Resurging from the ashes: A spectral study of seven candidate revived radio fossils in nearby low-mass galaxy clusters

¹ Istituto Nazionale di Astrofisica (INAF) – Istituto di Radioastronomia (IRA), Via Gobetti 101, 40129 Bologna, Italy
² Dipartimento di Fisica e Astronomia (DIFA), Università di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ Center for Radio Astronomy Techniques and Technologies, Rhodes University, Grahamstown 6140, South Africa
⁴ Istituto Nazionale di Astrofisica – Istituto di Astrofisica Spaziale e Fisica cosmica (IASF), Via A. Corti 12, 20133 Milano, Italy
⁵ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany
⁶ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
⁷ Istituto Nazionale di Astrofisica (INAF) – Osservatorio Astronomico di Padova (OAPD), Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
⁸ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁹ ASTRON, The Netherlands Institute for Radio Astronomy, Postbus 2, 7990 AA Dwingeloo, The Netherlands
¹⁰ Kapteyn Astronomical Institute, University of Groningen, PO Box 800 9700 AV Groningen, The Netherlands
¹¹ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Postbus 94249 1090 GE Amsterdam, The Netherlands
¹² Center for Advanced Interdisciplinary Research, Ss. Cyril and Methodius University in Skopje, 1000, Macedonia
¹³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Gießenbachstraße 1, D-85748 Garching bei München, Germany

^★ Corresponding author: lucabruno2501@gmail.com

Received: 10 September 2025
Accepted: 7 October 2025

Abstract

Context. Complex energy transfer processes in the intracluster medium (ICM) can revive fossil plasma (with spectral ages ≫100 Myr) that was initially generated by radio galaxies. This leads to the re-ignition of faint radio sources with irregular and filamentary morphologies and ultra-steep (α ≳ 1.5) synchrotron spectra, which can more easily be detected at low frequencies (∼100 MHz). These sources offer the opportunity of investigating the microphysics of the ICM and its interplay with radio galaxies, the origin of seed relativistic electrons, the merging history of the host cluster, and the phenomenology of radio filaments.

Aims. The study of revived sources has been hampered so far by the requirement of sensitive and high-resolution multi-frequency radio data at low frequencies to characterise their spatial properties and provide a proper classification. We analysed a sample of candidate revived sources that were identified among nearby (z ≤ 0.35) and low-mass (M₅₀₀ ≤ 5 × 10¹⁴ M_⊙) Planck clusters in the footprint of the Second Data Release of the LOw Frequency ARray (LOFAR) Two Metre Sky Survey (LoTSS-DR2).

Methods. By inspecting LoTSS-DR2 images at 144 MHz, we identified seven targets with patchy and filamentary morphologies. They were followed-up with the upgraded Giant Metrewave Radio Telescope (uGMRT) at 400 MHz. By combining LOFAR and uGMRT data, we obtained high-resolution images and spectral index maps that we used to interpret the nature of the sources.

Results. All targets show regions with very steep spectra, which confirms the effectiveness of our morphology-based selection in identifying fossil plasma. Based on their morphology, spectral properties, and optical associations, we investigated the origin of the targets. We found a variety of promising revived fossil sources but also showed that apparently intricate structures can easily be misclassified in the absence of high-resolution and multi-band data.