Abstract

We study the relation between obscuration and supermassive black hole (SMBH) accretion using a large sample of hard X-ray selected active galactic nuclei (AGNs). We find a strong decrease in the fraction of obscured sources above the Eddington limit for dusty gas (log lambda(Edd) >= -2) confirming earlier results, and consistent with the radiation-regulated unification model. This also explains the difference in the Eddington ratio distribution functions (ERDFs) of type 1 and type 2 AGNs obtained by a recent study. The break in the ERDF of nearby AGNs is at log lambda*(Edd) = -1.34 +/- 0.07. This corresponds to the lambda(Edd) where AGNs transition from having most of their sky covered by obscuring material to being mostly devoid of absorbing material. A similar trend is observed for the luminosity function, which implies that most of the SMBH growth in the local universe happens when the AGN is covered by a large reservoir of gas and dust. These results could be explained with a radiation-regulated growth model, in which AGNs move in the N-H-lambda(Edd) plane during their life cycle. The growth episode starts with the AGN mostly unobscured and accreting at low lambda(Edd). As the SMBH is further fueled, lambda(Edd), N-H and the covering factor increase, leading the AGN to be preferentially observed as obscured. Once lambda(Edd) reaches the Eddington limit for dusty gas, the covering factor and N-H rapidly decrease, leading the AGN to be typically observed as unobscured. As the remaining fuel is depleted, the SMBH goes back into a quiescent phase.