Do high redshift quasars have powerful jets?
  Double-lobed radio galaxies a few 100s of kpc in extent, like Cygnus A, are common at redshifts of 1 to 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 10s 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 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, ULASJ112001.48+064124.3 at z = 7.1 and SDSSJ1030+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 Msun black hole at a rate that violates the Eddington limit. 
A.C. Fabian, S.A. Walker, A. Celotti, G. Ghisellini, P. Mocz, K.M.   Blundell and R.G. McMahon
astro-ph.HE astro-ph.GA