Shanghai Astronomical Observatory Astrophysics Colloquium

Title：Formation of dust clumps in the torus of active galactic nuclei

Speaker：Prof.Xinwu Cao ( Zhejiang University)

Time：3:00 pm Apr. 30th (Thursday)

Tencent Meeting：46822606747 password: 6360

Location: Lecture Hall, 3rd floor

Abstract：The putative dusty torus is a key ingredient of the unification scheme of active galactic nuclei (AGN), but its origin remains a mystery. We put forward a new physical model to explain how a large number of small dusty gas clumps form and they collectively appear as a geometrically thick dynamic dusty torus. The circumnuclear hot gas flows toward the central black hole (BH) and forms a rotating disk on subparsec scales. A fraction of inflowing hot gas condenses to form small cold clumps due to thermal instabilities, when the accretion rate is sufficiently high. These cold dusty gas clumps are irradiated by the central accretion disk and reradiate as dust emission mostly in the infrared. We propose that the dusty torus in AGN consists of such cold clumps vertically supported by the radiation force against gravity. For clumps with suitable column density, the vertical component of the BH gravity is in quasistatic equilibrium with the infrared radiation force together with the vertical component of the disk radiation force. We further show that the hot gas in the rotating flow condenses to cold clumps only if its accretion rate is higher than about 1% of the Eddington rate. The radiation force is unable to lift the cold gas clumps up away from the midplane when the luminosity of the disk surrounding the BH is lower than 0.1% of the Eddington luminosity. These two features of our model may provide a physical explanation for the lack of evidence of dusty tori in low-luminosity AGN.

CV：Xinwu Cao is a professor in School of Physics, Zhejiang University. He got his Ph. D degree from the University of Science and Technology of China in 1994. During 1999-2019, he was a research professor in Shanghai Astronomical Observatory. He moved to Zhejiang University in 2019. His main research interests are the theories of the black hole accretion disks/outflows, and their applications in the black hole X-ray binaries and active galactic nuclei.  

https://person.zju.edu.cn/0019144

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