Revealing faint compact radio jets at redshifts above 5 with very long baseline interferometry

¹ Department of Astronomy, Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
² Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, H-1121 Budapest, Hungary
³ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, H-1121 Budapest, Hungary
⁴ Dipartimento di Fisica e Astronomia, Università di Bologna, Via P. Gobetti 93/2, 40129 Bologna, Italy
⁵ INAF Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy
⁶ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
⁷ Shanghai Astronomical Observatory, Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
⁸ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
⁹ HUN-REN–ELTE Extragalactic Astrophysics Research Group, Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary

Received: 7 June 2024
Accepted: 21 July 2024

Abstract

Context. Over the past two decades, our knowledge of the high-redshift (z > 5) radio quasars has expanded, thanks to dedicated high-resolution very long baseline interferometry (VLBI) observations. Distant quasars provide unique information about the formation and evolution of the first galaxies and supermassive black holes in the Universe. Powerful relativistic jets are likely to have played an essential role in these processes. However, the sample of VLBI-observed radio quasars is still too small to allow meaningful statistical conclusions.

Aims. We extend the list of the VLBI observed radio quasars to investigate how the source structure and physical parameters are related to radio loudness.

Methods. We assembled a sample of ten faint radio quasars located at 5 < z < 6 with their radio-loudness indices spanning between 0.9 − 76. We observed the selected targets with the European VLBI Network (EVN) at 1.7 GHz. The milliarcsecond-scale resolution of VLBI at this frequency allowed us to probe the compact innermost parts of radio-emitting relativistic jets. In addition to the single-band VLBI observations, we collected single-dish and low-resolution radio interferometric data to investigate the spectral properties and variability of our sources.

Results. The detection rate of this high-redshift, low-flux-density sample is 90%, with only one target (J0306+1853) remaining undetected. The other nine sources appear core-dominated and show a single, faint and compact radio core on this angular scale. The derived radio powers are typical of Fanaroff-Riley II radio galaxies and quasars. By extending our sample with other VLBI-detected z > 5 sources from the literature, we found that the core brightness temperatures and monochromatic radio powers tend to increase with radio loudness.