Dust In Active Galactic Nuclei

Dust is the cornerstone of the unification theory of active galactic nuclei (AGNs).This theory proposes that all AGNs are essentially―born equal‖: all types of AGNs aresurrounded by an optically thick dust torus and are basically the same object but viewedfrom different lines of sight. The large diversity in the observational properties of AGNsis simply caused by the viewing-angle-dependent obscuration of the nucleus: thoseviewed face-on are unobscured (allowing for a direct view of their nuclei) and recognizedas―type1‖AGNs, while those viewed edge-on are―type2‖AGNs with most of theircentral engine and broad line regions being hidden by the obscuring dust. It is this crucialrole played by dust in the unified model of AGNs that makes understanding dustproperties very important in understanding AGNs.However, little is known about the dust in the circumnuclear torus of AGNs. There isevidence suggesting that the size and composition of the dust in AGNs may differsubstantially from that of the Galactic interstellar dust, as reflected by the flat/―gray‖extinction curves, the2175extinction bump is not (or at least rarely) seen in AGNs,and the anomalous silicate emission or absorption features observed respectively in type1and type2AGNs. In this thesis we took a comparative study of the extinction and IRemission as well as the UV/IR spectroscopic properties and the inferred compositon, sizeand morphology of the dust in AGNs and the dust in the interstellar medium of the MilkyWay and other galaxies. Then we summarized our current understanding of the extinctionand infrared emission properties of the circumnuclear dust in AGNs as well as theinferred dust composition and size distribution. The main results are the following:1.Little is known about the wavelength dependence of the extionction caused by thecircumnuclear dust of AGNs. So far, the AGN extinction curves are mainly inferred fromcomposite quasar spectra and individual reddened AGNs. These two approaches derivecontradicting results: the former oftern reveals a―gray‖extinction, implying that the sizedistribution of the dust in the AGN circumnuclear environments is skewed towardssubstantially large grains; the later oftern suggests a steep–rising SMC-like extinction,indicating a preponderance of small grains near the nucleus.2.The2175bump not seen in AGNs suggests that its carrier (e.g. PAHs) may havebeen photodestroyed by energetic photons from the central engine; The3.4μm aliphatichydrocarbon absorption feature is seen in AGNs, but the exact carrier of this featureremains uncertain; The silicate features profiles of AGNs are rather diverse in peak wavelengths, widths, strengths, and band ratios of18μm O-Si-O feature to the9.7μmSi-O feature. Most of the type2AGNs display silicate absorption bands, while the silicatefeatures of type1AGNs appear in emission; The3.3,6.2,7.7,8.6and11.3μm PAHemission features are absent in AGNs, this is commonly interpreted as the destruction ofPAHs by extreme UV and soft X-ray photons in AGNs; The ice absorption features arenot expected in AGNs due to the high dust temperatures for ice mantles to survive.3.To constrain the dust size distribution and the size and geometry of the dust torus,various models have been proposed to explain the observed IR emission spectral energydistribution of AGNs, radiated by the circumnuclear dust heated by the AGN illumination.We reproduced the9.7μm and18μm silicate emission features of3C273, NGC3998andNGC7213, found that the silicate grains in AGNs are>1μm, much larger than the typicalsize (~0.1μm) of the Galactic interstellar dust.