The remarkably bright nature of radio-jetted active galactic nuclei (AGNs) allows us to observe them at extreme redshifts and thus use their properties as an observational tracer of cosmological principles (Wang et al. 2021). Blazars, in particular, benefit from increased apparent luminosities due to relativistic boosting effects (Cohen et al. 2007b), and can thus be detected over a wide range of the electromagnetic spectrum, although at the cost of being observationally compact objects with small angles between their jets and the line of sight. Conversely, AGNs with radio jets that are at a larger angle to the line of sight are harder to detect with increasing distance. Because they can be observed over such a wide range of redshifts, they provide a unique insight into cosmology, galaxy evolution, and the evolution of AGNs (Dunlop & Peacock 1990; Georgakakis et al. 2017). In addition to probing the universe by observing targets at different redshifts, one should also account for differences in their evolutionary stage, the environment they are embedded in at that time, and their interactions with that environment. Many radio surveys have already been performed to investigate radio-loud AGN populations (e.g. Becker et al. 1995; Condon et al. 1998; Cohen et al. 2007a; Intema et al. 2017) and most find self-consistent relative number ratios of radio galaxies and blazars up to a redshift of ~3 (Volonteri et al. 2011). Beyond this redshift, matters are complicated by uncertainties about the density evolution and the build-up of high black hole masses in the early universe (Blundell et al. 1999; Shankar et al. 2008). However, there seems to be a consensus that there is a relative lack of higher redshift radio galaxies even when accounting for evolutionary effects and detection limits (e.g. Wu et al. 2017; Hodges-Kluck et al. 2021, and references therein). The reason for the deficit is still not fully understood. The currently favoured explanation was suggested by Ghisellini et al. (2015): interaction of the extended radio emission with the cosmic microwave background (CMB) could efficiently quench the brightness of the extended radio lobes. Morabito & Harwood (2018) find evidence to support this model based on a comparison of simulations and observational data. In this scenario the CMB energy density dominates over the magnetic energy density at very high redshifts, so that the jet electrons interact with the CMB photons by inverse Compton (IC) scattering to cool, while the synchrotron radiation is suppressed. Although quenched, the steep-spectrum isotropic radiation of the extended structures could nevertheless be detected by telescopes operating in the long wavelength radio regime, which can test and guide the theoretical models. Ghisellini et al. (2015) proposed a number of suitable blazars to spotlight this issue, and published expected radio fluxes for different model parameters. The International LOFAR Telescope (ILT) (van Haarlem et al. 2013) offers the resolution, sensitivity, and observing wavelengths necessary to detect the extended emission from these blazars and to address the questions associated with its possible suppression. For this study, we processed and analysed one ILT observation of GB 1508+5714 at z = 4.30 (Hook et al. 1995), which is one of the most distant quasars with a detected X-ray jet (Yuan et al. 2003; Siemiginowska et al. 2003). Throughout the paper we use the following cosmological parameters: H0 = 71 km s−1 Mpc−1, Ωm = 0.27, and ΩΛ = 0.73, hence a luminosity distance of 39.8 Gpc and a conversion scale where 1″ is about 6.9 kpc for the given source.
Citation: Blundell et al. 1999
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