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This chapter explains how astronomers plan to find powerful radio sources tied to the very first supermassive black holes. It focuses on the Epoch of Reionization (EoR) — the era when the first stars, galaxies and black holes ionised the early Universe. Observations show that billion‑solar‑mass black holes already existed by redshift z≈7, but only a handful of similarly radio‑bright active galactic nuclei (AGN) are known at z>6. The authors argue that many more may exist but have been missed because of how we search for them.
The researchers combined results from modern cosmological models (both hydrodynamical and semi‑analytic simulations) with recent radio surveys from SKA pathfinder telescopes such as the Murchison Widefield Array (MWA), the Low‑Frequency Array (LOFAR), ASKAP and MeerKAT. Those models suggest a substantial, still‑undetected population of radio‑powerful AGN at z>6. At the same time, early survey work has already uncovered some notable distant radio sources — for example TN J0924−2201 at z=5.19, a radio galaxy at z=5.55 confirmed with ALMA (GLEAM J0856+0223, radio power L5GHz ≈ 2×1027 W Hz−1), and other candidates at z=6.44, z=6.82 and a likely blazar at z≈7.0.
A main point of the chapter is that current selection methods are biased or incomplete. Historically, people have used the ultra‑steep spectrum (USS) rule — picking sources whose radio brightness drops quickly with frequency — to find high‑redshift radio galaxies. This works at bright flux levels but fails at the faint levels now probed by deep surveys. At microjansky depths, USS selection often picks nearby star‑forming galaxies instead of the rare, distant radio AGN. The authors therefore propose a physically motivated strategy that looks at full broadband radio spectral energy distributions (SEDs), spectral curvature (how the spectrum bends with frequency), the expected growth of radio jets, compactness on the sky, and even radio‑only redshift estimates.
Why this matters: radio‑bright AGN at the EoR are important probes of early black‑hole growth and of the surrounding gas. Bright radio sources can serve as background beacons for 21‑cm absorption studies of neutral hydrogen. Those 21‑cm signals can directly measure the ionisation state of the intergalactic medium in the first billion years. The authors argue that the Square Kilometre Array Observatory (SKAO), with planned sensitivity, spatial resolution, and continuous spectral coverage from roughly 50 MHz to 15 GHz, will allow fine‑frequency sampling needed to apply these new selection methods and to find many more of the earliest radio AGN.
Important caveats are highlighted. The cosmological simulations used to predict radio AGN populations have unavoidable limits: they trade off volume for resolution, and processes such as black‑hole accretion, feedback and radio jet emission are modelled in a simplified, “subgrid” way. Direct predictions of radio emission are still uncertain. Observationally, surveys to date lack full, broadband spectral information for many sources, which creates selection biases. The chapter therefore presents the new strategy as a way to reduce those biases, while noting that significant uncertainty will remain until SKAO‑era surveys provide the deeper, wide‑band data needed to confirm how many radio‑powerful AGN lived in the EoR.