The accretion of quasars at the epoch of reionisation: JWST catches the primeval monsters slowly feasting

Quasars (QSOs) emit an enormous amount of light as a result of the accretion of matter onto supermassive black holes (SMBHs). Thanks to their luminosity, the most distant known QSOs allow us to trace the growth of SMBHs deep into the epoch of reionisation. In this work, we employed /NIRSpec observations of ten luminous (łog(L_ JWST 3000,Å /(̊m erg , -1 )),≥,45.6) QSOs at z,≥,5.8 to constrain their accretion properties, namely black hole mass, accretion disc (AD) luminosity, and Eddington ratio (M_̊m BH, L_̊m AD, λ_̊m Edd). We achieved this by employing different bolometric corrections and single-epoch relations for several rest-frame UV and optical broad emission lines as well as by taking advantage of AD modelling. This latter method provides self-consistent measurements of both M_̊m BH and L_̊m AD, with uncertainties on M_̊m BH and L_̊m AD (σ^ ̊m AD _ M_ ̊m BH ,∼,0.2 dex; σ^ ̊m AD _ L_ ̊m AD ,∼,0.1 dex), significantly smaller than the systematic uncertainties associated with single-epoch M_̊m BH (∼0.4 dex) and L_̊m AD derived via bolometric corrections (∼0.2 dex). Based on these results and assuming that our high-z QSOs are representative of optically selected bright blue QSOs and that the amount of dust reddening along the line of sight to these sources is negligible, in our sample we found an average Eddington ratio of łangle łog(λ_ ̊m Edd ) ̊angle,=,-0.9, with a standard deviation of ∼,0.3 dex. This implies a fraction of such systems accreting above the Eddington limit of łesssim,0.1%. In conclusion, this work i) demonstrates the suitability of AD modelling on JWST data to constrain the accretion properties of high-redshift (z,≳,4) QSOs thanks to the large spectral coverage of the JWST /NIRSpec instrument; ii) shows that AD modelling can yield robust M_̊m BH and L_̊m AD measurements in bright QSOs, with smaller uncertainties than the typical calibrations; and iii) provides compelling evidence of sub-Eddington accretion in bright high-z QSOs, challenging the widespread paradigm of near- or super-Eddington accretion occurring in these sources.