Discovery of 24 radio-bright quasars at $4.9 \leq z \leq6.6$ using low-frequency radio observations

TL;DR

This study reports the discovery of 24 high-redshift radio-bright quasars using combined optical dropout selection from DESI Legacy Imaging Surveys and low-frequency radio observations from LoTSS, significantly expanding the known sample at $z \,\geq\, 5$.

Contribution

It introduces a new method combining optical dropout selection with low-frequency radio data to efficiently find high-$z$ radio-bright quasars, doubling the known population at $z \,\geq\, 5$.

Findings

Discovered 24 new quasars at $4.9 \,\leq\, z \,\leq\, 6.6$.

Most of the quasars are radio-loud, more than doubling the known high-$z$ radio-loud quasar sample.

Optical and near-infrared properties of the sample are consistent with known radio-quiet quasars.

Abstract

High redshift quasars ($z>5$) that also shine brightly at radio wavelengths are unique signposts of supermassive black hole activity in the early universe. However, bright radio sources at $z\ge5$ are extremely rare and therefore we have started a campaign to search for new high-$z$ quasars by combining an optical dropout selection driven by the $g$, $r$, and $z$ bands from the Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Surveys with low-frequency radio observations from the LOFAR Two-metre Sky Survey (LoTSS). Currently, LoTSS covers a large fraction of the northern sky (5720 deg$^2$) to such a depth (median noise level of 83 $\mu$Jy beam$^{-1}$) that about 30% of the general quasar population is detected $-$ which is a factor of 5-10 more than previous large sky radio surveys such as NVSS and FIRST, respectively. In this paper, we present the discovery of 20 new quasars (and the independent confirmation of 4) between $4.9\leq z\leq 6.6$. Out of the 24 quasars, 21 satisfy the traditional radio-loudness criterion of $R=f_{5\text{GHz}}/f_{4400A} > 10$, with the full sample spanning $R\sim$6-1000, thereby more than doubling the sample of known radio-loud quasars at $z \ge 5$. Our radio detection requirement strongly decreases the contamination of stellar sources and allows one to select these quasars in a broad redshift range. Despite selecting our quasar candidates using fewer and less conservative colour restrictions, both the optical and near-infrared colours, Ly$\alpha$ emission line properties, and dust reddening, $E(B-V)$, measurements of our quasar sample do not deviate from the known radio-quiet quasar population, suggesting similar optical quasar properties of the radio-loud and radio-quiet quasar population at high-$z$. Our campaign demonstrates the potential for discovering new high-$z$ quasar populations through next generation radio continuum surveys.