Blazar candidates beyond redshift 4 observed with GROND

TL;DR

This study systematically searches for high-redshift blazar candidates using infrared-optical data from GROND, aiming to constrain supermassive black hole formation and growth in the early universe.

Contribution

It introduces a novel systematic approach to identify high-redshift blazar candidates based on radio-loudness and infrared-optical observations, providing new mass estimates.

Findings

Black hole masses peak at around 10^{9.3} solar masses.

All candidates show high accretion rates.

Results suggest rapid growth of massive black holes at high redshift.

Abstract

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.