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Compact steep-spectrum and peaked-spectrum radio sources

  • Review Article
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Abstract

Compact steep-spectrum (CSS) and peaked-spectrum (PS) radio sources are compact, powerful radio sources. The multi-frequency observational properties and current theories are reviewed with emphasis on developments since the earlier review of O’Dea (PASP 110:493–532, https://doi.org/10.1086/316162, 1998). There are three main hypotheses for the nature of PS and CSS sources. (1) The PS sources might be very young radio galaxies which will evolve into CSS sources on their way to becoming large radio galaxies. (2) The PS and CSS sources might be compact, because they are confined (and enhanced in radio power) by interaction with dense gas in their environments. (3) Alternately, the PS sources might be transient or intermittent sources. Each of these hypotheses may apply to individual objects. The relative number in each population will have significant implications for the radio galaxy paradigm. Proper motion studies over long time baselines have helped determine hotspot speeds for over three dozen sources and establish that these are young objects. Multi-frequency polarization observations have demonstrated that many CSS/PS sources are embedded in a dense interstellar medium and vigorously interacting with it. The detection of emission line gas aligned with the radio source, and blue-shifted Hi absorption, and [OIII] emission lines indicates that AGN feedback is present in these objects—possibly driven by the radio source. Also, CSS/PS sources with evidence of episodic AGN over a large range of time-scales have been discussed. The review closes with a discussion of open questions and prospects for the future.

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Notes

  1. See Sect. 3.3 for a discussion of the double–double radio galaxies.

  2. M* is the magnitude at the break of the Schechter luminosity function (Schechter 1976).

  3. The Eddington ratio (or fraction) is the ratio of the AGN bolometric luminosity to the Eddington luminosity \(\lambda _\mathrm{Edd} = L_\mathrm{bol}/L_\mathrm{Edd}\). The Eddington luminosity is the maximum possible luminosity due to accretion onto a compact object, assuming spherical accretion and a balance between gravity and radiation pressure.

  4. The Stromgren sphere is an idealized representation of the size of a nebula which has been ionized by a compact source of high-energy photons.

  5. This process is commonly referred to as jet-induced star formation, though it is expected that in powerful radio sources, the bow shock driven by the over-pressured cocoon is responsible for compressing the ambient clouds and triggering star formation (e.g., Rees 1989; Begelman and Cioffi 1989; De Young 1989; Daly 1990). However, there is also evidence for star formation triggered by direct jet impact, e.g., Minkowski’s Object (Brodie et al. 1985; van Breugel et al. 1985; Croft et al. 2006; Salomé et al. 2015; Lacy et al. 2017; Zovaro et al. 2020).

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Acknowledgements

We are very grateful to colleagues who provided thoughtful comments on the manuscript and/or figures for the review, especially T. An, G. Bicknell, J. Conway, W. Cotton, D. Dallacasa, C. Duggal, M. Giroletti, Y. Gordon, M. Kunert-Bajraszewska, A. Marecki, G. Migliori, R. Morganti, D. Mukherjee, P. Ogle, M. Orienti, T. Oosterloo, A. Polatidis, and L. Stawarz. We thank our referee, C. Tadhunter, for helpful comments which improved the paper. CO is grateful to M. Singha for help with some of the figures and to R. Antonucci for interesting discussions. CO is grateful to the Natural Sciences and Engineering Research Council (NSERC) of Canada for support. This research has made use of NASA’s Astrophysics Data System Bibliographic Services. This research has made use of the NASA/IPAC Extragalactic Database (NED), which is funded by the National Aeronautics and Space Administration and operated by the California Institute of Technology.

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Authors and Affiliations

  1. Department of Physics and Astronomy, University of Manitoba, Winnipeg, MB, R3T 2N2, Canada

    Christopher P. O’Dea

  2. Inter-University Centre for Astronomy and Astrophysics, Savitribai Phule Pune University Campus, Ganeshkind P.O., Pune, 411007, India

    D. J. Saikia

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  1. Christopher P. O’Dea
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  2. D. J. Saikia
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Correspondence to D. J. Saikia.

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O’Dea, C.P., Saikia, D.J. Compact steep-spectrum and peaked-spectrum radio sources. Astron Astrophys Rev 29, 3 (2021). https://doi.org/10.1007/s00159-021-00131-w

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