Low frequency radio properties of the z > ​5 quasar population

• Published in: Astronomy and astrophysics (Berlin) 2021-12, Vol.656, p.A137
• Main Authors: Gloudemans, A. J., Duncan, K. J., Röttgering, H. J. A., Shimwell, T. W., Venemans, B. P., Best, P. N., Brüggen, M., Calistro Rivera, G., Drabent, A., Hardcastle, M. J., Miley, G. K., Schwarz, D. J., Saxena, A., Smith, D. J. B., Williams, W. L.
• Format: Article
• Language: English
• Subjects: Evolution; Ionization; LF radio; LOFAR; Loudness; Low frequencies; Luminosity; Noise levels; Quasars; Radio astronomy; Radio observation; Red shift; Sky surveys (astronomy); Supermassive black holes
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202141722

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 deg 2 . We find that 41 sources (36%) are detected in LoTSS-DR2 at > 2 σ 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.