Can high-redshift AGN observed by JWST explain the EDGES absorption signal?

Abstract

The Experiment to Detect the Global Epoch of Reionization 21 cm Signal (EDGES) has reported evidence for an absorption feature in the sky-averaged radio background near 78 MHz. A cosmological interpretation of this signal corresponds to absorption of 21 cm photons by neutral hydrogen at $z \sim 17$. The large depth of the signal has been shown to require an excess radio background above the CMB and/or non-standard cooling processes in the IGM. Here, we explore the plausibility of a scenario in which the EDGES signal is back-lit by an excess radio background sourced from a population of radio-loud AGN at high redshift. These AGN could also explain the unexpected abundance of UV-bright objects observed at $z > 10$ by JWST. We find that producing enough radio photons to explain the EDGES depth requires that nearly all high-$z$ UV-bright objects down to $M_{\mathrm{UV}} \gtrsim -15$ are radio-loud AGN and that the UV density of such objects declines by at most 1.5 orders of magnitude between $z = $10 and 20. In addition, the fraction of X-ray photons escaping these objects must be $\lesssim 1\%$ of their expected intrinsic production rate to prevent the absorption signal being washed out by early IGM pre-heating. Reproducing the sharp boundaries of the absorption trough and its flat bottom requires that the UV luminosity function, the fraction of UV light produced by AGN, and the X-ray escape fraction have fine-tuned redshift dependence. We conclude that radio-loud AGN are an unlikely (although physically possible) candidate to explain EDGES because of the extreme physical properties required for them to do so.