Low frequency radio properties of thez > ​5 quasar population

Abstract

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.

The [CII] and FIR properties ofz> 6 radio-loud quasars

“…Radio-loud quasars are relatively rare as they constitute only 8-10% of the quasar population at z 6 (Bañados et al 2015b;Liu et al 2021;Gloudemans et al 2021). To date, there are only five radio-loud quasars known at z > 6: J1427+3312 at z = 6.12 (McGreer et al 2006), J1429+5447 at z = 6.18 (Willott et al 2010), J0309+2717 at z = 6.10 (Belladitta et al 2020), J2318-3113 at z = 6.44 (Ighina et al 2021a), and P172+18 at z = 6.83 (Bañados et al 2021).…”

Exploring the Radio Spectral Energy Distribution of the Ultraluminous Radio-quiet Quasar SDSS J0100+2802 at Redshift 6.3

“…We recalibrated and reimaged the data from this program with the same procedure as described above for the 2017 VLA observations. We also collect the observations from other VLA frequencies and VLBA (Wang et al 2016(Wang et al , 2017, and low-frequency data from LOFAR (Gloudemans et al 2021). These results are summarized in Table 1.…”

“…However, the observed-frame 32 GHz continuum flux density has large uncertainties due to the low S/N (∼3). Quasar J0100+2802 was also detected in the Low Frequency Array (LOFAR) Two Meter Sky survey (LoTSS) data release 2 (DR2; Shimwell et al 2022), which was published very recently Gloudemans et al (2021). The LoTSS data were taken in October 2016 at 144 MHz with a pointsource flux density of S 144 MHz = 0.88 ± 0.13 μJy.…”

Exploring the radio spectral energy distribution of the ultraluminous radio-quiet quasar SDSS J0100+2802 at redshift 6.3

“…We re-calibrated and re-imaged the data from this program with the same procedure as described above for the 2017 VLA observations. We also collect the observations from other VLA frequencies and VLBA (Wang et al 2016(Wang et al , 2017, and low frequency data from LOFAR (Gloudemans et al 2021). These results are summarized in Table 1.…”