Archaeology of active galaxies across the electromagnetic spectrum

TL;DR

This paper reviews recent optical and radio observations of active galactic nuclei (AGNs), highlighting their cyclical activity patterns over a wide range of timescales from years to hundreds of millions of years, and discusses how these findings inform galaxy evolution models.

Contribution

It summarizes recent observational and simulation-based insights into the lifecycle and activity cycles of AGNs, emphasizing the diversity of timescales involved.

Findings

AGN activity cycles range from years to hundreds of millions of years

Multiple cycles of AGN activity are supported by observational and simulation data

Understanding AGN cycles helps clarify their role in galaxy evolution

Abstract

Analytical and numerical galaxy-formation models indicate that active galactic nuclei (AGNs) likely play a prominent role in the formation and evolution of galaxies. However, quantifying this effect requires knowledge of how the nuclear activity proceeds throughout the life of a galaxy, whether it alternates with periods of quiescence and, if so, on what timescales these cycles occur. This topic has attracted growing interest, but making progress remains a challenging task. For optical and radio AGNs, a variety of techniques are used to perform a kind of "archaeology" that traces the signatures of past nuclear activity. Here we summarize recent findings regarding the lifecycle of an AGN from optical and radio observations. The limited picture we have so far suggests that these cycles can range from long periods of 10^7-10^8 yr to shorter periods of 10^4-10^5 yr, even reaching extreme events on timescales of just a few years. Together with simulations, observational results regarding the multiple cycles of AGN activity help to create a complete picture of the AGN lifecycle.