Radiation pressure and absorption in AGN: results from a complete unbiased sample from Swift

Outward radiation pressure can exceed the inward gravitational pull on gas clouds in the neighbourhood of a luminous active galactic nucleus (AGN). This creates a forbidden region for long-lived dusty clouds in the observed columnn density-Eddington fraction plane. (The Eddington fraction lambda(Edd) is the ratio of the bolometric luminosity of an AGN to the Eddington limit for its black hole mass.) The Swift/Burst Alert Telescope catalogue is the most complete hard X-ray selected sample of AGN and has 97 low-redshift AGN with measured column densities N-H and inferred black hole masses. Eddington fractions for the sources have been obtained using recent bolometric corrections and the sources have been plotted on the N-H-lambda(Edd) plane. Only one source lies in the forbidden region and it has a large value of N-H due to an ionized warm absorber, for which radiation pressure is reduced. The effective Eddington limit for the source population indicates that the high column density clouds in the more luminous objects lie within the inner few pc, where the central black hole provides at least half the mass. Our result shows that radiation pressure does affect the presence of gas clouds in the inner galaxy bulge. We discuss briefly how the N-H-lambda(Edd) plane may evolve to higher redshift, when feedback due to radiation pressure may have been strong.