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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: yshao@mpifr-bonn.mpg.de
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, PR China
³ SKA Observatory, Lower Withington Macclesfield, Cheshire SK11 9FT, UK
⁴ National Radio Astronomy Observatory, Socorro, NM 87801-0387, USA
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Department of Astronomy, Cornell University, Space Sciences Building, Ithaca, NY 14853, USA
⁷ Leiden Observatory, Leiden University, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
⁸ International Centre for Radio Astronomy Research, Curtin University, GPO Box U1987 Perth, WA 6845, Australia

Received: 20 October 2021
Accepted: 30 November 2021

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 R_2500 Å for our sample, which ranges from 12₋₁⁺¹ to 674₋₅₁⁺⁶¹.