The radio spectral turnover of radio-loud quasars at z > 5

Abstract

We present Karl G. Jansky Very Large Array S- (2–4 GHz), C- (4–8 GHz), and X-band (8–12 GHz) continuum observations toward seven radio-loud quasars at z > 5. This sample has previously been found to exhibit spectral peaks at observed-frame frequencies above ∼1 GHz. We also present upgraded Giant Metrewave Radio Telescope (uGMRT) band-2 (200 MHz), band-3 (400 MHz), and band-4 (650 MHz) radio continuum observations toward eight radio-loud quasars at z > 5, selected from our previous GMRT survey, in order to sample their low-frequency synchrotron emission. Combined with archival radio continuum observations, all ten targets show evidence for spectral turnover. The turnover frequencies are ∼1–50 GHz in the rest frame, making these targets gigahertz-peaked-spectrum or high-frequency-peaker candidates. For the nine well-constrained targets with observations on both sides of the spectral turnover, we fit the entire radio spectrum with absorption models associated with synchrotron self-absorption and free-free absorption (FFA). Our results show that FFA in an external inhomogeneous medium can accurately describe the observed spectra for all nine targets, which may indicate an FFA origin for the radio spectral turnover in our sample. As for the complex spectrum of J114657.79+403708.6 at z = 5.00 with two spectral peaks, it may be caused by multiple components (i.e., core-jet) and FFA by the high-density medium in the nuclear region. However, we cannot rule out the spectral turnover origin of variability. Based on our radio spectral modeling, we calculate the radio loudness R2500 Å for our sample, which ranges from 12−1+1 to 674−51+61.

Constraining the radio properties of the z = 6.44 QSO VIK J2318−3113

“…Together with a kinematic age similar to VIK J2318−3113 (∼1700 years; Momjian et al 2021), its properties suggest that, also in this case, the radio emission originates from a relatively young jet. Similar conclusions were also reached for the majority of the other z > 5 RL QSOs (e.g., Frey et al 2008Frey et al , 2011Shao et al 2022).…”

“…1. With this model, the resulting peak frequency of VIK J2318−3113 is at 4.8 ± 0.6 GHz (q = −0.49 +0.06 −0.07 and α = −1.54 +0.29 −0.34 ), which corresponds to about 650 MHz in the observed frame, similar to the values observed in other z > 5 RL QSOs (e.g., Shao et al 2022).…”

A powerful (and likely young) radio-loud quasar at z = 5.3

“…Squares represent all the sources defined as blazars in the literature on the basis of their radio and/or X-ray properties. Yellow circles represent all the sources identified as peaked spectrum objects in the literature (Coppejans et al 2017;Shao et al 2020Shao et al , 2022. PSO J191+86 is one of the most luminous RL QSOs at these redshifts, confirming the power of its radio emission.…”

Radio-loud Quasars above Redshift 4: Very Long Baseline Interferometry (VLBI) Imaging of an Extended Sample

“…However, because of the lack of low-frequency measurements, parameters of such a fit would be poorly constrained. Indeed, when low-frequency synchrotron emission is properly sampled, highredshift (z > 5) radio-loud quasars tend to show evidence for spectral turnover at rest-frame frequencies ∼1-50 GHz (Shao et al 2022). Peaked continuum radio spectra are also common for bright z > 3 radio quasars (Sotnikova et al 2021).…”

“…These include objects like the unbeamed versions of gigahertz-peaked spectrum (GPS) and compact steep-spectrum sources, and the symmetric ones are called compact symmetric objects (CSOs) or medium symmetric objects. They are found at z > 4 as well (e.g., Coppejans et al 2017;Shao et al 2022). In contrast to blazars, the radio emission of these types of AGN is dominated by the outer regions of the jet, while the "core" emission coinciding with the optical position, due to the absence of Doppler-boosting, often remains undetected (e.g., An & Baan 2012;O'Dea & Saikia 2021, and references therein).…”