Observed SED of the extended emission of the source PKS J0637−752 at z = 0.65 (grey points), based on which the physical parameters of the model have been set (grey lines). The red lines show the same model but at the redshift of PSO J0309+27, z = 6.1. The X-ray, optical-to-NIR, and radio measurements or upper limits of the extended jet emission in PSO J0309+27 are shown as red points and arrows, and those of PKS J0637−752 are shown as grey points and arrows. In grey we also show the upper limits from Fermi/LAT for both sources. We stress that the only difference between the input parameters of the two curves is the redshift. At low z the extended X-ray emission is dominated by the synchrotron component (dashed lines) of the second population of electrons accelerated up to ultra-high energies (γ ∼ 10^8 − 9). At high z, on the other hand, electrons are cooled down by the IC/CMB interaction much faster than they can be accelerated, and thus the IC/CMB emission (dotted lines) dominates in the X-rays and the synchrotron emission is significantly quenched.