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Radio AGN in the local universe: unification, triggering and evolution

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The Astronomy and Astrophysics Review Aims and scope

An Erratum to this article was published on 22 July 2016

Abstract

Associated with one of the most important forms of active galactic nucleus (AGN) feedback, and showing a strong preference for giant elliptical host galaxies, radio AGN (\(L_{1.4\,\mathrm{GHz}} > 10^{24}\) W \(\hbox {Hz}^{-1}\)) are a key sub-class of the overall AGN population. Recently their study has benefitted dramatically from the availability of high-quality data covering the X-ray to far-IR wavelength range obtained with the current generation of ground- and space-based telescope facilities. Reflecting this progress, here I review our current state of understanding of the population of radio AGN at low and intermediate redshifts (\(z < 0.7\)), concentrating on their nuclear AGN and host galaxy properties, and covering three interlocking themes: the classification of radio AGN and its interpretation; the triggering and fuelling of the jet and AGN activity; and the evolution of the host galaxies. I show that much of the observed diversity in the AGN properties of radio AGN can be explained in terms of a combination of orientation/anisotropy, mass accretion rate, and variability effects. The detailed morphologies of the host galaxies are consistent with the triggering of strong-line radio galaxies (SLRG) in galaxy mergers. However, the star formation properties and cool ISM contents suggest that the triggering mergers are relatively minor in terms of their gas masses in most cases, and would not lead to major growth of the supermassive black holes and stellar bulges; therefore, apart from a minority (<20 %) that show evidence for higher star formation rates and more massive cool ISM reservoirs, the SLRG represent late-time re-triggering of activity in mature giant elliptical galaxies. In contrast, the host and environmental properties of weak-line radio galaxies (WLRG) with Fanaroff–Riley class I radio morphologies are consistent with more gradual fuelling of the activity via gas accretion at low rates onto the supermassive black holes.

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Notes

  1. It is important to distinguish between the 3CR sample (Bennett 1962a, b, see Table 1) and the 3CRR sample of Laing et al. (1983). The latter has more restrictive selection criteria: flux densities \(S_{178\,\mathrm{MHz}} > 10.9\) Jy, declinations \(\delta > 10^{\circ }\), and Galactic latitudes \(|b| > 10^{\circ }\); the 3CRR sample selection is also based on higher quality radio data.

  2. See http://www.jb.man.ac.uk/atlas for a more detailed discussion of radio classifications schemes.

  3. Since the GPS and related CSS sources show strong morphological similarities with their more extended counterparts (O’Dea 1998; Tzioumis et al. 2002), and are thought to represent radio sources in a young evolutionary phase (Fanti et al. 1995; Owsianik et al. 1998; Polatidis and Conway 2003), in most of this review I will not distinguish them from the extended sources. However, note that the detection of GSP/CSS sources in flux-limited samples may be affected by selection effects related to the strong interactions between the compact radio sources and dense ISM in the central regions of the galaxies (Tadhunter et al. 2011; Morganti et al. 2011; Dicken et al. 2012).

  4. This object would also be classified as a BLRG on the basis of its broad Balmer emission lines.

  5. This includes the 3 objects classified by Capetti et al. (2011) as extreme low excitation radio galaxies (ELEGs).

  6. Buttiglione et al. (2010) and Capetti et al. (2013) also distinguish a class of radio AGN showing extremely low excitation emission line spectra that they label extreme low excitation galaxies (ELEGs). However, in what follows I make no distinction between LEGs and ELEGs and label them collectively as LEGs.

  7. Note that this sample extends to lower redshifts than the \(0.05< z < 0.7\) 2Jy sample of Dicken et al. (2009) described in Table 1.

  8. One object deserves particular mention here: 3C84 (also known as NGC1275). This object was originally classified as an FRI by Fanaroff and Riley (1974), but shows broad Balmer emission that lead to a BLRG optical classification. Also, as expected for a BLRG, it has luminous narrow lines, albeit of low ionisation (hence the LEG classification of 3C84 by Buttiglione et al. 2010). On the basis of these properties, 3C84 could be considered as the only FRI object in the combined 3CR and 2Jy sample classified as an SLRG. However, its radio structure is highly peculiar, with a strong, highly variable flat-spectrum core, an inner steep spectrum double structure, and an outer halo (Pedlar et al. 1990). Therefore, its radio morphological classification must be regarded as uncertain; it is certainly not a typical FRI source.

  9. Note that Haas et al. (2005) derive a larger attenuation factor of \(\sim \)7 based on a smaller, more heterogeneous sample.

  10. Note that the mid-IR AGN detection rates are based on photometric measurements with the IRAC instrument on the Spitzer satellite. In this case, rather than using high spatial resolution to detect the compact nuclei, the AGN are detected as an excess in the mid-IR continuum over the flux in the starlight predicted on the basis of an extrapolation of the near-IR starlight flux, with the latter measured from HST observations using the same aperture as used for the Spitzer/IRAC measurements (see Ramírez et al. 2014a).

  11. Using updated optical spectral classifications from the data in Buttiglione et al. (2009, (2010) and other sources.

  12. The exceptions are WLRG that have a strong starburst heated dust component or substantial contamination of their far-IR emission by non-thermal jet emission.

  13. This is consistent with the idea that the relatively low equivalent widths of the optical emission lines in BL Lac objects is due to their strongly beamed optical continuum emission, rather than their low emission line luminosities.

  14. Discounting the 3C84, which, as noted above, has a peculiar radio morphology.

  15. This is the typical scale on which the narrow-line emitting gas accessed by the nuclear apertures used for the spectroscopic classification is emitted. Many radio galaxies show lower-surface-brightness extended emission line nebulosities on much larger scales (up to \(\sim \)100 kpc: Baum et al. 1988; Tadhunter et al. 1989a), but these are not used for the optical spectral classifications of the sources.

  16. Note that, for a typical cloud in the NLR, the time taken for the [OIII] emission to fade following a substantial decline in the ionising flux is negligible compared with the light crossing time of the NLR (Capetti et al. 2013).

  17. Note that WLRG with hybrid FRII/FRI radio morphologies were counted as WLRG/FRII sources when making this estimate.

  18. Dust lanes have not been counted as tidal features. Note that only two SLRG in the intermediate redshift 2Jy sample—3C105 (PKS0404\(+\)05) and PKS0252-71—show no evidence for tidal features or close double nuclei. It is significant that one of these objects (3C105) is affected by an unusually high level of dust extinction at optical wavelengths (\(\sim \)1 magnitude in the \(r'\)-band), which makes it harder to detect faint morphological features, while the other (PKS0252-71) is at higher redshifts and was observed in relatively poor seeing conditions. Therefore the true rate of occurrence of tidal features or close double nuclei in the 2Jy SLRG could approach 100 %.

  19. The 4000-Å  break measures the ratio of fluxes measured in wavelength bins above and below the metal line blanketing break for old stars at 4000 Å. The version of D4000 defined by Tadhunter et al. (2002) uses continuum bins that avoid strong emission lines (\(3750< \lambda < 3850~\AA \)  and \(4150< \lambda < 4250~\AA \)).

  20. The nebular continuum comprises recombination, 2-photon and free-free emission from the warm ionised gas in the NLR: see Dickson et al. (1995) for details.

  21. Minor mergers are generally defined to be those in which one object involved in the merger has a mass of 25 % or less of that of the other.

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Acknowledgments

I am grateful to all my collaborators on the 2Jy project over the last 25 years for their valuable contributions, in particular Raffaella Morganti, Dan Dicken, Cristina Ramos Almeida, Martin Shaw, Sperello di Serego Alighieri, Bob Dickson, Katherine Inskip, Bob Fosbury, Andy Robinson, Joanna Holt, Martin Hardcastle, Beatriz Mingo and Montse Villar-Martín. I also thank Raffaella Morganti and Francesco Palla for suggestions that have improved the manuscript. I acknowledge the NASA Astrophysics Data System, which has greatly assisted me in constructing the bibliography for this review.

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  1. University of Sheffield, Sheffield, UK

    Clive Tadhunter

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Correspondence to Clive Tadhunter.

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An erratum to this article is available at http://dx.doi.org/10.1007/s00159-016-0097-7.

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Tadhunter, C. Radio AGN in the local universe: unification, triggering and evolution. Astron Astrophys Rev 24, 10 (2016). https://doi.org/10.1007/s00159-016-0094-x

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