"Curioser and curiouser! cried Alice (she was so much surprised that for the moment she quite forgot how to speak good English). Now I'm opening out like the largest telescope that ever was!" Alice's Adventures in Wonderland, Lewis Caroll, 1865
In this review we have shown that HzRGs are fascinating objects in their own right and that they provide important diagnostics for studying the early Universe. Knowledge about HzRGs and their evolution is fundamental to understanding the formation and evolution of galaxies and the large scale structure of the Universe.
There are still many aspects of HzRGs that are not understood. Here are some open questions.
There are good prospects for making progress during the next decade. Since its inception, the study of radio galaxies, has been observationally driven. Several forefront astronomical facilities now being constructed or planned that will give new insights into the nature of HzRGs and their environments.
First, with a combination of sensitivity and spatial resolution, the new low-frequency radio arrays, LOFAR (Rottgering et al. 2006) and the LWA (Kassim et al. 2006), will open up the frequency window below ~ 50 MHz for HzRG studies. LOFAR will survey the sky to unprecedented depth at low-frequencies and will therefore be sensitive to the relatively rare radio sources that have extremely steep spectra. Because of the vs z relation (Section 2.4), LOFAR is likely to detect HzRGs at z ~ 8, if they exist. Studies of detailed low-frequency spectra and their spatial variations will provide new information about the mechanism responsible for the vs z relation. Presently combination of the new radio surveys with planned new deep optical and infrared wide-field surveys, such as PAN-STARRS (Hodapp et al. 2004) and those with the VST and VISTA (Arnaboldi et al. 2007) will be used to identify HzRGs and provide photometric data.
Another task for sensitive radio arrays, such as LOFAR, the EVLA (Napier 2006) and eventually the Square Kilometre Array (SKA), will be to survey the radio emission of galaxies in protoclusters. The new arrays will study radio emission produced by relativistic jets and be able to detect and investigate radio emission from the brightest star forming protocluster galaxies.
Secondly, ALMA (Brown et al. 2004) and the EVLA, with their unprecedented sensitivities and resolutions at millimetre and sub-millimetre wavelengths, will revolutionise the study of molecular gas and dust. Several different CO transitions can be observed, allowing entire "CO ladders" to be constructed and the density and temperature structure of the molecular gas to be unraveled. Fainter molecular lines can be used to trace even denser gas than that studied until now. Important information about the dust composition and the gas to dust ratios is likely to be obtained.
ALMA's sensitivity at millimetre wavelengths should also facilitate observations of the atomic CI Carbon lines in HzRGs. This would provide an important constraint on the global metalicity of the gas. The fine-structure line of C+ at rest = 157.74 µm line is one of the main cooling lines in nearby galaxies, and has now also been detected in several of the most distant quasars known.
Thirdly, we can expect considerable progress in disentangling the detailed evolutionary history both of HzRGS and of radio-selected protoclusters. This evolutionary detective work will be pursued by combining spectroscopic data from the next generation of spectrographs on 8m-class telescopes with imaging results from the new camera, WFC3 (Leckrone et al. 1998), on the Hubble Space Telescope. For example, the detection of supernovae in z ~ 2 protoclusters will become possible. On a longer timescale, tracing the detailed history of the formation and evolution of HzRGS and the surrounding protoclusters will be helped enormously with the advent of 30m -class ground-based telescopes in the optical and near-infrared and the James Webb Space Telescope (JWST) (Gardner et al. 2006) in the near and mid-infrared.
Fourthly, the next generation X-ray telescope, such as XEUS (Bavdaz et al. 2006) or Constellation-X (Tananbaum 2006), will make observations of X-rays from HzRGs an important tool for studying galaxy formation. It will have sufficient sensitivity to perform spectroscopic studies of hot gas in HzRGs. The 0.3 - 10 keV X-ray band contains the inner (K-shell) lines for all of the abundant metals from carbon to zinc as well as many L-shell lines. These atomic transitions provide important new plasma diagnostics of the HzRG hot gas.
Fifthly, and perhaps most exciting, the potential discovery of HzRGS with z > 6 could open up a unique new window for studying the very early Universe during the "Epoch of Reionisation". Recent observational constraints on suggest that cosmic reionization may have taken place between z ~ 11 and z ~ 6 (Fan et al. 2006). The existence of HzRGs within the near edge of cosmic reionization could be used as sensitive probes of intermediate- to small-scale structures in the neutral IGM, through redshifted HI absorption observations (Section 3.3.1) (Carilli et al. 2002, Furlanetto and Loeb 2002), complementary to the very large scale that can be studied in HI emission.
Finally, we point out that in trying to understand HzRGs or in using them to probe the properties of the early Universe, we should exercise humility, remembering that 96% of the energy density in the universe is in a form of dark matter and dark energy, that are not directly observable.
HzRGS pinpoint the progenitors of cosmic megacities in the local Universe. Studying them is akin to investigating how human civilisations were urbanised. During the last few decades we have obtained some glimmerings of how and why this cosmic urbanisation occurred. During the next few decades we shall learn a great deal more about the origin of the megacities, the nature of the first city dwellers, their organisational structure and their successes and failures.
Acknowledgements. We are grateful for suggestions, useful comments and/or figures from Wil van Breugel, Ilana Feain, Jaron Kurk, Nicole Nicole Nesvadba, Roderik Overzier, Michiel Reuland, Birgitte Rocca, Huub Röttgering, Bram Venemans and Montse Villar-Martin. GM acknowledges support from an Academy Professorship of the Royal Netherlands Academy of Arts and Sciences.