| Abstract
| - We present the results of a new LUCI/Large Binocular Telescope near-infrared (NIR) spectroscopic observation of PSO J030947.49+271757.31 (hereafter PSO J0309+27), the highest redshift blazar known to date ( z ∼ 6.1). From the CIV λ1549 broad emission line, we found that PSO J0309+27 is powered by a 1.45 −0.85+1.89 × 10 9 M⊙ supermassive black hole (SMBH) with a bolometric luminosity of ∼8 × 10 46 erg s −1 and an Eddington ratio equal to 0.44 −0.35+0.78. We also obtained new photometric observations with the Telescopio Nazionale Galileo in J and K bands to better constrain the NIR spectral energy distribution of the source. Thanks to these observations, we were able to model the accretion disk and to derive an independent estimate of the black hole mass of PSO J0309+27, confirming the value inferred from the virial technique. The existence of such a massive SMBH just ∼900 million years after the Big Bang challenges models of the earliest SMBH growth, especially if jetted active galactic nuclei are indeed associated with a highly spinning black hole, as is currently assumed. In a Eddington-limited accretion scenario and assuming a radiative efficiency of 0.3, typical of a fast rotating SMBH, a seed black hole of more than 10 6 M⊙ at z = 30 is indeed required to reproduce the mass of PSO J0309+27 at a redshift of 6. This requirement suggests either earlier periods of rapid black hole growth with super-Eddington accretion or a scenario in which only part of the released gravitational energy goes toward heating the accretion disk and feeding the black hole.
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