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The origin of radio emission from radio-quiet active galactic nuclei


The central nuclei of galaxies, where supermassive black holes (SMBHs) are thought to reside, can experience phases of activity when they become active galactic nuclei (AGNs). An AGN can eject winds and jets and produce radiation across the entire electromagnetic spectrum. The fraction of the bolometric emission in the radio spans a factor of approximately 105 across the different classes of AGNs. The weakest radio sources, radio-quiet (RQ) AGNs, are typically 1,000 times fainter than the radio-loud (RL) AGNs, and represent the majority of the AGN population. In RQ AGNs, the absence of luminous jets allows us to probe radio emission from a wide range of possible mechanisms: star formation, AGN-driven wind, free-free emission from photoionized gas, low-power jets and the innermost accretion disk coronal activity. All these mechanisms can now be probed with unprecedented precision and spatial resolution, owing to the current and forthcoming generation of highly sensitive radio arrays.

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Journal peer review information: Nature Astronomy thanks Ann Kapinska and Andrea Merloni for their contribution to the peer review of this work.

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This review is the result of several fruitful discussions raised during the meeting ‘The radio–X-ray connection in accreting objects’ (21–25 May 2018, Tenuta Monacelle, Monopoli, Italy). The authors thank all the participants of the meeting: J. Bally, N. Brandt, G. Brunetti, A. Capetti, S. Corbel, L. Dai, J. Davelaar, B. De Marco, J. Ferreira, J. Gomez, M. Hardcastle, Y. Inuoe, M. Jarvis, P. Kharb, A. Kimball (in particular for her original idea that inspired Fig. 2), M. Koss, C. Mundell, D. Pasham, U. Peretz, M. Perucho, R. Plotkin, I. Prandoni, J. Poutanen, T. Roberts, D. Williams, C. Tadhunter, S. Tchekhovskoy, F. Ursini, D. Worrall and N. Zakamska. We also thank D. Altamirano, S. Antoniucci, P. Casella, E. Chiaraluce, S. Hoenig, A. Merloni, M. Middleton, M. Pahari, M. Perez-Torres and D. Williams.

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The authors declare no competing interests.

Correspondence to Francesca Panessa.

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Further reading

Fig. 1: Prototype radio maps of RQ AGNs with extended radio morphologies.
Fig. 2: Sketch of the four main different physical mechanisms producing radio emission in RQ AGNs.
Fig. 3: Radio luminosities at 1.4 GHz versus the velocity width containing 90% of the [O iii] line.
Fig. 4: SFR derived from the FIR luminosity versus the SFR from the monochromatic radio power, P1.4GHz, for the E-CDFS sample.
Fig. 5: Discrete cross-correlation function (CCF) plot showing time lag against the correlation coefficient for X-ray to radio (black crosses) for NGC 7213.
Fig. 6: X-ray luminosity (2–10 keV, erg s−1) versus millimetre-band luminosity (90–100 GHz, erg s−1) for accreting compact objects.
Fig. 7: Flow chart to guide the interpretation of the radio emission in local RQ AGNs.