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Radio AGN have always held an important place in our understanding of AGN and their link to galaxy evolution. Among the first active galaxies to be discovered following the opening of the radio window in first half of the 20th century (Bolton, Stanley & Slee, 1949), it was soon recognised that they are associated with giant elliptical galaxies (Matthews, Morgan & Schmidt, 1964). Their extraordinary luminosities at radio wavelengths also mean that they stand out at high redshifts. In this way, they act as important signposts to the early Universe, in particular probing the evolution of the most massive galaxies in some of the highest density environments (McCarthy, 1993, Miley & de Breuck, 2008). Most recently it has been acknowledged that the mechanical energy imparted by the expanding jets and lobes is one of the most important forms of AGN-induced feedback. This is because it prevents the hot X-ray emitting gas of the host galaxies and clusters from cooling to form stars, and thereby influences the shape of the high luminosity end of the galaxy luminosity function (e.g. Benson et a., 2003, McNamara & Nulsen, 2007). In addition, the jets drive massive outflows of warm, neutral and molecular gas that can potentially influence the star formation histories of the central bulge regions of galaxies (Holt, Tadhunter & Morganti, 2003, Morganti et al., 2005, Holt, Tadhunter & Morganti, 2008, Morganti et al., 2013).

The physics of the exquisite jets and lobes of relativistic particles that produce the radio emission, and how these components interact with their gaseous environments, has been a major focus for many detailed studies at radio wavelengths. However, much of our current understanding of the fuelling and triggering of the activity in radio AGN has been derived from observations at other wavelengths. These include: optical and infrared observations of the host galaxies; X-ray, optical and infrared observations of the nuclei; and X-ray and optical observations of the large-scale environments.

Aside from their high radio luminosities, perhaps the most important feature that the majority of radio AGN have in common is that they are associated with massive early-type galaxies. This feature holds considerable advantages for investigating the triggering of the AGN activity, since it allows relatively “clean” searches to be made for the morphological, star formation, and gas content signatures of the triggering events. This is in contrast to Seyfert galaxies, for example, which are more commonly associated with spiral galaxy hosts (e.g. Adams, 1977); the morphological complexity of such hosts, coupled with the large gas reservoirs of their quiescent disks and associated star formation activity, make it challenging to disentangle the triggering events from the normal, non-AGN-related evolution of their host galaxies.

Two of the key outstanding questions for radio AGN are as follows.

Answering these questions is crucial, for example, if we want to properly incorporate radio AGN into models of galaxy evolution. It has become clear that addressing them requires a multi-wavelength approach that encompasses not only deep radio and optical observations, but also observations at X-ray and infrared wavelengths. Therefore this is a field that has particularly benefitted from the availability of large space observatories such as Chandra, XMM, Spitzer and Herschel over the last 20 years. At the same time, deeper and higher resolution optical observations with 8m-class telescopes and the Hubble Space Telescope (HST) respectively have allowed the host galaxy morphologies to be examined in unprecedented depth. Concerted efforts have also been made to improve the completeness of optical spectroscopic classifications for samples of radio AGN.

In this article I review the considerable progress that has been made in the study of radio AGN and their host galaxies based on deep multi-wavelength observations with ground- and space-based observatories over the last 20 years. I take the approach of concentrating on modest-sized samples of radio AGN selected from radio surveys with relatively bright radio flux limits, in particular the southern 2Jy sample of Dicken et al. (2009), and the northern 3CR 1 sample of Buttiglione et al. (2009); full details of the selection criteria and references for these samples are given in Table 1. I also concentrate on objects at low- to intermediate-redshifts (z < 0.7) because this ensures a high degree of completeness in optical spectroscopic classifications and in the detection of individual objects at X-ray and infrared wavelengths. I aim to fill an important gap between highly detailed studies of individual iconic radio AGN in the local universe such as Centaurus A and Cygnus A, and the more statistical, but less detailed, studies of much larger samples of radio AGN selected using a combination of deep wide field optical spectroscopic and radio surveys.

Table 1. Selection criteria and references for the two main samples of radio AGN considered in this review. Note that both of these samples were selected from larger master samples with less restrictive selection criteria, the references for which are given in column 3. Columns 4 to 8 give the references for the optical, radio, near-IR, mid- to far-IR (MFIR), and X-ray data for the samples.

Sample Selection Mast. Radio Opt. NIR MFIR Xray

2Jy S2.7 GHz > 2Jy 1,9, 2,3, 6,7 10 5,11, 14,15,
Dicken 0.05 < z < 0.7 40 4,5, 8,9   8,12, 38
et al. (2009) δ < +10   39     13  
  α2.74.8 > +0.5            
(Fν ∝ ν−α)            

3CR S178 MHz > 9Jy 16,17, 19,20, 23,24, 26,27, 30,31, 33,34,
Buttiglione z < 0.3 18 21,22 25 28,29 32,11, 35,36,
et al. (2009) δ > −5         12,34 37,38

Reference key: 1. Wall & Peacock (1985); 2. Morganti et al. (1993, and references therein); 3. Morganti et al. (1997a); 4. Morganti et al. (1999); 5. Dicken et al. (2008); 6. Tadhunter et al. (1993, and references therein); 7. Shaw et al. (1995), 8. Morganti et al. (1997b); 9. Tadhunter et al. (1998); 10. Inskip et al. (2010); 11. Dicken et al. (2012); 12.Dicken et al. (2014); 13. Dicken et al. (2016); 14. Siebert et al. (1996); 15. Mingo et al. (2014); 16. Bennett (1962a); 17. Bennett (1962b); 18. Spinrad et al. (1985); 19.; 20. Black et al. (1992); 21. Leahy et al. (1997); 22. Hardcastle et al. (1997, and references therein); 23. Buttiglione et al. (2009); 24. Buttiglione et al. (2010); 25. Buttiglione et al (2011); 26. Lilly & Longair (1984); 27. Madrid et al. (2006); 28. Donzelli et al. (2007); 29. Baldi et al. (2010); 30. Haas et al. (2004); 31. Ogle et al. (2006); 32. Dicken et al. (2010); 33. Hardcastle et al. (2006); 34. Hardcastle et al. (2009); 35. Massaro et al. (2010); 36. Massaro et al. (2012); 37. Massaro et al. (2015); 38. Ineson et al. (2015); 39. Tzioumis et al. (2002); 40. di Serego Alighieri et al. (1994).

No single review can encompass all aspects of radio AGN. In particular, this review will not consider the detailed physics of the synchrotron-emitting jets and lobes, radio galaxies at high redshifts (z > 0.7), or the feedback effect of the expanding radio components. These aspects are covered by excellent reviews elsewhere: Miley (1980) and Worrall (2009) review the detailed jet/lobe physics from radio and X-ray perspectives; Miley & de Breuck (2008) review high redshift radio galaxies; McNamara & Nulsen (2007) review the impact of the radio sources on the hot ISM of the host galaxies and galaxy clusters; and Fabian (2012) presents a broad overview of the AGN feedback effect. Readers might also find the detailed reviews by Israel (1998) and Carilli & Barthel (1996) on, respectively, the archetypal FRI and FRII sources Centaurus A and Cygnus A useful. Finally, Heckman & Best (2014) present a comprehensive overview of the key results on nearby AGN (z < 0.2) from the Sloan Digital Sky Survey (SDSS), which helps to place radio AGN in the broader context of other AGN populations.

Throughout this review I assume a cosmology with H0 = 71 km s−1 Mpc−1, Ωm = 0.73 and Ωλ = 0.27, and a spectral index of α = 0.7 (for Fν ∝ ν−α) when converting radio fluxes between different frequencies.

1 It is important to distinguish between the 3CR sample (Bennett, 1962a, Bennett, 1962b, see Table 1) and the 3CRR sample of Laing, Riley & Longair (1983). The latter has has more restrictive selection criteria: flux densities S178 MHz > 10.9 Jy, declinations δ > 10, and Galactic latitudes |b| > 10; the 3CRR sample selection is also based on higher quality radio data. Back.

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