The Nature of Optically Dull Active Galactic Nuclei in COSMOS

TL;DR

This study investigates optically dull AGNs in the COSMOS field, revealing that most are not passive but show signs of weak AGN activity, with their optical dullness mainly due to host galaxy dilution or intrinsic weakness, not obscuration.

Contribution

It provides a detailed multiwavelength analysis of optically dull AGNs, showing they are often diluted by host galaxies or intrinsically weak, challenging previous assumptions about their nature.

Findings

Most optically dull AGNs are not passive but show weak AGN signatures.

Approximately 70% are host-diluted, with bright or edge-on hosts within the spectroscopic aperture.

About 30% are intrinsically weak with high X-ray to optical flux ratios.

Abstract

We present infrared, optical, and X-ray data of 48 X-ray bright, optically dull AGNs in the COSMOS field. These objects exhibit the X-ray luminosity of an active galactic nucleus (AGN) but lack broad and narrow emission lines in their optical spectrum. We show that despite the lack of optical emission lines, most of these optically dull AGNs are not well-described by a typical passive red galaxy spectrum: instead they exhibit weak but significant blue emission like an unobscured AGN. Photometric observations over several years additionally show significant variability in the blue emission of four optically dull AGNs. The nature of the blue and infrared emission suggest that the optically inactive appearance of these AGNs cannot be caused by obscuration intrinsic to the AGNs. Instead, up to ~70% of optically dull AGNs are diluted by their hosts, with bright or simply edge-on hosts lying preferentially within the spectroscopic aperture. The remaining ~30% of optically dull AGNs have anomalously high f_x/f_o ratios and are intrinsically weak, not obscured, in the optical. These optically dull AGNs are best described as a weakly accreting AGN with a truncated accretion disk from a radiatively inefficient accretion flow.