2014MNRAS.442L..81F

Double-lobed radio galaxies a few hundreds of kpc in extent, like Cygnus A, are common at redshifts of 1-2, arising from some 10 per cent of the most powerful active galactic nuclei (AGN). At higher redshifts, they are rare, with none larger than a few tens of kpc known above redshift z ∼ 4. Recent studies of the redshift evolution of powerful jetted objects indicate that they may constitute a larger fraction of the AGN population above redshift 2 than that appears from a simple consideration of detected GHz radio sources. The radio band is misleading as the dramatic (1 + z)4 boost in the energy density of the cosmic microwave background (CMB) causes inverse Compton scattering to dominate the energy losses of relativistic electrons in the extended lobes produced by jets, making them strong X-ray, rather than radio, sources. Here, we investigate limits to X-ray lobes around two distant quasars, ULAS J112001.48+064124.3 at z = 7.1 and SDSS J1030+0524 at z = 6.3, and find that powerful jets could be operating yet be currently undetectable. Jets may be instrumental in the rapid build-up of billion M_☉ black hole at a rate that violates the Eddington limit.