Blazar candidates beyond redshift 4 observed with GROND
  The search for extremely massive high redshift blazars is essential to put
strong constraints on the supermassive black holes formation theories. Up to
now, the few blazars known to have a redshift larger than 4 have been
discovered serendipitously. We try a more systematic approach. Assuming
radio-loudness as a proxy for the jet orientation, we select a sample of
extremely radio-loud quasars. We measure their black hole masses with a method
based on fitting the thermal emission from the accretion disc. We achieve a
precision of a factor of two for our measures, thanks to the observations
performed with the Gamma-Ray Burst Optical Near-Infrared Detector (GROND). The
infrared to optical GROND data allow us to observe directly the peak of the
disc emission, thus constraining the overall disc luminosity. We obtain a small
range of masses, that peaks at 10^{9.3}Msun. If some of our candidates will be
confirmed as blazars, these results would introduce interesting constraints on
the mass function of extremely massive black holes at very high redshift.
Moreover, all our blazar candidates have high accretion rates. This result,
along with the high masses, opens an interesting view on the need of a fast
growth of the heaviest black holes at very high redshift.
