2022ApJ...938...67R

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 ($\mathrm{log}{\lambda }_{\mathrm{Edd}}\gtrsim -2$) 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 $\mathrm{log}{\lambda }_{\mathrm{Edd}}^{* }=-1.34\,\pm \,0.07$. This corresponds to the λ_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-λ_Edd plane during their life cycle. The growth episode starts with the AGN mostly unobscured and accreting at low λ_Edd. As the SMBH is further fueled, λ_Edd, N_H and the covering factor increase, leading the AGN to be preferentially observed as obscured. Once λ_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.