Archaeology of active galaxies across the electromagnetic spectrum
  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.
