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tokens: the emitting electrons through the inverse Compton mechanisms and that weakens their radio emission. However, the regions of the jet producing the 1.4 GHz radio flux (≳7 GHz rest frame) are not affected by the ‘quenching’ of the radio due to the CMB radiation. This is because they have a magnetic energy density much larger than the CMB one, even taking into account the Γ 2 enhancement due to the relativistic motion of the emitting plasma. At z = 4, the CMB energy density is U CMB ∼ 2.6 × 10 −10 erg cm −3 . In the comoving frame, this is enhanced by a factor ∼Γ 2 , thus reaching U ′ = 2.6 × 10 −8 (Γ/10) 2 erg cm −3 . Most of the observed radiation from the jet is produced in a compact region, where the magnetic field is around 3 G, and the observed self-absorption frequency is ν t ∼ 3 × 10 12 Hz (rest frame, see e.g. Ghisellini Tavecchio 2015 ). In the case of a flat radio spectrum, the self-absorption frequency scales as R −1 , where R is the distance from the black hole. This is the same dependence of the dominant component of the magnetic field B . Therefore in the region self-absorbing at 7 GHz (rest frame) B should be ∼6 mG, and its energy density U B = B 2 /(8π) ∼ 1.4 × 10 − 6 . Since , there is no ‘quenching’ of the synchrotron emission of the jet. 4.1 The proposed scenario To solve the tension between predicted and observed sources, we propose a scenario that follows the ideas put forward by Fabian ( 1999 ). At redshifts larger than ∼4, jetted sources hosting a black hole with mass M ≳ 10 9 M ⊙ 1 are completely (i.e. 4π) surrounded with obscuring material (see the cartoon in Fig. 1 ). Only the jet can pierce through this material and break out. Observers looking down the jet can see the nuclear emission from the accretion disc and the broad emission lines. For observers looking with viewing angles even only slightly larger than θ j ≈ 1/Γ, the optical emission (including broad lines) is absorbed, the flux is fainter, and the source cannot enter the SDSS Catalog. The absorbed radiation is reradiated in the infrared. Even if the hotspots or the lobes were indeed emitting a radio flux above the 1 mJy level at the observed 1.4 GHz frequency, there would be no source in the SDSS to match with if the jet is only slightly misaligned. The quasi-spherical dusty structure (hereafter ‘obscuring’ or ‘dark bubble’) can cover the nuclear region until the accretion disc radiation pressure blows it away. This can occur at a threshold luminosity . 5 DISCUSSION Let us assume that the obscuring bubbles exist not only in jetted sources, but are common to all high-redshift quasars, including radio-quiet ones. The evolution in time of the obscuring bubbles and the central black hole mass could however be different in jetted and non-jetted sources. In fact, the presence of a jet could affect the accretion efficiency η d , defined as : part of the dissipation of the gravitational energy could amplify the magnetic field instrumental to launch the jet. In other