Spectropolarimetric Evidence for Radiatively Inefficient Accretion in an Optically Dull Active Galaxy

TL;DR

This study uses spectropolarimetry to investigate optically dull AGNs, finding evidence for radiatively inefficient accretion flows rather than obscuration as the cause of missing emission lines.

Contribution

It provides spectropolarimetric evidence that optically dull AGNs can host radiatively inefficient accretion flows, challenging the simple obscuration model.

Findings

Weak emission lines in one AGN are not due to obscuration.

Detected polarized continuum suggests intrinsic AGN emission without emission lines.

Supports the presence of radiatively inefficient accretion flows in optically dull AGNs.

Abstract

We present Subaru/FOCAS spectropolarimetry of two active galaxies in the Cosmic Evolution Survey. These objects were selected to be optically dull, with the bright X-ray emission of an AGN but missing optical emission lines in our previous spectroscopy. Our new observations show that one target has very weak emission lines consistent with an optically dull AGN, while the other object has strong emission lines typical of a host-diluted Type 2 Seyfert galaxy. In neither source do we observe polarized emission lines, with 3-sigma upper limits of P_BLR < 2%. This means that the missing broad emission lines (and weaker narrow emission lines) are not due to simple anisotropic obscuration, e.g., by the canonical AGN torus. The weak-lined optically dull AGN exhibits a blue polarized continuum with P = 0.78 +/- 0.07% at 4400 A < lambda_rest < 7200 A (P = 1.37 +/- 0.16% at 4400 A < lambda_rest < 5050 A). The wavelength dependence of this polarized flux is similar to that of an unobscured AGN continuum and represents the intrinsic AGN emission, either as synchrotron emission or the outer part of an accretion disk reflected by a clumpy dust scatterer. Because this intrinsic AGN emission lacks emission lines, this source is likely to have a radiatively inefficient accretion flow.