Promising directions for future research in this field include the following.
Understanding the WLRG/FRII sources. It is important to use observations of larger samples of WLRG/FRII sources to determine whether they represent a truly uniform population in terms of their host galaxy masses, detailed optical morphologies, large-scale environments, star formation properties and gas contents. By comparison with the properties of SLRG/FRII objects, this will help to establish whether all, or a only a subset, of the WLRG/FRII sources represent objects in which the AGN has recently switched off. In addition, further detailed spectral ageing studies at radio wavelengths will be key to establishing whether the radio sources of some WLRG/FRII objects represent relics, consistent with the switch-off idea.
What types of mergers trigger luminous radio AGN activity? The star formation properties and gas contents of the majority of SLRG/FRII sources suggest that they have been triggered in galaxy mergers that are relatively modest, at least in terms of their cool gas masses. Such mergers have the potential to deliver total reservoirs of cool gas that are sufficiently massive to sustain quasar activity on the requisite timescale. However, further numerical simulations are required to establish what types of modest mergers can trigger luminous AGN activity, considering a range of galaxy properties (e.g. total stellar masses), not just the cool gas contents. On the observational side, it will also be important to use deep high resolution imaging and/or integral field observations of the tidal features to establish the stellar masses of the galaxies merging with the giant elliptical hosts of the radio AGN.
The nature of the lower luminosity radio AGN populations. This review has concentrated the high-luminosity end of the radio source population (L1.4 GHz > 1024 W Hz−1). However, the majority of the radio AGN detected in large-area surveys such as NVSS and FIRST have lower luminosities. Although the SDSS has been used to establish the optical spectral classes of such sources, we know relatively little about their detailed radio and optical morphologies. Therefore further deep, high resolution imaging observations of samples of lower-luminosity radio sources at both radio and optical wavelengths are required to establish their relationship with the high-luminosity radio AGN population, and hence investigate the natures of their AGN and how they are triggered.
High spatial resolution studies of the cool gas kinematics in the near-nuclear regions. The new generation of mm-wavelength interferometers such as ALMA has the capability to study the kinematics of the cool gas in the central regions of the nearest radio AGN on scales of ∼ 0.1 — 1 kpc via observations of the CO emission lines. On such scales, the dynamical timescale of the gas (≤ 107 yr) is comparable with, or less than, the typical lifetimes of the AGN. In this case, detailed mapping of the gas kinematics has the potential to reveal how the gas is transported from kpc-scale dust lanes to sub-100pc-scales — one of the key outstanding problems in AGN studies. The observations will be challenging because the cool gas masses are modest in most cases (see section 3.4). However, it will be particularly interesting to compare the kinematics of the cool gas in nearby SLRG/FRII and WLRG/FRII objects, to determine whether the relatively dim AGN in the WLRG/FRII (perhaps related to the switch-off phase) are linked to a particular dynamical state of the near-nuclear gas.
Acknowledgements 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, Bob Dickson, Sperello di Serego Alighieri, 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.