On the disappearance of broad-line region in low-luminosity active galactic nuclei: the role of the outflows from advection dominated accretion flows

TL;DR

This paper investigates why broad-line regions disappear in low-luminosity active galactic nuclei, proposing that the cooling of outflows from advection dominated accretion flows influences BLR formation.

Contribution

It introduces a model linking the presence of BLRs to the cooling efficiency of outflows from ADAFs and the structure of accretion disks in low-luminosity AGNs.

Findings

BLRs can form from cooled outflows if the transition radius is small.

Disappearance of BLRs occurs when the inner cold disk evaporates at very low luminosities.

The model explains the presence or absence of BLRs based on accretion rate and disk structure.

Abstract

The broad-line region (BLR) disappears in many low-luminosity AGNs, the reason of which is still controversial. The BLRs in AGNs are believed to be associated with the outflows from the accretion disks. Most of the low-luminosity AGNs (LLAGNs) contain advection dominated accretion flows (ADAFs), which are very hot and have a positive Bernoulli parameter. ADAFs are therefore associated with strong outflows. We estimate the cooling of the outflows from the ADAFs, and find that the gases in such hot outflows always cannot be cooled efficiently by bremsstrahlung radiation. The ADAF may co-exist with the standard disk, i.e., the inner ADAF connects to the outer thin accretion disk at radius R_tr, in the sources accreting at slightly lower than the critical rate. For the ADAFs with >0.001 L_edd, a secondary small inner cold disk is suggested to co-exist with the ADAF due to the condensation process. We estimate the Compton cooling of the outflow, of which the soft seed photons either come from the outer cold disk or the secondary inner cold disk. It is found that the gas in the outflow far from the ADAF may be efficiently cooled to form BLR clouds due to the soft seed photons emitted from the cold disks, provided the transition radius of the ADAF to the outer cold disk is small or/and the secondary small cold disk has a luminosity >0.003L_edd. The BLR clouds can still be formed in the outflows from the outer cold thin disks, if the transition radius is not very large. For the sources with <0.001L_edd, the inner small cold disk is evaporated completely in the ADAF and outer thin accretion disk may be suppressed by the ADAF, which leads to the disappearance of the BLR. The physical implications of this scenario on the double-peaked broad-line emitters are also discussed.