The radio properties of z>3.5 quasars: Are most high-redshift radio-loud active galactic nuclei obscured?

TL;DR

This study investigates the radio properties of high-redshift (z>3.5) quasars, revealing an increased fraction of flat-spectrum sources and suggesting most are obscured in UV/optical bands, with implications for their evolution and orientation.

Contribution

It provides new insights into the spectral evolution and obscuration of high-redshift radio-loud quasars, highlighting the increase in flat-spectrum sources and their potential orientation-based obscuration.

Findings

Fraction of flat-spectrum quasars increases with redshift

Radio source sizes decrease with increasing redshift

Most high-redshift radio-loud quasars are obscured in UV/optical bands

Abstract

We explore the radio properties of powerful (rest-frame luminosity $\gtrsim10^{28}$ 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 $\sim 10^{27}$ 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, assuming 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 ($\lesssim 25^\circ$), 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.