Clumpy winds and the obscuration of Active Galactic Nuclei

Abstract

The role of star-formation driven outflows in the obscuration of the central source in the Active Galactic Nuclei (AGN) is discussed. The outflow from a sub-parsec scale accretion disc is numerically modelled for parameters appropriate to the Galactic Centre. The resulting obscuration pattern is very patchy, with some lines of sight becoming optically thick to Thomson scattering. A fixed observer would see column depth changing by factors of many over time scales of order months to hundreds of years, depending on the physical size of the outflow region. Such winds may be relevant for obscuration of some AGN and especially "changing look AGN". However, averaged over the sky as seen from the central source, these winds are always optically thin unless wind outflow rates are super-Eddington. A simple scaling argument shows that this is true not only for stellar-driven winds but for any AGN winds. We therefore conclude that AGN winds are unable to account for the vast majority of optically thick obscured AGN (a significant fraction of all AGN). We suggest that the most likely source of optically thick obscuration in AGN is a warped parsec scale accretion disc.