Abstract
The very existence of a dozen of high-redshift (z>4) blazars indicates that a
much larger population of misaligned powerful jetted AGN was already in place
when the Universe was <1.5 Gyr old. Such parent population proved to be very
elusive, and escaped direct detection in radio surveys so far. High redshift
blazars themselves seem to be failing in producing extended radio-lobes,
raising questions about the connection between such class and the vaster
population of radio-galaxies. We show that the interaction of the jet electrons
with the intense cosmic microwave background (CMB) radiation explains the lack
of extended radio emission in high redshift blazars and in their parent
population, possibly accounting for the apparently missing misaligned
counterparts of high redshift blazars. We then model the spectral energy
distribution of blazar lobes following simple prescriptions, finding that most
of them should be detectable by low frequency deep radio observations, e.g., by
LOw-Frequency ARray for radio astronomy (LOFAR) and by relatively deep X-ray
observations with good angular resolution, e.g., by the Chandra satellite. We
finally show that when misaligned, the jet emission is faint (de-beamed) and
missed by current large sky area surveys. Since the isotropic lobe radio
emission is also quenched by the CMB cooling, sources with even very powerful
jets can go undetected in current radio surveys, and misclassified as
radio-quiet AGNs.
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