Clumpy tori illuminated by the anisotropic radiation of accretion discs in active galactic nuclei

TL;DR

This study investigates how clumpy tori illuminated by anisotropic accretion disc radiation can explain the observed correlation between dust covering factor and AGN properties, highlighting the importance of clumpy structures over smooth models.

Contribution

It introduces and tests eight models combining dust distribution and radiation anisotropy, demonstrating that clumpy torus models better match observations than smooth models.

Findings

Clumpy torus models explain observed correlations effectively.

Smooth models fail to produce the required covering factors.

Significant scatter remains, indicating additional factors influence the observations.

Abstract

In this paper, we try to explain the observed correlation between the covering factor (CF) of hot dust and the properties of active galactic nuclei (AGNs), e.g., the bolometric luminosity ($L_{\rm{bol}}$) and black hole mass ($M_{\rm{BH}}$). Combining the possible dust distribution in the torus, the angular dependence of the radiation of the accretion disc, and the relation between the critical angle of torus and the Eddington ratio, there are eight possible models investigated in our work. We fit the observed CF with these models to determine the parameters of them. As a result, clumpy torus models can generally explain the observed correlations of tori, while the smooth models fail to produce the required CFs. However, there is still significant scatter even for the best-fitting model, which is the combination of a clumpy torus illuminated by the anisotropic radiation of accretion disc in an AGN. Although some of the observed scatter is due to the uncertainties in measuring $L_{\rm{bol}}$ and $M_{\rm{BH}}$, other factors are required in more realistic model. The models examined in this paper are not necessary to be the physical model of tori. However, the reasonable assumptions selected during this process should be helpful in constructing physical models of tori.