Low frequency radio properties of thez > ​5 quasar population

Abstract

Optically luminous quasars atz > 5 are important probes of super-massive black hole (SMBH) formation. With new and future radio facilities, the discovery of the brightest low-frequency radio sources in this epoch would be an important new probe of cosmic reionization through 21-cm absorption experiments. In this work, we systematically study the low-frequency radio properties of a sample of 115 known spectroscopically confirmedz > 5 quasars using the second data release of the Low Frequency Array (LOFAR) Two Metre Sky survey (LoTSS-DR2), reaching noise levels of ∼80 μJy beam−1(at 144 MHz) over an area of ∼5720 deg2. We find that 41 sources (36%) are detected in LoTSS-DR2 at > 2σsignificance and we explore the evolution of their radio properties (power, spectral index, and radio loudness) as a function of redshift and rest-frame ultra-violet properties. We obtain a median spectral index of −0.29−0.09+0.10by stacking 93 quasars using LoTSS-DR2 and Faint Images of the Radio Sky at Twenty Centimetres (FIRST) data at 1.4 GHz, in line with observations of quasars atz < 3. We compare the radio loudness of the high-zquasar sample to a lower-zquasar sample atz ∼ 2 and find that the two radio loudness distributions are consistent with no evolution, although the low number of high-zquasars means that we cannot rule out weak evolution. Furthermore, we make a first order empirical estimate of thez = 6 quasar radio luminosity function, which is used to derive the expected number of high-zsources that will be detected in the completed LoTSS survey. This work highlights the fact that new deep radio observations can be a valuable tool in selecting high-zquasar candidates for follow-up spectroscopic observations by decreasing contamination of stellar dwarfs and reducing possible selection biases introduced by strict colour cuts.