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 107–108 yr to shorter periods of 104–105 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.

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Fig. 1: The prototypical example of ‘light echo’ and a proposed lifecycle.
Fig. 2: The duty cycle of activity and quiescence predicted from simulations of chaotic cold accretion.
Fig. 3: Model of integrated radio spectra showing how a power-law radio spectrum is modified by the ageing of the source and the switching off of the central AGN.
Fig. 4: Example of an AGN remnant (B2 0924+30) discovered thanks to the morphology of its radio emission90: all possible signatures of ongoing activity (core, jets and hot spots) are absent, and only diffuse, low-surface-brightness emission is detected.
Fig. 5: Radio galaxy B0925+420, showing three phases of activity.
Fig. 6: LOFAR image (one pointing) centred on the Boötes area87.

Image  W. Williams / R. van Weeren / H. Röttgering / D. Hoang.

Fig. 7: A serendipitously discovered AGN remnant100 in a LOFAR field with properties rarely seen before.
Fig. 8: Two views of the LOFAR radio telescope.


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I would like to thank T. Oosterloo, M. Gaspari, L. Godfrey, M. Brienza, J. Harwood, A. Shulevski, L. Ciotti and the LOFAR Survey Team for help, comments and discussions. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013)/ERC Advanced Grant RADIOLIFE-320745. LOFAR was designed and constructed by ASTRON and has facilities in several countries that are owned by various parties (each with their own funding sources), and that are collectively operated by the International LOFAR Telescope foundation under a joint scientific policy.

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Morganti, R. Archaeology of active galaxies across the electromagnetic spectrum. Nat Astron 1, 39–48 (2017).

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