The effect of radiation pressure on dusty absorbing gas around active galactic nuclei

Many active galactic nuclei (AGN) are surrounded by gas which absorbs the radiation produced by accretion on to the central black hole and obscures the nucleus from direct view. The dust component of the gas greatly enhances the effect of radiation pressure above that for Thomson scattering so that an AGN which is sub-Eddington for ionized gas in the usual sense can appear super-Eddington for cold dusty gas. The radiation pressure enhancement factor depends on the AGN spectrum but ranges between unity and about 500, depending on the column density. It means that an AGN for which the absorption is long-lived should have a column density of NH > 5 × 1023λ cm−2, where λ is its Eddington fraction Lbol/LEdd, provided that NH > 5 × 1021 cm−2. We have compared the distribution of several samples of AGN – local, Chandra Deep Field-South and Lockman Hole – with this expectation and find good agreement. We show that the limiting enhancement factor can explain the black hole mass–bulge mass relation and note that the effect of radiation pressure on dusty gas may be a key component in the feedback of momentum and energy from a central black hole to a galaxy.