Are supermassive black holes shrouded by ‘super-Oort’ clouds of comets and asteroids?

Abstract

The last decade has seen a dramatic confirmation that in situ star formation is possible inside the inner parsec of the Milky Way. Here we suggest that giant planets, solid terrestrial-like planets, comets and asteroids may also form in these environments, and that this may have observational implications for active galactic nuclei (AGN). Like in debris discs around main-sequence stars, collisions of large solid objects should initiate strong fragmentation cascades. The smallest particles in such a cascade – the microscopic dust – may provide a significant opacity. We put a number of observational and physical constraints on AGN obscuring tori resulting from such fragmentation cascades. We find that tori fed by fragmenting asteroids disappear at both low and high AGN luminosities. At high luminosities, L∼LEdd, where LEdd is the Eddington limit, the AGN radiation pressure blows out the microscopic dust too rapidly. At low luminosities, on the other hand, the AGN discs may avoid gravitational fragmentation into stars and solids. We also note that these fragmentation cascades may be responsible for astrophysically ‘large’ dust particles of m sizes that were postulated by some authors to explain unusual absorption properties of the AGN tori.