From thin to thick: The impact of X-ray irradiation on accretion disks in active galactic nuclei

We argue that the X-ray and UV flux illuminating the parsec-scale accretion disk around luminous active galactic nuclei (AGNs) is super-Eddington with respect to the local far-infrared dust opacity. The far-infrared opacity may be larger than in the interstellar medium of the Milky Way due to a combination of supersolar metallicity and the growth of dust grains in the dense accretion disk. Because of the irradiating flux, the outer accretion disk puffs up with a vertical thickness h ∼ R. This provides a mechanism for generating a geometrically thick obscuring region from an intrinsically thin disk. We find obscuring columns ∼10²²-10²³ cm^-2, in reasonable agreement with observations.