Warped accretion discs and the unification of active galactic nuclei

Abstract

Orientation of parsec-scale accretion discs in active galactic nuclei (AGNs) is likely to be nearly random for different black hole feeding episodes. Because AGN accretion discs are unstable to self-gravity on parsec scales, star formation in these discs will create young stellar discs, similar to those recently discovered in our Galactic Centre. The discs blend into the quasi-spherical star cluster enveloping the AGN on time-scales much longer than a likely AGN lifetime. Therefore, the gravitational potential within the radius of the black hole influence is at best axisymmetric rather than spherically symmetric. Here we show that, as a result, a newly formed accretion disc will be warped. For the simplest case of a potential resulting from a thin stellar ring, we calculate the disc precession rates, and the time-dependent shape. We find that, for a realistic parameter range, the disc becomes strongly warped in a few hundred orbital times. We suggest that this, and possibly other mechanisms of accretion disc warping, have a direct relevance to the problem of AGN obscuration, masing warped accretion discs, narrow Fe Ka lines, etc.