AGNs in V band MV from -15 to -2. By generality of the correlation, it is confirmed that the basic property concerning dust sublimation in AGN is almost common in nearby Seyfert 1 galaxies along the wide range of UV/optical luminosity, that is for example, the dust grain size or SED of dust warming flux. I also compared the derived 49 inner radii of dust tori with the BLR size in Bentz et al. (2006). As Suganuma et al. (2006) showed, the inner radii of dust tori were distributed just outside of outer region of BLR. The drawing of unified model were again confirmed observationally with large homogeneous sample at the point of radial distribution of BLR and dust torus.

On the other hand, paying attention to the multi-epoch measurements of the time delay in an individual AGN, the time delay seemed to be variable beyond its error. Especially NGC4151 and a few Seyfert galaxies showed apparent variation of Δt beyond the 3σs. This is the first detection of the variation of the inner radius of dust torus with such clarity. The most rapid variation of the time delay of NGC 4151 was occurred during 309 days, and amount of variation of the time delay corresponded to a light-travel distance of 6.35×10(11)km. If such a decrease of inner radius of dust torus would occur with redistribution of dust grains supplied from outer regions of, the infall velocity would be 2.4×104km/s. This seems highly unlikely when we compare this infall velocity with a few 103km/s for the velocity dispersion of BLR clouds which exist just near the inner radius of dust torus. Consequently, it is concluded that reformation of dust grains did occur in the central region where they had been sublimated.

Following the time-changing track of the time delay and Mν of NGC4151, it also seemed that they did not always follow the general regression line of Δt∝L(0.5)(UV/opt), though they showed generally strong correlation with other Seyfert galaxies. If dust reformation or sublimation occur right after variation in UV/optical flux, the track would be drawn along the same direction of the general regression line. This would implicate that the dust sublimation or reformation is occur with a certain amount of time lag behind the UV/optical variation due to, for instance, clumpy and self-shielding structure of dust torus.

本論文は5章からなる。第1章はイントロダクションであり、活動銀河核について、一般的に受け入れられている描像や過去の観測がまとめられている。第2章は本論文で行った観測の詳細について、第3章はそのデータ解析手法について、第4章は得られた光度曲線や活動銀河核の光度などの結果がまとめられている。第5章において結果の解釈・議論を行い、論文全体のまとめも行っている。

活動銀河核は、中心にある超巨大ブラックホールのごく近傍の降着円盤、そのまわりに広がった広輝線放射領域(Broad Line Emitting Region)、さらにそれらをとりまく塵を含むトーラス(Dust Torus・以下塵トーラスと表記)という構造をしていると考えられている。降着円盤や広輝線放射領域からの放射の一部は塵トーラスによってさえぎられるため、活動銀河核は見込む角度によりスペクトルに広輝線が見られる1型と見られない2型に分類される、とする統一モデルが標準的であり多くの専門家に受け入れられている。塵は降着円盤からの紫外線・可視光放射によって温められ、熱的再放射を行う。塵の最高温度は昇華温度で決まると考えられ、例えばグラファイトを主成分とする塵では約1800Kになり、その熱的放射が近赤外線として観測される。

活動銀河核においては、塵トーラスは降着円盤よりも外側にあるため、塵の出す近赤外線放射は紫外線・可視光の変光に比べ遅延時間を持って追随する、反響現象(reverberation)が観測される。この反響現象は1980年代より観測されており、活動銀河核の統一モデルを支持する強力な観測的証拠の1つとなっている。また遅延時間は降着円盤から塵トーラスの内縁までの距離にほぼ比例するため、遅延時間と可視光・紫外線の光度と良い相関関係が期待され、実際、過去の研究によって、サンプル数は少ないものの相関が確認されていた。

本論文は、これら反響現象と活動銀河核の性質のより詳細な理解を目的として、ハワイ・ハレアカラ山頂の口径2m光赤外線望遠鏡を用いたMAGNUMプロジェクトによって、近傍のセイファート銀河の活動銀河核17個を最長8年間にわたって観測した結果をまとめたものである。

観測はUBVIJHKの7バンドで行われたが、特に可視光ではVバンド、近赤外線ではKバンドが中心となっている。データ解析はMAGNUMプロジェクトによって共同で開発されてきた手法に沿って行われている。ただし本論文においては、近赤外線波長域への降着円盤成分の混入や狭輝線放射の混入を除去することを統一して行っており、また遅延時間の測定においても、シミュレーションによる確認も含め、丁寧な解析手法を統一的に用いている。

その結果、2つの主な結論を得た。第一には、17個のセイファート銀河のVバンド絶対光度LVと遅延時間Δtの相関関係が、統一モデルで期待されるΔt∝(LV)(1/2)の関係によく従い、その絶対等級(Vバンド)の範囲も-15.5>MV>-22.5という広い範囲であることを示した。この相関はこれまでで最も大きくかつ均一な観測に基づくサンプルについて例外なく示されたもので、近傍セイファート銀河の活動銀河核の塵トーラスに関わる性質が多くの天体で共通であることを示している。またこの相関を広輝線放射領域についての相関関係と比べると、塵トーラスが広輝線放射領域のすぐ外側を覆うような関係となっており、両者の連続性を示唆している。

第二の結論は、同一の活動銀河核の観測において、遅延時間が有意に変化をしていることを発見したことである。特にNGC4151の活動銀河核について詳細な議論がなされており、例えば最も大きな遅延時間Δtの変化として309日間に65.1日から40.6日へと24.5日の減少が観測された。この減少量は塵の粒子の落下と考えるには不自然であり、トーラス内縁部で塵粒子の再生成などの現象が起きているためと考えられる。また絶対光度LVと遅延時間Δtの変化は必ずしもΔt∝(LV)(1/2)の関係を保って変化をしておらず、一定の時間差が期待されることがわかった。これは、例えば塵の雲が自己遮蔽的な形状をしていることによって理解できる。

以上本論文は、活動銀河核の塵トーラスについて、過去でもっとも明快な絶対光度と遅延時間の相関関係を示し、また遅延時間の変化を観測的に初めて示した画期的なものであり、高く評価できる。

なお、本論文は、小林行泰・吉井譲・峰崎岳夫・青木勉・塩谷圭吾・菅沼正洋・富田浩行・坂田悠・菅原章太・Bruce Petersonとの共同研究であるが、論文提出者が主体となって観測・解析・結果のまとめを行ったもので、論文提出者の寄与が十分であると判断する。

したがって、博士(理学)の学位を授与できると認める。