Archaeology of Galactic Nuclei Activity

TL;DR

This paper reviews methods and findings related to reconstructing the historical activity of supermassive black holes in galactic nuclei through various observational signatures across the electromagnetic spectrum.

Contribution

It introduces the concept of active galactic nuclei archaeology and discusses recent observational evidence for activity changes and their impact on surrounding environments.

Findings

Detection of relic radio structures indicating past activity

Evidence of ionisation echoes from gas clouds

Observations of switching between radiative and kinetic activity modes

Abstract

Considerable observational evidence suggests that the activity of supermassive black holes in galactic nuclei is transient. The term ``active galactic nuclei archaeology'' has even been coined. This implies the possibility of reconstructing the history of activity, such as changes in the nuclear luminosity over time across various regions of the electromagnetic spectrum, by analysing how this activity manifested itself on galactic and extragalactic spatial scales. These phenomena include relic radio structures, gas clouds illuminated by the ``ionising echo'' of past activity, and Fermi/eROSITA bubbles. We provide a review of the results of galactic nucleus activity studies, focusing on its observable impact on the intergalactic medium and circumgalactic environment. Our main focus is on optical observations of ionisation cones and evidence of switching between radiative (ionisation cones) and kinetic (radio jets) modes of nuclear activity.