Low frequency radio properties of the<i>z</i> &gt; 5 quasar population

Journal article

Optically luminous quasars at z > 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 confirmed z > 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 > 2σ significance 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+0.10 by 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 at z < 3. We compare the radio loudness of the high-z quasar sample to a lower-z quasar sample at z ∼ 2 and find that the two radio loudness distributions are consistent with no evolution, although the low number of high-z quasars means that we cannot rule out weak evolution. Furthermore, we make a first order empirical estimate of the z = 6 quasar radio luminosity function, which is used to derive the expected number of high-z sources 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-z quasar candidates for follow-up spectroscopic observations by decreasing contamination of stellar dwarfs and reducing possible selection biases introduced by strict colour cuts.

Authors: Gloudemans, AJ; Duncan, KJ; Röttgering, HJA; Shimwell, TW; Venemans, BP

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: EDP Sciences
• Journal: Astronomy & Astrophysics
• Volume: 656
• Pages: A137-A137
• Publication date: 2021-10-14
• Acceptance date: 2021-10-07
• DOI: 10.1051/0004-6361/202141722
• EISSN: 1432-0746
• ISSN: 0004-6361
• Language: English
• Keywords: Spectral line; Galaxy; Astrophysics; LOFAR; Population; Astronomy; Quasar; Sky; Spectral index; Radio telescope; Reionization; Redshift; Radio galaxy; Physics