Radio-loud active galactic nuclei at high redshifts and the cosmic microwave background

The interaction between the emitting electrons and the cosmic microwave background (CMB) affects the observable properties of radio-loud active galactic nuclei (AGN) at early epochs. At high redshifts z, the CMB energy density [UCMB ∝ (1 + z)4] can exceed the magnetic one (UB) in the lobes of radio-loud AGN. In this case, the relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron emission. This makes more distant sources less luminous in radio and more luminous in X-rays than their closer counterparts. In contrast, in the inner jet and the hotspots, where UB > UCMB, synchrotron radiation is unaffected by the presence of the CMB. The decrease in radio luminosity is thus more severe in misaligned (with respect to our line of sight) high-z sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux-limited radio surveys, where they are possibly underrepresented. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB is less important at low radio frequencies. Therefore, objects not detected so far at a few GHz could be picked up by low-frequency deep surveys, such as Low-Frequency Array and Square Kilometre Array. Until then, we can estimate the number of high-z radio-loud AGN through the census of their aligned proxies, i.e. blazars, since their observed radio emission arises in the inner and strongly magnetized compact core of the jet and it is not affected by inverse Compton scattering off CMB photons.