A New Catalogue of Type 1 AGN and its Implication on the AGN Unified Model

TL;DR

This study identifies a significant number of previously undetected low-luminosity type 1 AGNs, expanding the known population and challenging existing models of AGN unification based on their properties and orientation.

Contribution

It provides a new, more complete catalogue of type 1 AGNs with weak broad-line regions, revealing complex dependencies of the type 1 fraction on black hole mass and luminosity.

Findings

Type 1 AGN fraction varies with black hole mass and luminosity.

New catalogue increases known type 1 AGNs by 49%.

X-ray observations confirm the nature of newly identified AGNs.

Abstract

We have newly identified a substantial number of type 1 active galactic nuclei (AGN) featuring weak broad-line regions (BLRs) at z < 0.2 from detailed analysis of galaxy spectra in the Sloan Digital Sky Survey Data Release 7. These objects predominantly show a stellar continuum but also a broad H-alpha emission line, indicating the presence of a low-luminosity AGN oriented so that we are viewing the central engine directly without significant obscuration. These accreting black holes have previously eluded detection due to their weak nature. The new BLR AGNs we found increased the number of known type 1 AGNs by 49%. Some of these new BLR AGNs were detected at the Chandra X-ray Observatory, and their X-ray properties confirm that they are indeed type 1 AGN. Based on our new and more complete catalogue of type 1 AGNs, we derived the type 1 fraction of AGNs as a function of [OIII] 5007 emission luminosity and explored the possible dilution effect on the obscured AGN due to star-formation. The new type 1 AGN fraction shows much more complex behavior with respect to black hole mass and bolometric luminosity than suggested by the existing receding torus model. The type 1 AGN fraction is sensitive to both of these factors, and there seems to be a sweet spot (ridge) in the diagram of black hole mass and bolometric luminosity. Furthermore, we present a hint that the Eddington ratio plays a role in determining the opening angles.