2004MNRAS.348..647K

Near-infrared (IR) photometry for 20 radio-loud z > 3 quasars, 16 of which are radio-selected, are presented. These data sample the rest-frame optical/ultraviolet (UV) continuum, which is commonly interpreted as emission from an accretion disc. In a previous study, we compared the rest-frame optical/UV continuum shapes of 15 optically bright (V < 17.5), z > 3 quasars with those of 27 low-redshift (z ∼ 0.1) quasars that were matched to the high-redshift sample in evolved luminosity (i.e. having luminosities ranging from 1 to 7 times the characteristic luminosity, L *, where L *∼ (1 +z)^∼3) to look for signs of evolution in the central engines. We found the continuum shapes at z ∼ 0.1 and z > 3 similar, consistent with no significant change in the ratio m/ M, where m is the accretion rate with respect to the Eddington rate and M is the black-hole mass. This study expands our earlier high-redshift sample to lower luminosity, away from extreme objects and towards a luminosity overlap with lower-redshift samples. The distribution of rest-frame optical/UV continuum shapes for this fainter sample is broader, extending further to the red than that of the brighter z > 3 sample. Three quasars from this fainter sample, two radio-selected and one optically selected, have optical continuum slopes α < -1 (F_ν∼ν^α). The optically selected one, LBQS 0056+0125, appears to be reddened by dust along the line of sight or in the host galaxy, whereas the radio-selected ones, PKS 2215+02 and TXS 2358+189, could derive their red continua from the contribution of a relatively strong synchrotron component to the rest-frame optical. These objects may represent a bridge to a population of very red high-redshift quasars to which ongoing or future near-IR, optical and deep X-ray surveys will be sensitive.