Subarcsecond view on the high-redshift blazar GB 1508+5714 by the International LOFAR Telescope^★

¹ Institut für Theoretische Physik und Astrophysik, Universität Würzburg, Emil-Fischer-Straße 31, 97074 Würzburg, Germany
e-mail: alexander.kappes@uni-wuerzburg.de
² INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, 23807 Merate, Italy
³ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁴ Space Science Division, Naval Research Laboratory, Washington, DC 20375, USA
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching b. München, Germany
⁶ Department of Astronomy, University of Michigan, 1085 S University, Ann Arbor, MI 48109, USA
⁷ INFN – Sezione Milano–Bicocca, Piazza della Scienza 3, 20126 Milano, Italy
⁸ Dipartimento di Scienza e Alta Tecnologia, Università dell’Insubria, Via Valleggio 11, 22100 Como, Italy
⁹ Departament d’Astronomia i Astrofísica, Universitat de València, C/ Dr. Moliner, 50, 46100 Burjassot, València, Spain
¹⁰ Observatori Astronòmic, Universitat de València, C/ Catedràtic Beltrán 2, 46091 Paterna, València, Spain
¹¹ Dipartimento di Fisica G. Occhialini, Univ. Milano–Bicocca, P.za della Scienza 3, 20126 Milano, Italy
¹² Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
¹³ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
¹⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
¹⁵ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹⁶ Department of Physics and Astronomy, School of Natural Sciences, University of Manchester, Manchester M13 9PL, UK
¹⁷ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain

Received: 6 July 2021
Accepted: 25 April 2022

Abstract

Context. Studies of the most distant active galactic nuclei (AGNs) allow us to test our current understanding of the physics present in radio-jetted AGNs across a range of environments, and probe their interactions with these environments. The decrease in apparent luminosity with distance is the primary difficulty to overcome in the study of these distant AGNs, which requires highly sensitive instruments.

Aims. Our goal is to employ new long wavelength radio data to better parametrise the broad-band spectral energy distribution (SED) of GB 1508+5714, a high-redshift (z = 4.30) AGN. Its high redshift, high intrinsic luminosity and classification as a blazar allow us to test emission models that consider the efficient cooling of jet electrons via inverse Compton losses in interactions with the dense cosmic microwave background (CMB) photon field at high redshifts. A significant detection of this effect in GB 1508+5714 may partly explain the apparent sparsity of high-redshift radio galaxies in wide-field surveys, detections of this kind are only becoming possible with the current generation of Square Kilometre Array (SKA) precursors.

Methods. We used the LOw-Frequency ARray (LOFAR) to image the long wavelength radio emission around the high-redshift blazar GB 1508+5714 on arcsecond scales at frequencies between 128 and 160 MHz. This allowed us to compare the spatially resolved structure with higher frequency observations, and construct spectral index maps to study the spectral properties of the different components.

Results. The LOFAR image shows a compact unresolved core and two resolved emission regions around 2 arcsec to the east and to the west of the radio core. We find structure consistent with previous Very Large Array (VLA) observations, as well as a previously unreported emission region to the east. The region in the west shows a spectral index of −1.2_−0.2^+0.4 while the region in the east indicates a spectral index of ≲−1.1. The radio core features aflat spectral index of 0.02 ± 0.01.

Conclusions. We interpret the arcsecond-scale radio structure of GB 1508+5714 as a FR II-like radio galaxy at a small viewing angle, and the western component as the region containing the approaching jet’s terminal hot spot while the eastern diffuse component near the core can be interpreted as the counter-hot spot region. Our SED modelling shows that a scenario featuring significant quenching effects caused by interaction with the CMB provides a good description of the data, and notably explains the suppressed radio emission.