On the radio properties of the highest redshift quasars

We present deep radio observations of the most distant complete quasar sample drawn from the Sloan Digital Sky Survey. Combining our new data with those from literature, we obtain a sample which is ∼100 per cent complete down to S1.4 GHz= 60 μJy over the redshift range 3.8 ≤z≤ 5. The fraction of radio detections is relatively high (∼43 per cent), similar to what observed locally in bright optical surveys. Even though the combined radio and optical properties of quasars remain overall unchanged from z∼ 5 to the local Universe, there is some evidence for a slight overabundance of radio-loud (RL) sources at the highest redshifts when compared with the lower-z regime. Exploiting the deep radio Very Large Array observations, we present the first attempt to directly derive the radio luminosity function of bright quasars at z≳ 4. The unique depth – both in radio and optical – allows us to thoroughly explore the population of optically bright Fanaroff–Riley type II (FR II) quasars up to z∼ 5 and opens a window on the behaviour of the brightest FR I sources. A close investigation of the space density of RL quasars also suggests a differential evolution, with the more luminous sources showing a less-pronounced cut-off at high z when compared with the less-luminous ones.