Active galactic nuclei ghosts: A systematic search for faded nuclei

TL;DR

This study systematically identifies 88 nearby AGN candidates showing signs of fading or rising activity, revealing that most are fading, which supports models of AGN evolution and galaxy mergers.

Contribution

It presents a new catalog of fading and rising AGN candidates using multi-wavelength data, highlighting the prevalence of fading phases and their association with galaxy interactions.

Findings

96% of candidates are fading sources

Approximately 50% are linked to merging or interacting systems

Around 30% exhibit jets and a torus-like dust geometry

Abstract

Physical processes such as re-ignition, enhancement, and fading of active galactic nuclei (AGN) are not entirely understood because the timeline of these events is expected to last many years. However, it is well known that the differences in the energy budget between AGN components, like the optical ionizing region and the mid-infrared (MIR) dust echoes, can be interpreted as a hint on AGN evolution. Here we present a catalog of 88 AGN candidates showing hints on the fading and rising of their activity in the nearby Universe. We use AGN scaling relations to select them from an initial sample of 877 candidates using publicly available optical, X-ray, and MIR luminosities. We then use the multi-wavelength information to discard sources contaminated with extranuclear emission and those with an X-ray luminosity not well corrected for absorption. We find that 96% of our candidates are fading sources. This result suggests a scenario where the Universe had its peak of AGN activity somewhere in the past and is dominated by a fading phase at the present time. Alternatively, the fading phase is longer than the rising phase, which is consistent with galaxy merger simulations. Around 50% of these fading candidates are associated with merging or interacting systems. Finally, we also find the existence of jets in ~30% of these candidates and that the preferred AGN dust geometry is torus-like, instead of wind-like. Our results are compatible with the fading of nuclear activity, expected if they are in an inefficient state.