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

TL;DR

This study uses a complete sample of AGN from Swift to examine how radiation pressure influences the distribution of gas clouds near black holes, finding most clouds are within the effective Eddington limit and affected by radiation pressure.

Contribution

It provides the first observational evidence that radiation pressure impacts the location of gas clouds in AGN, based on a complete hard X-ray selected sample.

Findings

Only one AGN lies in the forbidden region due to ionized warm absorber

High column density clouds are within a few parsecs of the black hole

Radiation pressure influences gas cloud presence in the galaxy bulge

Abstract

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/BAT 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.