A brief introduction to the AGN torus observation and theory
Torus is an important structure through which the major differences between type-I and type- II AGNs can be explained in the original AGN unification scheme. There are different models assuming a certain geometry and dust composition, including continuous models, clumpy models, and composite models. But they all face some observational challenges. The studies on the X-ray reflection features such as Fe K𝛼 line and Compton hump, and the IR SED and spectral feature have got many important results on the nuclear obscuration properties. In the field of time-domain astronomy, variations of the obscuring material in the torus have been found, which is evidence of clumpy torus. From the near- and mid-IR reverberation Mapping, the inner size of the obscuring torus can be measured and a tight correlation with AGN luminosity is obtained. Observations of the outer regions of the torus have come from mid-IR interferometry imaging. The polar dust has also been found in many local AGN using high resolution observations. The ALMA sub-mm observations also lead to a better understanding of the properties of the dusty molecular tori, their connection to the host galaxy and the feedback mechanisms. In this talk, I will give a brief introduce about how is our current picture of the torus formed and some observational evidences.