High-resolution radio imaging of TGSS J1530+1049, a radio galaxy in a dense environment at z = 4

¹ Department of Astronomy, Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
² Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
³ HUN-REN–ELTE Extragalactic Astrophysics Research Group, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁴ Institute of Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudzidzka 5, 87-100 Toruń, Poland
⁵ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117, Budapest, Hungary
⁶ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁷ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS, Delft, The Netherlands
⁸ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
¹⁰ Leiden Observatory, University of Leiden, Niels Bohrweg 2, 2333 CA Leiden, The Netherlands
¹¹ Observatório Nacional/MCTI, Rua General José Cristino, 77, Sao Cristóvao, Rio de Janeiro, RJ 20921 400, Brazil
¹² TNO, Oude Waalsdorperweg 63, 2597 AK Den Haag, The Netherlands
¹³ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Ajalvir, km 4, E-28850 Torrejón de Ardoz, Madrid, Spain
¹⁴ Instituto de Astrofísica de La Plata, CONICET – UNLP, Paseo del Bosque, B1900FWA, La Plata, Argentina
¹⁵ Facultad de Ciencas Astronómicas y Geofísicas, Universidad Nacional de La Plata, B19000FWA, La Plata, Argentina
¹⁶ DTx – Digital Transformation CoLab, Building 1, Azurém Campus, University of Minho, 4800-058 Guimarães, Portugal
¹⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, PT4150-762 Porto, Portugal

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Received: 18 November 2025
Accepted: 15 April 2026

Abstract

Context. High-redshift radio galaxies can provide important insights into structure formation and galaxy evolution during earlier cosmological epochs. TGSS J1530+1049 was selected as a candidate high-redshift radio galaxy based on its very steep radio spectrum. Subsequent observations with the James Webb Space Telescope (JWST) presented in a companion paper have shown that it is located at a redshift of z = 4.0. The JWST data furthermore showed that the radio source is part of one of the densest structures of galaxies and ionized gas known at these redshifts. The complex system qualitatively resembles a massive (cluster) galaxy that formed early through a rapid succession of mergers.

Aims. TGSS J1530+1049 is an unresolved source down to an ∼0.6″ scale in multiple radio surveys. To reveal its high-resolution radio structure and allow for a detailed comparison with JWST observations, we studied its morphology at various angular scales with different radio interferometric instruments.

Methods. We observed TGSS J1530+1049 at a milliarcsecond- (mas) scale angular resolution with the European VLBI Network (EVN), and at an ∼100-mas scale resolution with the enhanced Multi-Element Remotely Linked Interferometer Network (e-MERLIN).

Results. We recovered a complex north–south oriented structure with steep-spectrum radio-emitting features, which are associated with the lobes and hot spots of a jetted active galactic nucleus. However, the centre of the radio galaxy proved to be too faint at centimetre wavelengths to be unambiguously detected in our observations. Nevertheless, its linear size (∼5.5 kpc) and radio power (L_1.4 GHz ≈ 3 × 10²⁷ W Hz⁻¹) place it among the so-called medium-sized symmetric objects, which are a smaller and/or confined version of larger radio galaxies. A comparison between its radio morphology and that of the ionized gas as observed with the NIRSpec integral field unit on JWST shows that the two are closely aligned. However, the optical emission line gas extends out to ∼25 kpc, which is well beyond the detected radio structures.