# AGN accretion disk vs. torus

The torus is the donut of dust encircling the Active Galactic nucleus. The accretion disk is inside the torus. Is there a boundary between the two? At what point does a torus become an accretion disk? What are their differences?

The Figure above is Figure 1 of this paper. It shows the scales of various features of the unified AGN model in units of $\log_{10}(r/\text{pc})$. Remember that a parsec is about 3.26 light-years or 200,000 AU (or 5000 times Pluto's mean orbital distance). The $10^{-5}$ mark is roughly the Schwarzschild radius for a $10^8\text{M}_\odot$ black hole.
If I understand the literature right, the disk "ends" when it reaches the self-gravity radius $R_{sg}$ where local gravity from the disk exceeds the vertical component of the black hole gravity and the disk becomes unstable. $$R_{sg}\approx 1680 \left[\frac{M_{BH}}{10^9 M_\odot}\right]^{-2/9} \alpha^{2/9} \left[\frac{L_{AGN}}{L_{Edd}}\right]^{4/9} \left[\frac{\epsilon}{0.1}\right]^{-4/9} R_S$$ where $\alpha$ is the viscosity parameter $\approx 0.01-0.1$. For a $10^9 M_\odot$ black hole with $L_{AGN}/L_{Edd}\approx 0.1$ $R_{sg}\approx 0.04$ pc. This appears to be in rough agreement with observations of quasars.