Ultraluminous quasars at high redshift show evolution in their radio-loudness fraction in both redshift and ultraviolet luminosity

We take a sample of 94 ultraluminous, optical quasars from the search of over 14 486 deg² by Onken et al. in the range 4.4 < redshift < 5.2 and match them against the Rapid ASKAP Continuum Survey (RACS) observed on the Australian Square Kilometre Array Pathfinder (ASKAP). From this most complete sample of the bright end of the redshift ∼5 quasar luminosity function, there are 10 radio continuum detections, of which eight are considered radio-loud quasars. The radio-loud fraction for this sample is 8.5 ± 2.9 per cent. Jiang et al. found that there is a decrease in the radio-loud fraction of quasars with increasing redshift and an increase with increasing absolute magnitude at rest frame 2500 Å. We show that the radio-loud fraction of our quasar sample is consistent with that predicted by Jiang et al.,extending their result to higher redshifts.