2024A&A...689A.174C

We explore the radio properties of powerful (rest-frame luminosity ≳10²⁸ W Hz^–1 at 500 MHz) high-redshift (z ≳ 3.5) quasars. The aim of this study is to gain a better understanding of radio-loud sources at the epoch when they reach the highest space density. We selected 29 radio-loud quasars at low radio frequencies (76 MHz). Their radio spectra, covering the range from 76 MHz to 5 GHz, are generally well reproduced by a single power law. We created samples that were matched in radio luminosity at lower redshift (from z ∼ 1.3 to z ∼ 2.8) to investigate any spectral evolution. We find that the fraction of flat-spectrum radio quasars (FSRQs) strongly increases with redshift (from ∼8% at z = 1.2 to ∼45% at z > 3.5). This effect is also observed in quasars with lower luminosities (down to ∼10²⁷ W Hz^–1). The increase in the fraction of FSRQs with redshift corresponds to a decrease in the steep- spectrum radio quasars. This result can be explained when we assume that the beaming factor and the slope of the luminosity function do not change with redshift, if high-redshift radio-loud sources can be recognized as quasars only when they are seen at a small viewing angle (≲25°), while most of them, about 90%, are obscured in the UV and optical bands. We also found a trend for the size of radio sources to decrease with increasing redshift. Because projection effects are insufficient to cause this trend, we suggest that the large amount of gas causing the nuclear obscuration also hampers the growth of the more distant sources.