Disk Outflows and High-Luminosity True Type 2 AGN

TL;DR

This paper analyzes the conditions under which active galactic nuclei can lack broad emission lines due to disk outflows, predicting that such true type 2 AGN can occur at very high luminosities and Eddington ratios based on mass conservation constraints.

Contribution

It provides a theoretical framework linking mass conservation in accretion disks to the absence of broad lines, applicable across various disk outflow models and explaining high-luminosity true type 2 AGN.

Findings

Intrinsic broad line emission can disappear at luminosities up to 4x10^{46} erg s^{-1}.

True type 2 AGN are more likely at Eddington ratios below about 1%.

Mass conservation constraints explain the absence of broad lines in certain AGN.

Abstract

The absence of intrinsic broad line emission has been reported in a number of active galactic nuclei (AGN), including some with high Eddington ratios. Such "true type 2 AGN" are inherent to the disk-wind scenario for the broad line region: Broad line emission requires a minimal column density, implying a minimal outflow rate and thus a minimal accretion rate. Here we perform a detailed analysis of the consequences of mass conservation in the process of accretion through a central disk. The resulting constraints on luminosity are consistent with all the cases where claimed detections of true type 2 AGN pass stringent criteria, and predict that intrinsic broad line emission can disappear at luminosities as high as about 4x$10^{46}$ erg s$^{-1}$ and any Eddington ratio, though more detections can be expected at Eddington ratios below about 1%. Our results are applicable to every disk outflow model, whatever its details and whether clumpy or smooth, irrespective of the wind structure and its underlying dynamics. While other factors, such as changes in spectral energy distribution or covering factor, can affect the intensities of broad emission lines, within this scenario they can only produce true type 2 AGN of higher luminosity then those prescribed by mass conservation.